Paul Estevenon

Thibault Kaczmarek

Fabien Vadot

Pier Thomas Dumas

Lorenzo Solari

Éléonore Welcomme

Stephanie Szenknect

Adel Mesbah

Philippe Moisy

Christophe Poinssot

Pier Lorenzo Solari

Eleonore Welcomme

Nicolas Dacheux

Formation of CeSiO 4 from cerium( iii ) silicate precursors

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Cerium is often considered as a convenient surrogate in order to determine the behavior of plutonium in various systems, because Ce(III) and Ce(IV) chemistry are close to their plutonium counterparts and relatively representative of Pu(IV)/Pu(III) redox couple (E°(Ce(IV)/Ce(III) = 1.72 V/ENH; 1 E°(Pu(IV)/Pu(III) = 1.047 ± 0,003 V/ENH 2 ). In particular, Ce(III) silicate phases may be used to simulate isostructural Pu(III) silicates, which have been reported in few studies only.

3-5

CeSiO 4 is also isostructural of ZrSiO 4 , HfSiO 4 and AnSiO 4 (for An = Th, Pa, U, Np, Pu and Am) [START_REF] Keller | [END_REF][7][8][9] (space group I4 1 /amd). The formation of actinide silicate colloids was observed at room temperature for thorium, uranium(IV) and neptunium(IV) and was proved to modify the mobility of the actinides in the environment. [10][11][12][13][14] The potential formation of Pu-based colloids in conditions representative of spent nuclear fuels repository settings deserves to be investigated. In this context, the study of cerium silicates appears as a compulsory first step to evaluate the behavior of plutonium in silicate-rich environments submitted to various redox conditions. The preparation and characterization of Ce(III) silicates (Ce 2 SiO 5 , Ce 4.67 (SiO 4 ) 3 O, A-Ce 2 Si 2 O 7 and G-Ce 2 Si 2 O 7 ) by high temperature routes has been extensively studied due to their potential applications. [15][16][17][18][19][20][21][22][23][24][25] Indeed, these compounds have been identified as promising luminescent materials (in the blue/violet region) for scintillators and detectors applications. [21][22][23][24][25] They have also been identified as very efficient ions exchangers for radionuclides separation 20 and their formation has been reported as secondary phases for cerium doped zircon ceramics and cerium doped nuclear waste glasses. 26, 27 However, the behavior of Ce(III) silicate phases in wet reactive media has not been studied so far. CeSiO 4 has been reported by Schlüter et al. 28 as a natural occurring mineral phase formed in pegmatite rocks of the Stetind mount (Norway). The first synthesis of this silicate was reported by Dickson and Glasser in 2000 9, 29 . It consisted in the hydrothermal treatment of a mixture containing Ce(NO 3 ) 3 , SiO 2 and CaO under inert atmosphere between 55 and 180°C. The formation of CeSiO 4 samples using a wet chemistry route from Ce(III) nitrate or chloride at temperatures ranging from 40°C to 150°C in weakly basic conditions has been reported recently 30 . These results highlighted that the use of Ce(III) was required to prepare CeSiO 4 . Indeed, all the attempts to form CeSiO 4 from Ce(IV) precursors failed due to the precipitation of cerium tetrahydroxide, leading to CeO 2 by ageing. Consequently, the use of reactive conditions that may disfavor cerium hydrolysis constitutes a key parameter to prepare pure CeSiO 4 . The formation of CeSiO 4 from crystalline Ce(III) silicate phases through a dissolution and oxidation process could also be considered as an alternative way of synthesis. Moreover, due to cerium representativity as a plutonium surrogate, this study may provide crucial information on plutonium silicate chemistry and, therefore, on the behavior of plutonium in the environment. In this context, this paper reports a comprehensive study of the formation of CeSiO 4 by chemical degradation of solid Ce(III) silicate precursors. The samples were then pelletized by isostatic pressing under 5 MPa at room-temperature and heated at high temperature under Ar -4% H 2 atmosphere in order to favor the cerium reduction, as described in Table 1. The as-prepared samples were characterized by Powder X-Ray Diffraction (PXRD), infrared and Raman spectroscopies and Scanning Electron Microscopy (SEM). The presence of cerium oxides was never detected in the samples. Moreover, Rietveld refinement of the diagrams did not show significant differences of the lattice parameters compared to those reported in the literature 19 (Table 1, Table S2 and Figure 1). Infrared and Raman spectra were also recorded for each compound before and after hydrothermal treatment (Figure S1 and Figure S2).

Preparation of CeSiO 4 from Ce(III) silicate precursors. Ce(III) silicate based precursors were placed in contact with 0.75 mol.L -1 HNO 3 or HCl solution in air (no dissolution was evidenced in these conditions). The pH was then adjusted to the final value with 8 mol•L -1 NaOH solution. The mixtures were put in Teflon lined reactors in Parr autoclaves and then treated in hydrothermal conditions for 1 to 20 days between 60°C to 250°C and under autogenous pressure (reference conditions were fixed to 7 days and 150°C, respectively) (Table S1). The obtained precipitates were then separated from the supernatant by centrifugation for 12 min at 14 000 rpm, washed twice with deionized water and once with ethanol, and finally dried overnight at 60°C in an oven. The samples were characterized by PXRD, infrared and Raman spectroscopies, SEM and XAS.

Characterization

The synthesized compounds were analyzed by PXRD with the help of a Bruker D8 advance diffractometer equipped with a lynxeye detector and using Cu Kα radiation (λ = 1.54184 Å) in a reflection geometry (parallel beam). PXRD patterns were recorded between 5° and 100° (2θ) with steps of 0.019° and a total counting time of 2.5 to 3 hours per sample. Pure silicon was used as a standard material to extract the instrumental parameters. All the collected data were refined by the Rietveld method using the Fullprof_Suite package. 31 During the refinements, different profile and structure parameters were allowed to vary, such as the zero shift, unit-cell parameters, scale factor, and overall displacement factor. However, the occupancy of each site was fixed to the calculated values. Raman spectra were recorded with a Horiba-Jobin Yvon Aramis device equipped with an edge filter and a Nd:YAG laser (532 nm) that delivered 60 mW at the surface of the sample. In order to avoid any laser-induced degradation of the compound, the power was turned down by the means of optical filters. The laser beam was then focused on the sample using an Olympus BX 41 microscope with a X50LMP objective, resulting in a spot area of ∼1 μm 2 and a power of 475 µW for CeSiO 4 samples and of 3.8 mW for Ce(III) silicates. For each spectrum, a dwell time ranging from 30 to 600 s was used. All the recorded spectra corresponded to the average of four scans in order to minimize the measurement error. Moreover, the data were collected on different areas for each sample. FTIR spectra were recorded in the 300-4000 cm -1 range with a Perkin-Elmer FTIR Spectrum 100 device. Powdered samples were deposited on the surface of an ATR crystal without any prior preparation. The spectra collected in such operating conditions exhibited a resolution lower than 4 cm -1

. Four scans were performed to average the measurement error. SEM observations were directly conducted on small powder samples without any prior preparation such as metallization, using a FEI Quanta 200 electronic microscope, equipped either with an Everhart-Thornley Detector (ETD) or a Back-Scattered Electron Detector (BSED), under high vacuum conditions with a very low accelerating voltage (2 -3.1 kV). These conditions were chosen in order to create a beam deceleration effect that led to high resolution images. The extended X-ray absorption fine structure (EXAFS) and X-Ray Absorption Near Edge Structure (XANES) spectroscopy measurements were carried out at the MARS Beamline at the SOLEIL synchrotron facility (Saint Aubin). The spectra were collected in the fluorescence mode at room temperature. Measurements were performed at the cerium L III edge (5723 eV). Data reduction and extraction of EXAFS oscillation was performed using the Athena and Artemis package. 33 The threshold energy, E 0 , was defined as the . The shell fits were performed in R-space using k 1 -, k 2 -, and k 3 -weighting. Theoretical phase shifts and backscattering amplitudes were obtained with the ab initio code FEFF8.2 34 using the CeSiO 4 structures.

9 ) usually led to the simultaneous precipitation of amorphous SiO 2 and solubilization of cerium as Ce 3+ in solution. Only small amounts of cerium were precipitated as CeO 2 or CeSiO 4 in these conditions (mass yield below 20% compared to quasi-quantitative precipitation at higher pH). Furthermore, hydrothermal treatments performed in alkaline media (i.e. for pH  11) did not lead to the formation of CeSiO 4 due to the low dissolution rate of the Ce(III) silicate based precursors. This could be directly correlated to the low solubility of these precursors in alkaline conditions due to kinetics and/or thermodynamic factors. From this point of view, performing hydrothermal treatment with an initial pH close to pH = 7 appeared as the best compromise to prepare CeSiO 4 .

Results and discussion

An important decrease of the pH value during the hydrothermal treatment (from initial pH = 7 to final pH = 2) was observed in conditions that favored the formation of CeSiO 4 (Table S1). However, it remains unclear whether this pH evolution was linked to the oxidation of Ce(III) species or controlled by CeSiO 4 solubility equilibrium. Among the precursors used for these Ce 4.67 (SiO 4 ) 3 O and A-Ce 2 Si 2 O 7 were the best reactants to form pure CeSiO 4 . Indeed, using Ce 2 SiO 5 as starting reactant generally led to mixtures of CeO 2 and CeSiO 4 (Figure S3). This result was explained by the high sensibility of The impact of the temperature of the hydrothermal treatment on the yield of formation of CeSiO 4 was also studied through various experiments between 60°C and 250°C starting from Ce 4.67 (SiO 4 ) 3 O and A-Ce 2 Si 2 O 7 precursors (Figure 5 and Figure S6). CeSiO 4 was formed between 60°C and 150°C from both precursors. At T = 250°C, a large amount of crystallized CeO 2 was formed while only small amounts of CeSiO 4 were obtained. Such a temperature limit is in good agreement with the one obtained from aqueous Ce(III) precursors. microstructure characteristic of sintered samples, whereas the final CeSiO 4 grains exhibited the square-based bipyramid morphology characteristic of zircon-type materials (Figure 6). The same changes were also observed during the conversion of Ce 4.67 (SiO 4 ) 3 O. Therefore, the formation of CeSiO 4 from Ce(III) silicate precursors surely required the progressive dissolution of the starting precursor, the oxidation of Ce(III) to Ce(IV) in solution and then, the precipitation of CeSiO 4 .

Preparation of CeSiO 4 in hydrochloric media

In order to underline the potential role of nitric acid during the synthesis of CeSiO 4 , especially through the development of redox reactions with cerium, several experiments were made from Ce 4.67 (SiO 4 ) 3 O, A-Ce 2 Si 2 O 7 and G-Ce 2 Si 2 O 7 in hydrochloric media. It should be noted that both hydrochloric and nitric acids are strong acids in this concentration range and that the complexing power of nitrate and chloride anions may be inferred to be similar. [START_REF] Cotton | Lanthanide and actinide chemistry[END_REF] In contrast, nitric acid is often an oxidizing agent while hydrochloric acid is usually considered as a more reducing medium. Hydrothermal treatments were carried out in air for 7 days at 150°C, with pH values ranging from pH = 2 to 7. All of these experiments led to the partial conversion of the starting precursors into CeSiO 4 (Figure 7

Characterization of CeSiO 4 samples

To summarize, the multiparametric study developed to prepare pure CeSiO 4 allowed to determine optimal conditions, which correspond to the following guidelines: precursors due to their favorable behavior (in terms of kinetics) during dissolution tests; -nitric medium, to be preferred over hydrochloric one; -hydrothermal treatment in air with a starting pH value adjusted to 7.0, a temperature of 150°C and maintained for at least 3 days. For such optimized conditions, the Rietveld refinement of XRD data led to the following unit cell parameters: a = 6.9523(2) Å and c = 6.2036(2) Å, i.e. V = 300.06(2) Å 3 (Figure 9). These values is in good agreement with those obtained when using aqueous Ce(III) precursors ). Moreover, infrared and Raman spectra recorded on these samples were characteristic of the features obtained for CeSiO 4 30 (Table S3, Figure S8 and Figure S9).

In order to complete the characterization of pure CeSiO 4 , XANES and EXAFS experiments were performed at the Ce L III edge. The sample studied was prepared after hydrothermal treatment in nitric medium (150°C, 7 hours, pH = 7) starting from A-Ce 2 Si 2 O 7 . On the XANES spectra, the CeSiO 4 sample is characterized by a white line double peaks of roughly equal intensity with maxima at 5731 eV and 5738 eV (Figure 10). These double peaks are characteristic of tetravalent cerium corresponding to the 2p 3/2 → (4fL)5d, and 2p 3/2 → (4f 0 )5d electron transitions (L denotes relaxation transition of an electron from oxygen 2p to cerium 4f orbital) and their locations in good agreement with literature references, while trivalent cerium would be characterized by a single peak at 5727 eV (originating from the electron transition 2p 3/2 → (4f 1 )5d) (Figure 10). [38][39][40] Therefore, the XANES spectra tends to confirm the oxidation state +IV expected for CeSiO 4 . It could also be noticed that the shoulder usually observed at 5728 eV on the CeO 2 spectra (used as Ce(IV) reference in The spectra obtained from these simulations were found to be in good agreement with the experimental data (Figure 11 and Table 2). The first peak was assigned to the oxygen atoms present in the first coordination sphere of cerium whereas the two following peaks were attributed to the silicon atoms present in the second coordination sphere, which correspond to bidentate and monodentate silicate bonds, respectively. At longer distance, a third contribution was associated to the Ce-Ce interactions. All of these results confirmed the formation of a zircon-type CeSiO 4 (I4 1 /amd space group), resulting from the oxidation of Ce(III) into Ce(IV) under hydrothermal conditions.

Discussion regarding the formation mechanism of CeSiO 4

According to the SEM observations, it may be inferred that the formation of CeSiO 4 relies on the low dissolution rates of the Ce(III) silicate starting precursor. This mechanism could correspond to the oxidative dissolution of Ce(III) silicate starting precursors, leading to the formation of Ce(IV)-silicate aqueous species which then precipitate as CeSiO 4 . However, due to the suspected very low solubility of Ce(IV) silicate species, 30 the precipitation of CeSiO 4 in the near field of the precursor and then the formation of a passivating layer on its surface may be expected with this mechanism. However, since no passivating layer was observed, a chemical path through aqueous Ce(III) silicate intermediate species seems to be more likely.

The precursors dissolution could also correspond to the hydrolysis of the Si-O-Si bonds, which could exist for all Ce(III) silicate species, and then could lead to the release of Ce(III) silicate based complexes in the reactive media. These complexes are subsequently oxidized to form Ce(IV) silicate species which play a key role during the precipitation of CeSiO 4 , as already described for thorium and uranium (IV) silicates. [41][42][43] The formation of Ce(IV) species may be enhanced by oxidizing species such as nitrogen based species coming from the decomposition of nitric acid and O 2 from air.

According to this hypothesis, the corresponding formation mechanism may be explained by the following reactions: and only extrapolated ones for silicate complexes. 30 However, it is known that for actinides and lanthanides, the complexation for M(IV) is much more important than for M(III).

44

This higher complexation obtained for M(IV) compared to M(III) would, therefore, lead to an apparent decrease of the potential associated with the redox couple. Therefore, the temperature limit observed for the conversion of Ce(III) silicate into CeSiO 4 was probably due to the destabilization of aqueous Ce(III) (and/or Ce(IV)) silicate intermediates for T > 150°C, and also of aqueous Ce(IV) silicate species with respect to hydroxide ones. In this context, the small amounts of CeSiO 4 observed for T > 150°C were surely formed during the beginning of the hydrothermal treatment (i.e. during the rise of temperature). From a more general point of view, the formation of CeSiO 4 from Ce(III) silicate solid precursors appeared to be more efficient and easier than working directly from aqueous Ce(III) precursors. Indeed, this original way of preparation enabled to work on a wider pH range with a good recovery yield and without any constraint coming from the working atmosphere. This difference may result from the slow dissolution and oxidation (or oxidative dissolution) of Ce(III) starting precursors, leading to low concentrations in the reactive media which disfavored the cerium (IV) and cerium (III) hydrolysis.

Conclusions

CeSiO 4 was successfully formed from several solid Ce(III) based precursors, i.e. Ce 2 SiO 5 , Ce 4.67 (SiO 4 ) 3 O, A-Ce 2 Si 2 O 7 and G-Ce 2 Si 2 O 7 after hydrothermal treatment under air atmosphere. For several of them, their slow conversion allowed the preparation of single phase CeSiO 4 with higher recovery yields compared to those obtained when starting from mixtures of cerium and silicate solutions. The optimized conditions to form pure CeSiO 4 were based on hydrothermal treatment for 7 days at T ≤ 150°C (nitric medium) starting from A-Ce 2 Si 2 O 7 or Ce 4.67 (SiO 4 ) 3 O precursor. Since CeSiO 4 was not obtained from Ce(IV) precursors due to the formation of CeO 2 , as a consequence of the rapid hydrolysis of Ce(IV), the redox state of cerium in the starting precursor appeared as a key parameter. The formation of CeSiO 4 was directly correlated to the formation of Ce(III) silicate aqueous species which were slowly oxidized in situ during the hydrothermal treatment to form Ce(IV) silicate. In addition, the reactivity of such Ce(III)-based silicates in representative conditions of environmental situations (pH = 7 and 60°C  T  150°C) raises important questions about their evolution in conditions characteristic for several of their reported applications (including wet environments)

Figure 1 .

 1 Figure 1. PXRD diagrams of prepared Ce2SiO5, Ce4.67(SiO4)3O, A-Ce2Si2O7 and G-Ce2Si2O7, calculated and difference profiles after Rietveld refinement. XRD lines of sample holder are pointed out by an asterisk.

Figure 2 .

 2 Figure 2. PXRD patterns obtained after hydrothermal treatment (7 days, T = 150°C) under air atmosphere starting from Ce4.67(SiO4)3O precursor in nitric acid media and with an initial pH equal to 11.0 (1), 7.0 (2), 6.1 (3), 4.3 (4), 2.0 (5), CHNO 3 = 0.7 mol•L -1 (6) and CHNO 3 = 1.3 mol•L -1 (7). XRD lines of the sample holder are pointed out by an asterisk. Characteristic XRD lines of CeO2, CeSiO4 and Ce4.67(SiO4)3O were extracted from ref 32, 9 and 16, respectively.

Figure 3 .

 3 Figure 3. PXRD patterns obtained after hydrothermal treatment (7 days, T = 150°C) under air atmosphere starting from A-Ce2Si2O7 precursor in nitric acid media and with an initial pH equal to 11.1 (8), 8.1 (9), 7.0 (10), 4.0 (11), 2.0 (12), CHNO 3 = 0.7 mol•L -1 (13) and CHNO 3 = 1.3 mol•L -1 (14). XRD lines of sample holder are pointed out by an asterisk. Characteristic XRD lines of CeO2, CeSiO4 and A-Ce2Si2O7 were extracted from ref 32, 9 and 24, respectively.

  4 on a wide pH range: -from C HNO 3 = 0.7 mol•L -1 to pH = 8.4 starting from Ce 4.67 (SiO 4 ) 3 O (Figure 2); -from C HNO 3 = 1.3 mol•L -1 to pH = 8.1 starting from A-Ce 2 Si 2 O 7 (Figure 3); -from pH = 1.0 to pH = 8.5 starting from Ce 2 SiO 5 (Figure S3); -from pH = 1.0 to pH = 8.2 starting from G-Ce 2 Si 2 O 7 (Figure S4). The syntheses performed in very acidic media (i.e. for C HNO 3 > 0.3 mol•L -1

Figure 4 .

 4 Figure 4. PXRD patterns obtained when using A-Ce2Si2O7 as starting precursor, after hydrothermal treatment performed at 150°C under air atmosphere in nitric media and pH = 7 for 1 day (15), 3 days (16) and 7 days (10). XRD lines of sample holder are pointed out by an asterisk. Characteristic XRD lines of CeO2, CeSiO4 and A-Ce2Si2O7 were extracted from ref 32, 9 and 24, respectively.

7 couldFigure 5 .

 75 Figure5. PXRD patterns obtained when using Ce4.67(SiO4)3O as starting precursor, after hydrothermal treatment performed under air atmosphere in nitric media and pH = 7 at 60°C (52 days)(17), 150°C (7 days) (2) and 250°C (7 days)(18). Characteristic XRD lines of CeO2, CeSiO4 and Ce4.67(SiO4)3O were extracted from ref 32, 9 and 16, respectively.

30 Moreover,Figure 6 .

 306 Figure 6. SEM micrograph obtained for A-Ce2Si2O7 (T = 1350°C for 9 hours, Ar -4% H2) (a) and for CeSiO4 obtained after hydrothermal conditions (T = 150°C, t = 7 days) under air atmosphere from A-Ce2Si2O7, in nitric medium and pH = 7.0 (10) (b).

  -a and Figure S7).As observed for G-Ce 2 Si 2 O 7 , extending the duration of the hydrothermal treatment induced the increase of the yield of formation of CeSiO 4 (Figure7-b). Therefore, it was concluded that the kinetics of formation of CeSiO 4 were lower in hydrochloric media than in nitric media (Figure2and FigureS7). This difference could be attributed to the presence of nitrous acid, issued from the reduction of HNO 3 , during the oxidation of Ce(III), which speeds up the kinetics of formation of CeSiO 4 . SEM observations performed on G-Ce 2 Si 2 O 7 and CeSiO 4 confirmed the morphological change already noted in nitric medium. Indeed, from the features characteristic of a sintered sample obtained for G-Ce 2 Si 2 O 7 , the morphology turned into a square-based bipyramid morphology for CeSiO 4 (Figure8). Moreover, in these conditions, which allowed a slow oxidation process, an epitaxial growth of CeSiO 4 was observed leading to a prism-like morphology characteristic of the zircon-type silicates growth (the spherical particles observed in Figure 8-b may correspond to residual G-Ce 2 Si 2 O 7 or to SiO 2 ).

37

 37

Figure 7 .Figure 8 .

 78 Figure 7. PXRD patterns obtained after hydrothermal treatment (7 days and T = 150°C (a) or 21 days and T = 150°C (b)) under air atmosphere starting from G-Ce2Si2O7 precursor, in hydrochloric media and with initial pH equal to 7.2 (19), 3.3 (20), 2.2 (21), 7.2 (22), 3.1 (23) or 2.1 (24). Characteristic XRD lines of CeO2, CeSiO4 and G-Ce2Si2O7 were extracted from ref 32, 9 and 18, respectively.

Figure 9 .

 9 Figure 9. PXRD diagram, calculated and difference profile after Rietveld refinement obtained for CeSiO4 prepared under hydrothermal conditions (T = 150°C, t = 7 days) under air atmosphere in nitric medium and with pH = 7.0 starting from A-Ce2Si2O7 precursor (10).

  Ce 2 Si 2 O 7 or Ce 4.67 (SiO 4 ) 3 O used as Ce(III) starting

30 (a = 6 .

 306 9603(1) Å and c = 6.1946(2) Å, i.e. V = 300.11(2) Å 3 ) and with those reported by Skakle et al. 9 (a = 6.9564(3) Å and c = 6.1953(4) Å, i.e. V = 299.80(3) Å 3

Figure 10 )

 10 , was not observed for CeSiO 4 , probably because of the differences of Ce-O bonds local geometry in these two phases. In order to confirm the Ce environment in CeSiO 4 , pseudoradial distribution function was determined from the Fourier transform of the EXAFS spectrum (k = 2-10 Å -1 ). The data analysis was based on the model structure reported in the literature for the crystal structure of CeSiO 4 . 9 Only Debye-Waller factors were adjusted variables whereas coordination numbers and distances were fixed to crystallographic values.

Figure 10 .

 10 Figure 10. Ce LIII edge XANES spectrum (a) and respective first derivative (b) of Ce(III) and Ce(IV) references40 and CeSiO4 prepared under hydrothermal conditions (T = 150°C, t = 7 days) in nitric medium and with pH = 7.0, starting from A-Ce2Si2O7 precursor(10).

Figure 11 .

 11 Figure 11. Cerium LIII EXAFS spectrum 2 Å < k < 10 Å -1 (a) and corresponding Fourier transform (b) of CeSiO4 prepared under hydrothermal conditions (T = 150°C, t = 7 days) under air atmosphere in nitric medium and with pH = 7.0, starting from A-Ce2Si2O7 precursor (10).

Table 1 .

 1 Conditions of formation of Ce(III) silicate precursors and associated lattice parameters determined by Rietveld refinement. All of the reagents used were of analytical grade and supplied by Sigma-Aldrich. Na 2 SiO 3 was used as the silicate precursor. CeO 2 (99.9 %) and SiO 2 (99.5 %) were used as solid precursors to prepare high-temperature Ce(III) silicate phases. Ce 4.67 (SiO 4 ) 3 O (space group P6 3 /m), tetragonal Ce 2 Si 2 O 7 (space group P4 1 , named "A-Ce 2 Si 2 O 7 ") and monoclinic Ce 2 Si 2 O 7

	Compound	Space group	Grinding step	Thermal treatment	a (Å)	Unit cell parameters b (Å) c (Å)	β (°)
	Ce 2 SiO 5	P12 1 /c1 (14)			9.2775(3)	7.3942(3)	6.9665(3)	108.33(1)
	Ce 4.67 (SiO 4 ) 3 O	P6 3 /m (176)	1h -30 Hz	9h -1350°C Ar -4% H 2	9.6505(4)		7.0738(3)
	A-Ce 2 Si 2 O 7	P4 1 (76)			6.7965(3)		24.7258(14)
	G-Ce 2 Si 2 O 7	P2 1 /n (14)	1h -30 Hz	9h -1550°C Ar -4% H 2	8.7245(4)	13.0735(6)	5.4031(3)	90.13(1)

Syntheses

Reactants. (space group P2 1 /n, named "G-Ce 2 Si 2 O 7 "). The samples were prepared by mixing CeO 2 and SiO 2 in stoichiometric conditions with respect to the targeted materials. Homogenization of the powders was performed by mechanical milling step (30 Hz, 1 hour) in a tungsten carbide milling vessel thanks to a Retsch MM 200 vibration mill mixer.

Preparation of CeSiO 4 in nitric media

  All attempts to prepare CeSiO 4 were made from Ce 2 SiO 5 , Ce 4.67 (SiO 4 ) 3 O, A-Ce 2 Si 2 O 7 and G-Ce 2 Si 2 O 7 precursors and Na 2 SiO 3 as complementary silicon source in order to maintain the Ce:Si stoichiometry of CeSiO 4 .

	These experiments were performed in nitric acid with
	8.4 ×10	-4 mol of cerium and silicon in 4 mL (leading to a
	concentration of 0.21 mol•L	-1 when considering the full
	dissolution of cerium and silicon), initial acid conditions
	ranging from C HNO 3 = 1.3 mol•L

-1

to pH = 11.7, air atmosphere and with a hydrothermal treatment of 7 days at 150°C. For each precursor, hydrothermal treatments allowed the formation of CeSiO

  67(SiO4)3O as starting precursor, after hydrothermal treatment performed under air atmosphere in nitric media and pH = 7 at 60°C (52 days)(17), 150°C (7 days) (2) and 250°C (7 days)(18). Characteristic XRD lines of CeO2, CeSiO4 and Ce4.67(SiO4)3O were extracted from ref 32, 9 and 16, respectively. be correlated to the temperature of the thermal treatment required to form this phase (9 hours, 1550°C, Ar -4% H 2 ), syntheses of Ce 4.67 (SiO 4 ) 3 O were also performed in the same conditions. This heating treatment did not induce any significant change in the behavior of the Ce(III) silicate precursor compared to the heating treatment performed at 1350°C. Thus, the low reactivity of G-Ce 2 Si 2 O 7 could be linked to the nature of the phase rather than to its better crystallinity or lower reactive surface area compared to others precursors. Experiments have been also performed in order to determine if the mechanical grinding step (performed before the Ce(III) solid silicate synthesis) alone was sufficient to explain the formation of CeSiO 4 . To this aim, hydrothermal syntheses were made at 150°C from stoichiometric amounts of CeO 2 and SiO 2 mechanically milled together for 1 hour at 30 Hz and with different pH values. CeSiO 4 did not form during these experiments. Therefore, the use of Ce(III) silicate precursor seems to be necessary to prepare CeSiO 4 . This result is in good agreement with the ab initio calculation which predicts that CeSiO 4 is less stable than the mixture of CeO 2 and SiO 2 .

35

Because experiments performed in air led to the formation of pure CeSiO 4 , it appeared that the working atmosphere was not a key parameter controlling the formation of CeSiO 4 , contrarily to what was observed with the aqueous Ce(III) species.

29, 30

Table 2 .

 2 Structural parameters determined for CeSiO4 sample. The values fixed for the simulations have been marked by an asterisk. ΔEk=0 = 4 eV ; F = 0.13 ; S0² = 0.8.

		R (Å)	N	σ	2 (Ų)
	Ce-O 1	2.2740 *	4 *	0.007 (5)
	Ce-O 2	2.3645 *	4 *	0.006 (5)
	Ce-Si 1	3.0976 *	2 *	0.001 (1)
	Ce-Si 2	3.8075 *	4 *	0.003 (13)
	Ce-Ce	3.8075 *	4 *	0.013 (13)

  15, 16, 18-25 . It can also pave the way to a better understanding of the potential reactivity of actinide(III) silicate phases such as the solid Pu(III) silicate phases obtained by Fortner et al. by vapor phase hydration of borosilicate glasses 51, 52 and, therefore, of the behavior of plutonium in nuclear waste repository conditions.

Acknowledgements

The authors would like to thank R. Podor, J. Lautru and V. Trillaud (from ICSM) for supporting SEM experiments.

Conflicts of interest

There are no conflicts to declare.


Paul Estevenon

Eleonore Welcomme

Stephanie Szenknect

Adel Mesbah

Philippe Moisy

Christophe Poinssot

Nicolas Dacheux

Preparation of CeSiO 4 from aqueous precursors under soft hydrothermal conditions

Even though CeSiO4 was synthesized one time through a hydrothermal treatment, the conditions leading to its formation remain largely unknown. In order to define the optimized conditions of synthesis, a multiparametric study was developed by varying the pH of the solution, the temperature, the nature of the reactants and of the complexing ions in solution. This study highlighted that CeSiO4 could not be obtained starting from Ce(IV) reactants. An optimal set of conditions was defined to prepare single phase samples. Pure CeSiO4 was obtained through a hydrothermal treatment at 150°C using a starting mixture of 1 mol•L -1 Ce(III) nitrate and Na2SiO3 solutions and by adjusting the initial pH to 8. The chemical limitations observed during the synthesis of CeSiO4 suggested that the formation of this phase may result from the slow insitu oxidation of a Ce(III) silicate complex during the hydrothermal treatment.

Introduction

The synthesis of Ce(III) silicates has been extensively studied mainly because of their optoelectronic, catalytic and ion exchanger properties. [1][2][3][4][5][6][7][8][9][10][11] The formation of cerium(III) silicates was also reported as secondary phases for cerium doped silicon nitride ceramics and cerium doped nuclear waste glasses. 12 However, even if the formation of cerium(III) silicates is ubiquitous, only few information are available concerning the properties of the cerium(IV) silicate, because high temperature methods commonly used to prepare silicate based phases did not allow to form CeSiO 4 . 3 Indeed, thermodynamic calculations predicted that this phase was unstable compared to the mixture of corresponding oxides, CeO 2 and SiO 2 .

13

Silicate species are abundant in environmental conditions, 14 especially in potential geological sites dedicated to underground repository of radioactive waste. In such conditions, actinide elements could be in contact with silicate species coming from cement, glass waste form, granite or clay. This raises important questions regarding the interaction of silicate species and actinides and associated potential impact on actinide mobility in environmental conditions. Tetravalent actinide elements are often associated to very low mobility in environmental conditions due to the low solubility of actinide bearing hydroxides, i.e. An(OH) 4 , in neutral and alkaline media. [START_REF] Neck | [END_REF] Such a solubility could be significantly modified in the presence of silicate species in solution. Indeed, uranium(IV) and thorium(IV) are well known to form natural thorite (ThSiO 4 ), coffinite (USiO 4 , second more abundant U(IV) bearing phase) and uranothorite solid solutions by alteration of the respective oxides. [16][17][18][19][START_REF] Janeczek | Materials Research Society, Symposium Proceedings[END_REF] Moreover, actinide silicate colloids have been observed at room temperature for actinide(IV): Th(IV), U(IV) and Np(IV). Several authors showed that colloidal transport influenced significantly the mobility of actinides in environmental conditions. [START_REF] Hennig | [END_REF][22][23][24][25] In the case of plutonium(IV), the formation of PuSiO 4 and of plutonium silicate colloids was also suggested. 26 The mechanism associated to their formation remained, however, largely unknown.

Cerium is commonly used as a surrogate for actinides, especially for plutonium due to some similarities in terms of ionic radius and chemical properties. First, CeSiO 4 was reported to be isostructural with ZrSiO 4 (space group I4 1 /amd), HfSiO 4 and AnSiO 4 (with An = Th, Pa, U, Np, Pu and Am). [27][28][29][30] It has been reported as a natural occurring mineral phase of the zircon group by Schlüter et al. in 2009 31 and named stetindite.

The formation of synthetic CeSiO 4 was formerly reported by Dickson and Glasser in 2000 during the study of cement alteration. 30, 32 To the best of our knowledge, this protocol was the only reported in literature to prepare this phase. It consisted in the hydrothermal treatment (55  T  180°C for 14 to 700 days) of a mixture containing cerium(III) nitrate solution, SiO 2 and CaO dissolved in aqueous solution under inert atmosphere. From PXRD analyses, these authors reported the formation of a mixture of CeSiO 4 and CeO 2 for T ≥ 130°C and of CeSiO 4 as the only crystallized phase for T ≤ 85°C. However, due to the poor crystallinity of CeO 2 precipitated at low temperatures, its presence in the final mixtures was not definitively discarded. Moreover, the role of CaO in these media remained largely unclear. This work was thus dedicated to the preparation of oxide free CeSiO 4 samples in order to get a set of thermodynamic experimental data associated to this phase. In this field, the comprehensive study of the formation of pure CeSiO 4 samples with different aqueous precursors is reported here.

Materials and methods

Preparation of CeSiO 4

All of the reagents used were of analytical grade and supplied by Sigma-Aldrich. Na 2 SiO 3 was used as silicate precursor. Ce(NO 3 ) NaOH. The experiments were performed either in air or under inert conditions (Ar-atmosphere glove box). All the mixtures were put in Teflon lined reactors in Parr autoclaves and then treated under hydrothermal conditions during 1 to 20 days with a temperature ranging from 40°C to 250°C and under autogenous pressure (Table S1). Reference conditions were fixed to 10 days of hydrothermal treatment at 150°C. Thus, the final precipitates were separated from the supernatant by centrifugation for 12 min at 14 000 rpm, washed twice with deionized water and once with ethanol, and then finally dried overnight at 60°C in an oven.

Characterizations

PXRD data were collected on the resulting powders using the Bruker D8 advance diffractometer equipped with a lynxeye detector and using Cu Kα radiation (λ = 1.54184 Å) in a reflection geometry (parallel beam). PXRD patterns were recorded between 5° and 100° (2θ) with steps of 0.019° and a total counting time of 2.5 to 3 hours per sample. Pure silicon was used as a standard material to extract the instrumental function. Therefore, all the collected data were refined by the Rietveld method using the Fullprof suite package. 33 During the refinements, different profile and structural parameters were allowed to vary, such as the zero shift, unit-cell parameters, scale factor, and overall displacement factor. However, the occupancy of each site was fixed to the calculated values. Raman spectra were recorded with a Horiba-Jobin Yvon Aramis device equipped with an edge filter and a Nd:YAG laser (532 nm) that delivers 60 mW at the sample surface. In order to avoid any laser-induced degradation of the compound, the power was turned down by the means of optical filters. The laser beam was then focused on the sample using an Olympus BX 41 microscope with an X50LMP objective, resulting in a spot area of ∼1 μm 2 and a power of 475 µW. For each spectrum, a dwell time ranging from 30 to 600 s was used. Four scans were performed to average the measurement error. For each sample, the data were collected on different areas. FTIR spectra were recorded in the 300-4000 cm -1 range thanks to a Perkin-Elmer FTIR Spectrum 100 device. Powdered samples were deposited on the surface of an ATR crystal without any prior preparation. The spectra collected in such operating conditions exhibited a resolution lower than 4 cm -1

. Four scans were performed to average the measurement error. SEM observations were directly conducted on small aliquots of samples without prior preparation such as metallization, using a FEI Quanta 200 electronic microscope, equipped either with an Everhart-Thornley Detector (ETD) or a Back-Scattered Electron Detector (BSED), under high vacuum conditions with a very low accelerating voltage (2 -3.1 kV). These conditions were chosen in order to create a beam deceleration effect that led to high resolution images. Speciation calculations were performed using the PHREEQC 2 software 34 and the thermochimie_PHREEQC_SIT_v9 database, 35 combined with the thermodynamic constants available in Table S2 in order to determine the speciation of cerium in the reactive media.

Results and discussion

As already mentioned, the only protocol reported for the preparation of CeSiO whereas the initial pH of the reactive media was varying from pH = 1 to pH = 12. Hydrothermal treatment were performed with temperatures and holding times ranging from 90°C to 250°C and from 7 to 10 days, respectively. All the attempts to prepare CeSiO 4 were unsuccessful. From PXRD characterization, only mixtures of CeO 2 and amorphous SiO 2 were observed (Figure 1). Moreover, the increase of the FWHM of the XRD lines associated to CeO 2 was noted when increasing the pH of the starting mixture, as the consequence of the decrease of the crystallite size. These results were explained by the rapid hydrolysis of Ce(IV) which happened even at low pH values and prevented any complexation of cerium by silicate species. 46 This explanation may be also supported by thermodynamic considerations. Indeed, the formation of CeSiO 4 is expected to be strongly disfavored compared to the mixture of CeO 2 and SiO 2 .

13

The IR spectra recorded for pH < 8 were characteristic of amorphous SiO 2 (Figure S1). On the contrary, they did not indicate the presence of SiO 2 or CeO 2 for pH > 8 but may correspond to the sorption or integration of silicate species in the structure of CeO 2 similar to those observed for the sorption of silicate species on the surface of actinides oxides. , initial pH of the reactive media varying from pH = 8.7 to 12 and for hydrothermal treatment performed during 7 to 10 days with temperatures between 150°C and 250°C. PXRD diagrams of the samples prepared are reported in Figure 2 for the various initial conditions examined. Unlike other tetravalent actinide silicates, CeSiO 4 was never formed using these conditions (Figure 2). This specific behavior of cerium(IV) could result from the stronger complexation constants of carbonate species with cerium and from the different stoichiometry of the limiting complexes obtained for cerium, Ce(CO 3 ) 6 8-50 and actinides, An(CO 3 ) 5 6-51 in carbonate media.

Both aspects would induce the decrease of the cerium availability as the consequence of the strong interactions with carbonate species (Table 1) leading to the formation of the cerium hexacarbonate complex and preventing other ligands to interact with the metal due to steric constraints. The hydrothermal treatments were performed during 10 days at 150°C under inert atmosphere (Ar). The PXRD of the samples obtained are reported in Figure 3. It is worth noting that the formation of CeSiO 4 was observed in a small range of pH, i.e. between 7 and 9. Moreover, it was not obtained as a pure phase. Indeed, the formation of CeSiO 4 was limited by that of CeO 2 whether at higher and at lower pH values. Complementary characterization by IR spectroscopy (Figure S2) revealed the presence of SiO 2 for pH < 7 whereas it suggested the integration of silica in the lattice of cerium , in nitric acid media and with an initial pH equal to 12.3 (11), 10.1 (12), 9.0 (13), 8.4 (14), 7.0 (15), 5.9 (16), 3.1 (17) and 1.3 (18). XRD lines of the sample holder are pointed out dioxide or the sorption of silicate ions on the surface of the samples for the pH > 9, as already described for thorium.

47

Working with higher cerium and silicate concentrations (typically

C Ce  C Si  1.0 mol•L -1
) did not affect the pH range in which CeSiO 4 was formed (Figure 4). However, the comparison of the data reported for both concentrations showed the significant increase of the relative content of CeSiO 4 (compared to CeO 2 ) in the mixture when increasing the initial cerium and silicate concentrations. This result was in good agreement with that obtained for ThSiO 4, 37, 38 for which high concentrations of reactants appeared to improve the yield of formation of the silicate phase. In order to fix the better conditions associated to the preparation of CeSiO 4 , Rietveld refinement of the PXRD data was performed (Figure 5). From the results reported in Figure 4 and in Figure 5, the maximum relative content of CeSiO 4 in the prepared mixtures was obtained for concentrations of reactants of 1.0 mol , we can conclude from this study that CaO does not play any important role during the synthesis. In order to increase the recovery yield of CeSiO 4 , several experiments were unsuccessfully performed by using over-stoichiometric amounts of silicate compared to cerium. Furthermore, additional syntheses performed in air led to lower reaction yields than those obtained under argon atmosphere. This difference was assigned to the rapid oxidation of Ce(III) into Ce(IV) under aerated conditions, [START_REF] Hayes | [END_REF] which led to the rapid precipitation of cerium tetrahydroxide, then finally to CeO 2 by ageing.

46

Impact of the heating temperature on the formation of CeSiO 4 in nitric acid media. In order to underline the impact of the temperature of the hydrothermal treatment on the yield of formation of CeSiO 4 , various experiments were performed between 90°C and 250°C during 10 days, with starting cerium and silicate concentrations of 0.21 mol•L -1 and initial pH values ranging from pH = 7 to pH = 9. The PXRD diagrams of the samples obtained are plotted in Figure 6 for several temperatures of hydrothermal treatment. It is worth noting that CeSiO 4 was formed only between 90°C and 150°C. At higher temperatures (T ≥ 170°C), only crystallized CeO 2 was formed whereas at lower temperatures, the amount of CeO 2 formed as a by-product of the synthesis was difficult to quantify because of its poorly crystallinity for T < 120°C. However, even for the lower temperatures studied, the formation of CeO 2 was evidenced by Raman spectroscopy, whereas that of amorphous SiO 2 was confirmed by IR spectroscopy (Figure S3). The optimal conditions to prepare CeSiO 4 were found to be pH = 8.0 and T = 150°C. Performing the syntheses at lower or higher pH values led to the significant decrease of the yield of formation of CeSiO 4 (and thus to the formation of higher amounts of CeO 2 and SiO 2 ). It has been observed that, at 90°C, the optimum pH value was shifted to lower values (i.e. pH = 7), probably because of the dependence of the water selfionization constant with temperature. Moreover, it is worth noting that the extension of the holding time of the hydrothermal treatment up to 210 days at 40°C or up to 52 days at 60°C also led to the formation of CeSiO 4 (cerium and silicate concentrations of 1.0 mol•L -1 and pH = 7) (Figure 7).

As it was reported for ThSiO 4 , 38, 55 the synthesis temperature of CeSiO 4 affected the lattice parameters of the crystallized phase. Indeed, the decrease of the a lattice parameter and the of the c lattice parameter were observed when the temperature increased (Figure 8). As inferred for ThSiO 4 , 38, 55 this evolution could result from the elimination of hydroxide groups inserted in the silicate phase when increasing heating temperature. Finally, the stability of CeSiO 4 under hydrothermal conditions was examined at 250°C by contacting CeSiO 4 with solution for different pH. No proof of the CeSiO 4 degradation was obtained during these experiments. Therefore, it may inferred that the impact of temperature on the preparation of pure CeSiO 4 did not result from the metastability of this phase. It is surely associated to the degradation of intermediate species required to form CeSiO 4 instead of CeO 2 in the final samples.

Preparation of CeSiO 4 in hydrochloric acid medium. Several syntheses were performed in hydrochloric media using CeCl 3 as starting precursor. The results followed roughly the same trend than in nitric acid solution. However, the formation of CeSiO 4 in these conditions was limited by the formation of cerium hydroxyl-chloride, Ce(OH) 2 Cl which formed in the same pH range (Figure 9). Ce(OH) 2 Cl is isostructural to Ln(OH) 2 Cl phases which were formed under hydrothermal conditions in experimental conditions close to those developed in this study. 57, 58 Due to its high solubility in nitric acid, a washing step of 10 minutes with 1 M nitric acid after the synthesis allowed to dissolve specifically the Ce(OH) 2 Cl phase leading to a CeSiO 4 and CeO 2 mixture (Figure 9). The direct comparison of the results obtained in nitric and hydrochloric media showed that oxidation of cerium(III) to cerium(IV) necessary to form CeSiO 4 was not only due to the presence of nitrate ions. Residual oxygen dissolved in water could contribute to this oxidation reaction. , with pHinitial = 8.7 and carbonate concentration of 0.21 mol•L -1 (34) or 1.0 mol•L -1 (35). Characteristic XRD lines of CeO2, CeSiO4, Ce2O(CO3)2 and natural burbankite, (Na,Ca)3(Ca,Sr,Ba,Ln)3(CO3)5, were extracted from references ref 30, 45, 61 and 60, respectively.

Preparation of CeSiO 4 in carbonate media. Several attempts to use carbonate ions as pH buffer in the reactive media and/or as complexing agent of cerium to increase its availability, thus its reactivity with silicate species did not lead to the formation of CeSiO 4 . The main limitation was the formation of cerium(III) carbonate based species, such as Ce(OH)CO 3 under hydrothermal conditions at 150°C as observed by Guo et al.

59

, and burbankite type phases 60 Na 4 Ce 2 (CO 3 ) 5 , for higher carbonate concentrations (Figure 10). Moreover, increasing the temperature of the hydrothermal treatment led to the decomposition of such cerium carbonate species to form cerium oxide (mixed with amorphous silica) without any trace of CeSiO 4 . These results indicated that the complexation of Ce(III) with carbonate ions was more favorable than with silicate species (see Figure 13 and Figure S9 thereafter).

Characterization of CeSiO 4 sample obtained in optimized conditions

From the multiparametric study developed, the set of optimal conditions to prepare pure CeSiO 4 (Figure 11) and optimize its yield of formation, corresponds to high Ce(III) and silicate concentrations (i.e. 1 mol•L -1

) and pH value adjusted to 8.0 with the help of NaOH. Hydrothermal treatment is performed at 150°C for 20 days under inert atmosphere (Ar). Rietveld refinements performed on the XRD diagrams confirmed that CeSiO 4 crystallized in the zircon structure (I4 1 /amd space group) with the following unit cell parameters: a = 6.9603(1) Å, c = 6.1946(2) Å, i.e. V = 300.11(2) Å 3 (Table S3). These values are close to those reported by Skakle et al.

30

(a = 6.9564(3) Å and c = 6.1953(4) Å, i.e. V = 299.80(3) Å 3 ).

In such conditions, neither silica nor cerium oxide features were observable from infrared and Raman spectrometric measurement (Figure S4 and Figure S5). Moreover, infrared and Raman spectra confirmed the same features for CeSiO 4 than for isostructural zircon-type silicate (Table 2).

The characterization of CeSiO 4 by Scanning Electron Microscopy clearly showed the formation of square based bipyramids crystals with less than 1 µm in length. This morphology is clearly characteristic of zircon-type actinide silicates (Figure 12).

32

As suggested by Dickson et al.

32

, heating of CeSiO 4 under inert atmosphere (Ar) or in air revealed that the phase was stable up to 1000°C. The unit-cell parameters obtained after heating at this temperature were found to a = 6.9446(1) Å, c = 6.1975 Å and V = 298.89(1) Å 3 (Figure S7 and Table S3). It showed the decrease of the a unit cell parameters and the increase of the c unit cell parameter, compared to those obtained just after hydrothermal treatment. Such a difference could result from the elimination of hydroxide groups in the structure.

38, 55

New insights regarding the formation of CeSiO 4

Several lessons can be extracted from the results presented in this paper, especially to explain the reasons of the formation CeSiO 4 and to propose associated mechanism. First, CeSiO 4 is formed when either cerium and silicate ions are available to react in solution, e.g. using Ce(III) precursor and working at pH = 8. The mechanism of formation surely involves the existence of cerium silicate complexes in solution, as already suggested during the preparation of ThSiO 4 . 38 Working at lower pH values is unfavorable to the formation of CeSiO 4 (and surely to the existence of this cerium silicate complexes) consequently to the precipitation of SiO 2 . [START_REF] Iler | The chemistry of silica: solubility, polymerization, colloid and surface properties, and biochemistry[END_REF] On the contrary, the precipitation of Ce(OH) 3 (and its oxidation to form CeO 2 ) could limit the availability of cerium for higher pH values. 46 The same considerations with Ce(IV) precursor lead to conclude that the coexistence of free cerium and silicate ions in solution is not possible, which prevents the formation of CeSiO 4 . Moreover, the direct formation of CeSiO 4 from ceria and silica is also . Thermodynamics data considered for these calculations are gathered in Table S3. limited from a thermodynamic point of view, due to its instability compared to the mixture of CeO 2 and SiO 2 .

13

The formation of CeSiO 4 from Ce(III) and silicate ions in solution may result from the very slow in-situ oxidation of the suspected Ce(III)-silicate complex. Increasing temperature (i.e. increasing oxidation rate) induce the formation of SiO 2 and CeO 2 (obtained by precipitation than ageing of Ce(OH) 4 ). Thus, it seems crucial to maintain Ce(IV) at a low concentration in solution in order to avoid the rapid establishment of oversaturation conditions regarding to Ce(OH) 4 . It is also the reason why CeSiO 4 was never prepared from Ce(IV), due to its very strong tendency to hydrolysis. This also suggests that CeSiO 4 could not be prepared as soon as CeO 2 was formed. The absence of evolution of PXRD diagrams when performing hydrothermal treatment of CeO 2 + CeSiO 4 mixtures at 250°C and for large pH range support the fact that the conversion of CeO 2 to CeSiO 4 is unfavorable.

In order to support these observations, speciation simulations were performed using PHREEQC 2 software aiming at determining the conditions which may favor the formation of silicate-based precursor in the starting solution. Since no complexation constant was available for both Ce(III)-and Ce(IV)-silicate species, the respective constants have been extrapolated. These results were based on the apparent linear relationship reported by Thakur et al. 67 between the first hydrolysis constant of a metal ion and its complexation by H 3 SiO 4 -. In order to be more representative, the constants were calculated considering only elements showing the same oxidation state than cerium (Table S4 and Figure S8). Based on these hypotheses, the complexation constants were evaluated to logβ°(Ce III (OSi(OH) 3 ) 2+ ) = 7. S2 and extrapolated for Ce(III)-and Ce(IV)silicate species. Due to some lacks in the thermodynamic data associated to the temperatures considered for the hydrothermal treatment, simulation for higher temperatures was not possible.

The results of these simulations showed that the formation of the Ce IV (OSi(OH) 3 ) 3+ complex in the reactive media is strongly limited for Ce(IV) due to its significant hydrolysis (Figure 13). Therefore, in these conditions, the formation of Ce-silicate species appears to be strongly limited because of the formation of Ce(IV) hydroxide species and their rapid evolution to CeO 2 .

Due to its lower tendency to hydrolyze, Ce(III)-based system does not follow the same behavior than Ce(IV). Consequently, Ce III (OSi(OH) 3 ) 2+ complex appears as a predominant specie on a wide pH range in reductive conditions (Figure 13). It could then be inferred that this silicate specie evolved by in situ oxidation leading to the formation of CeSiO 4 while the formation of cerium oxide is disadvantaged. Same simulations performed in the presence of carbonate ions confirmed that the formation of Ce III (OSi(OH) 3 ) 2+ is strongly limited by the existence of Ce(III)-carbonate complexes (Figure S9). This can explain why CeSiO 4 is not formed in such experimental conditions. Therefore, all the speciation calculations support the key role played by Ce(III)-silicate complex during the formation of CeSiO 4 . However, the results have to be considered with caution because of the poor knowledge of experimental data associated to cerium silicate complexes. This highlights the necessity of studying the aqueous chemistry of cerium in order to thoroughly understand the mechanism of CeSiO 4 formation.

Conclusions

The formation of CeSiO 4 was successfully achieved from cerium(III) precursor under inert atmosphere. The yield of CeSiO 4 synthesis was found to be strongly dependent on the concentration of the reactants and of the pH of the reactive media. The formation of CeSiO 4 may also require the formation of cerium-silicate complexes in solution. Also, temperature plays an important role in the preparation of CeSiO 4 , which is formed between 40°C and 150°C. The oxidation state of cerium has a huge impact, supported by kinetic and thermodynamic considerations. Indeed, Ce(IV) concentration has to be maintained to low level in order to avoid the formation of Ce(OH) 4 thus of CeO 2 by ageing. Furthermore, these observations shed new light on the reactivity of cerium(III) in the presence of silicate species in solution. With this aim, the identification of potential cerium(III) silicate species in solution, the determination of associated complexation constants and their role in the CeSiO 4 synthesis, constitute key issues. Since silicate ions are abundant in environmental conditions representative of underground repository (either in granite and clay environments), such information are important to consider in order to better understand the potential reactivity of actinide in silicate rich environments, especially for plutonium.

Figure 1 .

 1 Figure 1. PXRD patterns obtained for samples prepared under hydrothermal conditions (7 days, T = 250°C) with starting silicate and cerium(IV) concentrations of 4.2  10-2 mol•L -1 and initial pH value equal to 10.5 (1), 8.7 (2), 6.9 (3), 4.1 (4), 3.9 (5) and 2.4(6). Characteristic XRD lines of CeO2 and CeSiO4 were extracted from ref 30 and 45, respectively.

Figure 2 .

 2 Figure 2. PXRD patterns obtained for samples prepared under hydrothermal conditions (10 days, T = 150°C) with starting silicate and cerium(IV) concentrations of 0.21 mol•L -1 and initial pH value equal to 8.7 and with NaHCO3 concentration equal to 0.21 mol L -1 (7), 0.42 mol L -1 (8), 1.0 mol L -1 (9) and 2.1 mol L -1 (10). XRD lines of the sample holder are pointed out by an asterisk. Characteristic XRD lines of CeO2 and CeSiO4 were extracted from ref 30 and 45, respectively.

Figure 3 .

 3 Figure 3. PXRD patterns obtained for samples prepared under hydrothermal conditions (10 days, T = 150°C) with starting silicate and cerium(III) concentrations of 0.21 mol•L -1

  by an asterisk. Characteristic XRD lines of CeO2 and CeSiO4 were extracted from ref 30 and 45, respectively.

Figure 4 . 1 ,

 41 Figure 4. PXRD patterns obtained for samples prepared under hydrothermal conditions (10 days, T = 150°C) with starting silicate and cerium(III) concentrations of 1.0 mol•L -1, in nitric acid media and with an initial pH equal to 8.7(19), 8.6 (20), 8.2 (21), 7.8 (22), 7.4 (23) and 7.0 (24). Characteristic XRD lines of CeO2 and CeSiO4 were extracted from ref 30 and 45, respectively.

Figure 5 . 1 ,

 51 Figure 5. Relative contents of CeSiO4 and CeO2 (expressed in wt.%) determined by Rietveld refinement of the PXRD patterns of samples prepared under hydrothermal conditions (10 days, T = 150°C), with starting silicate and cerium(III) concentrations of 1.0 mol•L -1, in nitric acid media and for various initial pH values.

Figure 6 .

 6 Figure 6. PXRD patterns obtained for samples prepared with starting silicate and cerium(III) concentrations of 0.21 mol•L -1 , in nitric acid media and with an initial pH ranging from 7 to 8, after hydrothermal treatment during 10 days at 90°C (25), 130°C (26), 150°C (27), 170°C (28) and 250°C (29). Characteristic XRD lines of CeO2 and CeSiO4 were extracted from ref 30 and 45, respectively.Figure 7. PXRD patterns for samples prepared with starting silicate and cerium(III) concentrations of 1.0 mol•L -1 , in nitric acid media with initial an pH equal to 7 at various temperature: T = 40°C (30), and T = 60°C (31). XRD lines of sample holder are pointed out by an asterisk. Characteristic XRD lines of CeO2 and CeSiO4 were extracted from ref 30 and 45, respectively.Figure 8. Variation of the lattice parameters a and c of CeSiO4 versus the temperature of hydrothermal treatment. Dash lines correspond to the unit cell parameters obtained after heating the samples at 1000°C (considered as reference values).

Figure 7 .

 7 Figure 6. PXRD patterns obtained for samples prepared with starting silicate and cerium(III) concentrations of 0.21 mol•L -1 , in nitric acid media and with an initial pH ranging from 7 to 8, after hydrothermal treatment during 10 days at 90°C (25), 130°C (26), 150°C (27), 170°C (28) and 250°C (29). Characteristic XRD lines of CeO2 and CeSiO4 were extracted from ref 30 and 45, respectively.Figure 7. PXRD patterns for samples prepared with starting silicate and cerium(III) concentrations of 1.0 mol•L -1 , in nitric acid media with initial an pH equal to 7 at various temperature: T = 40°C (30), and T = 60°C (31). XRD lines of sample holder are pointed out by an asterisk. Characteristic XRD lines of CeO2 and CeSiO4 were extracted from ref 30 and 45, respectively.Figure 8. Variation of the lattice parameters a and c of CeSiO4 versus the temperature of hydrothermal treatment. Dash lines correspond to the unit cell parameters obtained after heating the samples at 1000°C (considered as reference values).

Figure 8 .

 8 Figure 6. PXRD patterns obtained for samples prepared with starting silicate and cerium(III) concentrations of 0.21 mol•L -1 , in nitric acid media and with an initial pH ranging from 7 to 8, after hydrothermal treatment during 10 days at 90°C (25), 130°C (26), 150°C (27), 170°C (28) and 250°C (29). Characteristic XRD lines of CeO2 and CeSiO4 were extracted from ref 30 and 45, respectively.Figure 7. PXRD patterns for samples prepared with starting silicate and cerium(III) concentrations of 1.0 mol•L -1 , in nitric acid media with initial an pH equal to 7 at various temperature: T = 40°C (30), and T = 60°C (31). XRD lines of sample holder are pointed out by an asterisk. Characteristic XRD lines of CeO2 and CeSiO4 were extracted from ref 30 and 45, respectively.Figure 8. Variation of the lattice parameters a and c of CeSiO4 versus the temperature of hydrothermal treatment. Dash lines correspond to the unit cell parameters obtained after heating the samples at 1000°C (considered as reference values).

Figure 9 . 1 ,

 91 Figure 9. PXRD patterns obtained for a sample prepared under hydrothermal conditions during 10 days at T = 150°C with starting silicate and cerium(III) concentrations of 0.21 mol•L -1 , in hydrochloric acid and with pH = 8.5. PXRD obtained before (32) and after washing step in 6 mol•L -1 HNO3. Characteristic XRD lines of CeO2, CeSiO4 and Sm(OH)2Cl were extracted from ref 30, 45 and 56, respectively.

Figure 10 .

 10 Figure 10. PXRD patterns for the samples prepared under hydrothermal conditions during 10 days at T = 150°C with starting silicate and cerium(III) concentrations of 0.21 mol•L -1

Figure 11 .

 11 Figure 11. PXRD diagram, calculated and difference profile after Rietveld refinement obtained for CeSiO4 prepared under hydrothermal conditions (T = 150°C, t = 20 days, Ar-atmosphere), in nitric medium, with CCe(III)  CSi  1 mol•L -1 and pHinitial = 8.0 (36).

Figure 13 .

 13 Figure 13. Speciation diagrams of Ce(III) (a) and Ce(IV) (b) determined by PhreeqC calculations at room temperature with CSi total  CCe total = 0.21 mol•L -1

Figure 12 .

 12 Figure 12. SEM micrograph of CeSiO4 sample prepared under hydrothermal conditions (T = 150°C, t = 20 days, Ar-atmosphere), in nitric medium, with CCe(III)  CSi  1 mol•L -1 and pHinitial = 8.0 (36).

  were performed at C Si  C Ce = 0.21 mol•L -1 at room temperature considering the data reported in Table

  3

  Sigma Aldrich ACS grade NaOH (98 %) before the experiments. Aqueous silicate solutions were prepared by dissolving Na 2 SiO 3 and the cerium precursor in acidic solutions (which nature depended on the cerium precursor used). The pH was then adjusted to the final expected value with 8 mol•L

	•6H 2 O (99.9 %) and CeCl 3 •7H 2 O (99.9 %) were used as
	Ce(III) precursors, whereas (NH 4 ) 2 Ce(NO 3 ) 6 (99.99 %) and
	Ce(SO 4 ) 2 (99.9 %) were considered for Ce(IV). NaHCO 3 was used as the carbonate source. 1.5 mol•L -1 HNO 3 , 1.5 mol•L -1 HCl
	and 0.75 mol•L

-1 H 2 SO 4 solutions were prepared by dilution of Sigma Aldrich ACS grade solutions: HNO 3 (70%), HCl (37%) and H 2 SO 4 (95 -98 %). 8 mol•L -1 NaOH solution was freshly prepared from -1

Preparation of CeSiO 4 from Ce(IV) precursors Experiments developed in the absence of carbonate species in the reactive

	Glasser.	4 was developed by Dickson and 30, 32 It consisted in preparing an equimolar mixture of
	lime, silica and cerium(III) nitrate in aqueous solution under
	inert atmosphere. This mixture was submitted to
	hydrothermal treatment between 55 and 180°C for 14 to 700
	days. The precise role of CaO and redox reactions involving
	cerium were not clearly understood. The syntheses of the
	tetravalent actinide silicates, which are analogs of CeSiO 4 , are usually performed in the presence of carbonate ions. 27, 36, 37
	However, it was recently reported that pure ThSiO 4 can be prepared without adding carbonate ions. 38 All these chemical
	routes have been explored with the aim to prepare pure
	CeSiO 4 . Especially, the protocols described by Costin et al. for 39-44 , or other recently uranothorite solid solutions Th x U 1-x SiO 4 37, 38 , and described by Dickson et al. for developed for ThSiO 4 CeSiO 4 30, 32 were adapted.

media. Attempts to prepare CeSiO 4 from Ce(IV) precursors were performed with starting cerium and silicon concentrations ranging from 4.2  10 -2 mol•L -1 and 0.21 mol•L -1

Table 1 .

 1 Constants of complexation of Ce(IV) and An(IV) with carbonate associated to the limit complexes at I = 0 (25°C).

	Ln/An	log(β°5)*	log(β°6)*	Reference
	Th	31.0 ± 0.7	---	52
	U	34.0 ± 0.9	---	53
	Np	35.6 ± 1.1	---	53
	Pu	35.7 ± 1.1	---	53
	Ce	 41.8 ± 0.5	42.2 ± 0.5	50
	* β°n is associated to the reaction An	4+ + n CO 3 2-= An(CO 3 ) n (2n-4)-

.

Preparation

of CeSiO 4 from Ce(III) precursors in solution. Impact of the initial pH and concentrations of reactants on the formation of CeSiO 4 in nitric acid media

. Since hydrolysis and complexation by carbonate ions are strongly unfavorable to the formation of pure CeSiO 4 , complementary experiments have been performed starting from Ce(III) precursors. With this aim, the syntheses were developed from mixtures of Ce(III) and silicate ions with starting concentrations of 0.21 mol•L -1 and initial pH values ranging from pH = 1.3 to 12.3.

Table 2 .

 2 Assignment of the bands associated to silicate groups observed by Raman and IR spectra for CeSiO4 and comparison with some other zircon-type compounds.

				Raman spectroscopy				Infrared spectroscopy
		ν 2		ν 4		ν 1		ν 3	ν 2		ν 4		ν 1		ν 3
	ZrSiO 4 HfSiO 4 ThSiO 4 62, 63 64-66 39 USiO 4 39 CeSiO 4 (this study)	439 cm 448 cm 438 cm 424 cm 416 cm	-1 -1 -1 -1 -1	608 cm 620 cm 592 cm 591 cm 592 cm	-1 -1 -1 -1 -1	974 cm 984 cm 887 cm 906 cm 903 cm	-1 -1 -1 -1 -1	1008 cm 1018 cm 914 cm -1 -1 -1 919 cm -1 919 cm -1	431 cm 430 cm 445 cm 437 cm 431 cm	-1 -1 -1 -1 -1	620 cm 610 cm 584 cm 578 cm 572 cm	-1 -1 -1 -1 -1	866 cm 890 cm 823 cm 850 cm 800 cm	-1 -1 -1 -1 -1	-1 -1 -1 1049 cm 1020 cm 986 cm -1 970 cm 984 cm -1

Acknowledgements

The authors would like to thank R. Podor, J. Lautru and V. Trillaud (from ICSM) for supporting SEM experiments.

Supporting information

Table S1 reporting the synthesis parameters for all the CeSiO 4 hydrothermal syntheses presented here. Table S2 gathering the thermodynamic constants used for PHREEQC simulation.

Conflicts of interest

There are no conflicts to declare.


Jean-Claude Bermond

Claudine Peyrat

De Bruijn and Kautz networks : a competitor for the hypercube?


Jean-Claude Bermond

Claudine Peyrat

Broascasting in de Bruijn networks


Anne-Laurence Boutillier

Laura Tzeplaeff

Luc Dupuis
email: ldupuis@neuro-cnrs.unistra.fr

The dark side of HDAC inhibition in ALS

In this article of EBioMedicine, Viviana Moresi and colleagues provide genetic evidence that skeletal muscle HDAC4 exerts a protective role for neuromuscular junction and muscle innervation in amyotrophic lateral sclerosis (ALS) [START_REF] Pigna | Histone deacetylase 4 protects from denervation and skeletal muscle atrophy in a murine model of amyotrophic lateral sclerosis[END_REF].

HDACs are enzymes able to remodel the chromatin through histone deacetylation. The hypothesis that alterations in chromatin remodeling participates in ALS progression comes from studies published 10 years ago showing that ALS disease progression in a mouse model is associated with a loss of the histone acetyl transferase CBP and decreased histone acetylation in motor neurons at disease onset [START_REF] Rouaux | Critical loss of CBP/ p300 histone acetylase activity by caspase-6 during neurodegeneration[END_REF]. However, counteracting pharmacologically decreased histone acetylation by broad HDAC inhibition had a moderate effect on whole animal survival [START_REF] Petri | Additive neuroprotective effects of a histone deacetylase inhibitor and a catalytic antioxidant in a transgenic mouse model of amyotrophic lateral sclerosis[END_REF][START_REF] Sugai | Benefit of valproic acid in suppressing disease progression ALS model mice[END_REF]. More particularly, such treatment efficiently protected motor neuron cell bodies, and slightly delayed the onset of motor decline, but was unable to prevent disruption of neuromuscular junctions [START_REF] Rouaux | Sodium valproate exerts neuroprotective effects in vivo through CREB-binding protein-dependent mechanisms but does not improve survival in an amyotrophic lateral sclerosis mouse model[END_REF]. It is therefore likely that HDAC inhibitors might negatively influence other cell types involved in ALS. Pigna and collaborators show here that muscle HDAC4 is a strong candidate for these negative effects of HDAC inhibition.

HDAC4 is a member of the class IIa family that is devoid of deacetylase activity, and likely leads to histone desacetylation through binding of Class I HDACs [START_REF] Lahm | Unraveling the hidden catalytic activity of vertebrate class IIa histone deacetylases[END_REF]. In neurons, HDAC4 is predominantly cytoplasmic, particularly concentrated in dendritic spines [START_REF] Darcy | Regional and subcellular distribution of HDAC4 in mouse brain[END_REF]. Specific neuronal loss of HDAC4 leads to defects in motor coordination, learning and anxiety disorder [START_REF] Kim | An essential role for histone deacetylase 4 in synaptic plasticity and memory formation[END_REF]. HDAC4 is also expressed in muscle, and has a critical developmental role through inhibition of the transcriptional activity of MEF2 [START_REF] Miska | HDAC4 deacetylase associates with and represses the MEF2 transcription factor[END_REF]. HDAC4 is highly upregulated upon denervation, as occurs in ALS, and is enriched at neuromuscular junctions where it activates the subsynaptic gene expression program [START_REF] Cohen | The histone deacetylase HDAC4 connects neural activity to muscle transcriptional reprogramming[END_REF][START_REF] Tang | A histone deacetylase 4/myogenin positive feedback loop coordinates denervationdependent gene induction and suppression[END_REF]. Muscle specific deletion of HDAC4 thus leads to defective induction of subsynaptic gene expression but without impacting muscle mass [START_REF] Moresi | Myogenin and class II HDACs control neurogenic muscle atrophy by inducing E3 ubiquitin ligases[END_REF][START_REF] Choi | A direct HDAC4-MAP kinase crosstalk activates muscle atrophy program[END_REF]. Surprisingly, such manipulation also leads to better muscle reinnervation after nerve injury, in part through relieving the expression of FGF-related retrograde signals to motor neurons [START_REF] Williams | MicroRNA-206 delays ALS progression and promotes regeneration of neuromuscular synapses in mice[END_REF]. In patients, high muscle HDAC4 expression was correlated to faster disease progression [START_REF] Bruneteau | Muscle histone deacetylase 4 upregulation in amyotrophic lateral sclerosis: potential role in reinnervation ability and disease progression[END_REF]. Given the broad roles of HDAC4 in the different cell-types, it was difficult to predict whether HDAC4 loss would be beneficial or deleterious in ALS.

Pigna and collaborators here used an elegant genetic strategy to dissect the role of HDAC4 in muscle in a mouse model of ALS. Using a conditional allele of Hdac4, they ablated HDAC4 in muscle through crossing with transgenic mice expressing CRE under the Myog promoter, and introduced a transgene allowing expression of mutant SOD1, a cause of familial ALS. They observed that muscle HDAC4 deletion exacerbates a number of disease parameters in this model: acceleration of weight loss, muscle atrophy and weakness leading to acceleration of disease onset although overall survival remained unaffected. Consistent with a critical role of HDAC4 in neuromuscular junction, muscle HDAC4 ablation led to smaller and more frequently denervated neuromuscular synapses in mutant SOD1 mice, and potently blunted the induction of the so-called denervation response mounted by muscle in response to inactivity. Notably, no effect was observed on motor neuron survival. Muscle proteolytic pathways were also less activated in ALS mice without muscle HDAC4. Interestingly, HDAC4 ablation led to transcriptional dysregulations related to metabolism, ubiquitin-dependent catabolism and skeletal muscle response to denervation, including the upregulation of mitochondrial uncoupling protein 1, whose muscle expression is sufficient to recapitulate most of the observed effects in these animals [START_REF] Dupuis | Muscle mitochondrial uncoupling dismantles neuromuscular junction and triggers distal degeneration of motor neurons[END_REF]. As an overall conclusion of the study, Pigna and collaborators demonstrate that the pathways activated by HDAC4 in muscle during ALS are responsible for compensatory reinnervation through the myogenin pathway. Ablation of this key player in the pathway blunts this response and negatively affects NMJ response to denervation, while potentially inducing hypermetabolism.

The study by Pigna and collaborators provides an elegant demonstration why class IIa HDAC inhibition may not be suitable to protect the neuromuscular function in ALS, consistent with a recent study that showed only a transient protection of motor performance after treatment of the same ALS model with the class II specific HDAC inhibitor MC1568 [START_REF] Buonvicino | Effects of class II-selective histone deacetylase inhibitor on neuromuscular function and disease progression in SOD1-ALS mice[END_REF]. The current study also nicely confirms 10 years old studies using the pan-HDAC inhibitor valproic acid that potently protected motor neurons, likely via class I HDACs inhibition-dependent effects, but did not prevent NMJ denervation at late stages, possibly because of class IIa HDACs inhibition-dependent effects. In the context of ALS, recent work has shown that HDAC6 inhibitors could provide interesting protection in various mouse models [START_REF] Guo | HDAC6 inhibition reverses axonal transport defects in motor neurons derived from FUS-ALS patients[END_REF] and inhibitors of class I HDACs remain also interesting targets for neuronal protection, especially as HDAC1 appears as a downstream target of both FUS and TDP-43 related ALS. Future work in this area should focus on increasing selectivity of HDAC inhibitors, and ameliorate targeting of the CNS to avoid the deleterious effects of HDAC inhibition in the periphery.

Acknowledgments

The authors apologize to the many researchers whose work is not specifically referenced due to space limitations. The authors are supported by Inserm, CNRS, Unistra, ANR-16-CE92-0031 (EPIFUS), ANR-16-CE16-0015 (ToFU), Alsace Alzheimer 67, France Alzheimer (AAP SM 2017 #1664), Fondation pour la recherche médicale and Axa Banque Patrimoniale research fund.

Author's contribution ALB, LT and LD performed literature search and wrote the manuscript.

Conflict of interest

The authors declare no conflict of interest.


A Marouf

N Simiriotis

J-B Tô

J-F Rouchon

Y Hoarau

M Braza

MORPHING FOR SMART WING DESIGN THROUGH RANS/LES METHODS

This article presents numerical simulation results obtained in the context of the H2020 European research project SMS, "Smart Morphing and Sensing for Aeronautical configurations" by using among other, hybrid RANS-LES methods, able to accompany the design of the wings of the future. The morphing concepts studied are partly bioinspired and are able to act in multiple time and length scales. They are proven efficient for the increase of the aerodynamic performances of A320 wings in reduced scale and near scale one, in synergy with the prototypes built within this project. The simulations have shown the ability of novel electroactive actuators performing slight deformation of the trailing edge region and optimal vibrations, to create suitable vortex breakdown of specific coherent structures and to enhance beneficial vortices, leading to thinning of the shear layers and the wake's width. The simulations quantified the optimal actuation ranges and the gains in lift increase, drag reduction and simultaneous attenuation of the noise sources past the trailing edge.

Introduction

The present article highlights the contribution of RANS-LES methods in the future wing design by means of electroactive morphing developed in the currently running H2020 European research project "Smart Morphing and Sensing for aeronautical configurations 1 ". This is a multidisciplinary project that employs intelligent electroactive actuators modifying the lifting structure of an aircraft and allowing optimal shapes with respect to the aerodynamic performance (high lift and low drag). A closed-loop is accomplished for the controller design of the morphing wings in this project by using a new generation of pressure sensors using fiber optics-Bragg grating allowing distributed pressure measurements and in situ real-time optimization of the aerodynamic characteristics. The coordinator's research institutes IMFT -Institut de Mécanique des Fluides de Toulouse and LAPLACE -Laboratoire Plasma et Conversion d'Energie, had studied since 2009 bio-inspired morphing wings thanks to electrically actuated materials able to deform and vibrate in different time and length scales, as dictated by the turbulence surrounding the wing and developed in its wake. These actuations operate in a similar way as the different classes of large hunting bird wings, winglets and feathers, able to simultaneously increase lift, decrease drag and reduce their noise when these birds hasten to the prey. By optimizing the fluid-structure interaction thanks to these novel actu-1 www.smartwing.org/SM/EU ators, harmful coherent vortices are attenuated by means of vortex breakdown and beneficial vortices are enhanced, producing a significant increase of the aerodynamic performance and decrease of the noise sources. The studies of the research team IMFT-LAPLACE created the hybrid electroactive morphing [START_REF] Scheller | Electroactive morphing for the aerodynamic performance improvement of next generation airvehicles[END_REF] by simultaneously actuating different classes of smart materials in low frequencies (order of 1 Hz) but high deformations (order of 15% of the chord's length) and higher frequencies (order of 500 Hz) but low deformations (order of 1% of the chord's length), like the wings and feathers of the birds respectively. A priority of the European aeronautical industries is to make the flight safer, cleaner, greener and quieter than ever before. The bio-inspired electroactive morphing provides lighter aerostructures with a much fast response than other hydromechanical actuators and different MEMS concepts of previous studies. A first set of bio-inspired morphing prototypes of the IMFT-LAPLACE team were part of the Royal Society Annual Exhibition in 2014 in the stand: "Smart wing design through turbulence control Science imitating Nature2 ", in collaboration with Imperial College 3 . Electroactive morphing wing prototypes of A320 have been studied by the research team IMFT-LAPLACE in collaboration with AIRBUS "Emerging Technologies and Concepts Toulouse" and in the context of the SMS project, in the subsonic wind tunnels of IMFT in reduced scale (chord of 70 cm) and near scale one (wing chord of 2.40m and high-lift flap chord of 1m), by means of Time-Resolved PIV (TRPIV), forces and pressure measurements. Following our previous studies, dynamic camber systems using Shape Memory Alloys (SMA) for low frequency-high deformation [START_REF] Chinaud | Trailing-edge dynamics and morphing of a deformable flat plate at high Reynolds number by time-resolved PIV[END_REF] and higher-frequency-low deformation trailing edge vibrations [START_REF] Scheller | Trailingedge dynamics of a morphing NACA0012 aileron at high Reynolds number by high-speed PIV[END_REF] by means of piezo-actuators, a new type of an A320 morphing wing was constructed, offering a simultaneous actuation at different time and length scales [START_REF] Jodin | Dynamics of a hybrid morphing wing with active open loop vibrating trailing edge by timeresolved PIV and force measures[END_REF]. This prototype explored the effects of both, low frequency-high deformation by using the SMA actuators together with piezo-electric actuators of Macro-Fiber-Composite (MFC) type. By using the piezo-actuators only, a lift increase of 2 % and drag reduction of 3 % have been obtained, where by simultaneous actuation of the SMA and piezos, a lift increase of order 10 % and a drag reduction of 5% have been reached, with simultaneous reduction of the noise sources of order 20 dB, corresponding to the coherent eddies shed past the trailing edge. The experiments and simulations were ac-complished at Reynolds numbers of 500,000 and 1 Million. The numerical studies in the SMS project, [START_REF] Simiriotis | Morphing of a supercritical wing by means of trailing edge deformation and vibration at high Reynolds numbers: experimental and numerical investigation[END_REF] examined the morphing efficiency obtained by trailing-edge low deformation (order of 1-3 mm) and higher frequency vibration (order of 400 Hz) at Reynolds number of 1 Million. The turbulence modelling approach OES, Organised Eddy Simulation, [START_REF] Braza | Turbulence Properties in the cylinder wake at high Reynolds number[END_REF], [START_REF] Bourguet | Anisotropic Organised Eddy Simulation for the prediction of non-equilibrium turbulent flows around bodies[END_REF], [START_REF] Szubert | Shockvortex shear-layer interaction in the transonic flow around a supercritical airfoil at high Reynolds number in buffet conditions[END_REF] has been used, in which the resolved turbulence contains the coherent structures and the modelled turbulence contains the chaotic turbulent motion. This approach had been proven efficient for the coherent structures development and the related instabilities, even in two-dimensional approach. The numerical results have shown that the morphing produces improved aerodynamic performances and reduces the amplitude of instability modes associated to aerodynamic noise. In the context of the SMS project, The optimal frequency and amplitude of the actuations obtained by the present numerical study are in use for the closed-loop for the optimal manipulation of the embedded actuators, able to cutoff the harmful instabilities at their birth, thus showing the strong synergy between the numerical simulations and the experimental studies for the design in the SMS project. Beyond the bio-inspired context, it is worthwhile noticing that the birds do not fly at transonic speeds corresponding to the cruise phase of a real aircraft. In these regimes, the numerical approach has proven essential for the design, prior to the experiments under way within the SMS project for this regime. In [START_REF] Tô | Effects of vibrating and deformed trailing edge of a morphing supercritical airfoil in transonic regime by numerical simulation at high Reynolds number[END_REF] we investigated the electroactive morphing for an A320 type of wing in transonic regime (Mach number of 0.78, Reynolds number of order 3 Million), with the same turbulence modelling method as previously mentioned. The study revealed that a deformation with a slight upwards deflection of the trailingedge by 2 degrees associated with a trailing-edge vibration of 300 Hz is able to attenuate the buffet instability and to enhance the life-to-drag of order 6%. This is due to the feedback effects created from the trailing edge and the wake area towards the SWBLI (Shock Wave Boundary Layer Interaction) region, produced by an eddy-blocking within the shear layers, by means of the trailing-edge vibration. This mechanism is similar to a shear sheltering one as in [START_REF] Hunt | Vortical interactions with interfacial shear layers[END_REF]. Our current numerical simulations in the SMS project concerning the transonic Reduced Scale (tRS) morphing prototype of an A320 wing have determined the optimal frequency ranges able to considerably increase the aerodynamic performance presented in table 1 and 2. to decrease Alloys (figure 2). This design ensures high-amplitude deformations (order of 10 cm) downwards in camber control and low frequency actuation (order of 1Hz) . Moreover, piezo-electric patches allow for higher frequency actuation of order 500 Hz) and low deformations of order 1-3 mm (figure 3). In addition, numerical simulations are carried out for the Large-Scale (LS) prototype of the SMS project, having a total chord of 2.40 m and a flap's chord of 1m, (figure 4) with morphing high-lift flap near scale one, in a two-element configuration of the A320 wing operating at the take-off position. These simulations accompany the design and the experiments concerning this prototype. The NSMB code [START_REF] Hoarau | Recent developments of the Navier Stokes Multi Block (NSMB) CFD solver[END_REF][START_REF] Vos | Recent advances in aerodynamics inside the NSMB (Navier Stokes Multi Block) consortium[END_REF][START_REF] Hoarau | Analyse physique par simulation numérique et modélisation des écoulements décollés instationnaires autour de surfaces portantes[END_REF] is used to perform CFD and CFDSM (Computational Fluid Dynamics -Structural Mechanics) simulations in the SMS project. The code solves unsteady and steady compressible Navier-Stokes equations using the finite volume method, a structured grid is used to simulate the flow around single and two-element wing-flap. This code includes a variety of efficient high-order numerical schemes and turbulence modelling closures in the context of URANS, LES and hybrid turbulence modelling. In the present study, the fourth-order central scheme and second order dual-time stepping for the temporal discretisation are selected. Artificial compressibility preconditioning was used to simulate the flow in a subsonic speed range for both Reduced and Large-Scale configurations. The NSMB code includes efficient fluid-structure coupling for moving and deformable structures. It is interfaced with specific structural modules taking into account the prototype's deformation according to different actuations and interfaced with the CFD part of the code. The mesh movement and deformation is carried out by means of the ALE (Arbitrary Lagrangian-Eulerian) method. The Organised Eddy Simulation OES [START_REF] Bourguet | Anisotropic Organised Eddy Simulation for the prediction of non-equilibrium turbulent flows around bodies[END_REF] and the hybrid Delayed Detached Eddy Simulation DDES-OES [START_REF] Shinde | Numerical simulation of the fluid-structure interaction in a tube array under cross flow at moderate and high Reynolds number[END_REF] with embedded OES approach in the statistical part of the DDES were selected to simulate the flow for both configurations. The design of the DDES grid around the wing requires a homogeneous local grid cells with ∆x ≈ ∆y ≈ ∆z following the recommendations in [START_REF] Spalart | Detached-Eddy Simulation[END_REF].

3 Results and discussion

Reduced Scale prototype

This part of the study focuses on the Reduced-Scale in clean configuration with a chord of 0.7 m at a Re = 10 6 and angle of attack 10 • . Using the DDES-OES, the homogeneous ambient terms are activated as in [START_REF] Spalart | Effective Inflow Conditions for Turbulence Models in Aerodynamic Calculations[END_REF]. The grid used for the Reduced Scale prototype contain 10 million cells for the OES model and 60 million cells for the DDES-OES model with a minimum local cell size less than 1 mm. A span of 0.8C (40 cells in the span direction ; C being the chord) is used for the OES model. In the DDES-OES, a span dimension of 0.3C (120 cells in the span direction) is selected as presented in the figure 6. The grids were created to respect the dimensions of the IMFT S4 wind tunnel in the vertical to the chord dimension. The streamwise numerical dimension from the inlet to the outlet is 11C and the wing is placed in the middle. Two-dimensional and three-dimensional simulations are carried out for the Reduced and the Large Scale prototypes.

A detailed parametric study has been accomplished in 2D in order to detect the optimal ranges whereas the 3D study has been performed for selected parameters only. Beyond the comparisons with the TRPIV [START_REF] Simiriotis | Morphing of a supercritical wing by means of trailing edge deformation and vibration at high Reynolds numbers: experimental and numerical investigation[END_REF], the numerical simulations are used to focus the experiments in the optimal actuation ranges. Figures 5 and6 illustrate the flow be- The spanwise velocity in the z direction has been extracted from different horizontal lines according to the vertical direction z in figure 7. A considerable attenuation of this is obtained, illustrating a practical suppression of the three-dimensionnality. This is due to the actuation frequency by 300 Hz obtained by the piezo-patches operating monochromatically along the span in the trailing-edge region. This actuation modifies the direction of the mean shear of the von Kármán vortices, leading to a subcritical elliptic instability shape of these coherent vortices, thus attenuating the secondary instability spanwisely [START_REF] Braza | Successive stages and the role of natural vortex dislocations in three-dimensional wake transition[END_REF]. Consequently, the wake thickness is thinner compared to the static case (no vibration).

Large-Scale prototype

In the context of the SMS European project, the Large-Scale (LS) prototype of a morphing A320 high-lift configuration including the wing-flap two-element system was studied in accordance with the IMFT S1 wind tunnel configuration, [START_REF] Marouf | Evaluation of the aerodynamic performance increase thanks to a morphing A320 wing with high-lift flap by means of CFD Hi-Fi approaches[END_REF], [START_REF] Marouf | Numerical investigation of frequency-amplitude effects of dynamic morphing for a high-lift configuration at high Reynolds number[END_REF]. The morphing effects for the Large Scale prototype. A subsonic regime at a Mach number M = 0.032 and Reynolds number of Re = 2.25 million is selected and an angle of attack for the fixed wing of α = 8.2 • which corresponds to the range of angles in the take-off position. A flap's deflection angle of δ = 10 • degrees is selected. 2D and 3D simulations have been performed for the static case, taken as reference to compare with the morphing case. The vibration frequencies vary in the range of 60 to 400 Hz with a fixed vertical actuation amplitude of 0.7 mm.

An optimal frequency range in respect of the aerody- namic performance increase is found near 300 Hz. The coherent structures corresponding to low frequency instabilities are von Kármán vortices interacting with anticlockwise vortices generated from the actuation of the flap's trailing-edge. These vortices break down the existing von Kármán eddies and lead to a wake thinning. Furthermore, smaller-scale vortices are generated from the trailing edge actuation, that propagate in the wake and produce a shear-sheltering effect as in [START_REF] Szubert | Shockvortex shear-layer interaction in the transonic flow around a supercritical airfoil at high Reynolds number in buffet conditions[END_REF]. This results to a constriction of the shear layers and to an attenuation of the predominant frequencies of the shear-layers vortices as shown in Figure 8. Regarding the aerodynamic coefficient, the drag is decreased and an enhancement of the lift is noticed. Figure 9 illustrates the Q criterion using the DDES-OES in [START_REF] Marouf | DDES and OES Simulations of a Morphing Airbus A320 Wing and Flap in Different Scales at High Reynolds[END_REF]. A breakdown of the vortex rows and appearance of the three-dimensional structures in the wake is related to the high Reynolds number and the angle of attack. Vortex dislocations of the primary von Kármán vortex rows are formed in the spanwise direction, illustrated by a junction of the "spinal column" (of two adjacent vortex rows [START_REF] Braza | Successive stages and the role of natural vortex dislocations in three-dimensional wake transition[END_REF]), in the spanwise direction in figure 10. It is found that an actuation at 300 Hz and amplitude of 0.7mm suppresses the vortex dislocations by generating anticlockwise vortex rows, resulting to a considerable wake thinning in figure 11. Figure 12 shows comparison of the mean lift coefficient in both static and morphing configurations. The CL varies as a function of the frequency in the range of (60 Hz -300 Hz) and the amplitude actuation in a range of (0.5mm -1.5mm) and is found increased in all morphing configurations.

Conclusion

This study provides discussion based on numerical results obtained in the case of two main morphing configurations of an A320 wing in the context of the H2020 N°7

23402 European project "Smart Morphing and Sensing for aeronautical configurations" coordinated by IMFT -Institut de Mécanique des Fluides de Toulouse. A single element configuration corresponding to the Reduced-Scale (RS) morphing prototype and a high-lift configuration with two-element wing-flap in take-off position corresponding to the Large-Scale (LS) prototype (near scale one) of this project are considered. In the present numerical study, the morphing is imposed in the neartrailing edge region as a slight deformation and vibration in frequency ranges (50 -400) Hz. The simulations with high camber effects concern another part of our studies, not presented in this article. The turbulence modelling approaches OES (Organised Eddy Simulation) and the hybrid DDES-OES (Delayed Detached Eddy Simulation with embedded OES in its statistical part) have been used. It has been shown that an actuation at 300 Hz produces suitable vortex breakdown, leading to shear layer and wake thinning, as well as to an attenuation of threedimensionality. These facts lead to a considerable increase of the aerodynamic performances for both morphing configurations of the RS and LS prototypes and attenuate the aerodynamic noise sources corresponding to the predominant frequencies generated by the coherent structures development. These benefits are produced thanks to the wake thinning through a breakdown of the coher-ent structures in the wake, by optimal manipulation of the turbulence structures near the trailing edge and the wake regions. The turbulence modelling approaches including Hybrid RANS/LES methods contributed to this relevant performance and accompanied the prototypes design realised in the SMS project.

Figure 1 :

 1 Figure1: Attenuation of the buffet frequency f b due to actuation and slight deformation of the trailing edge region in cruise phase, Mach=0.78, Re= 2.06 x 10 6 . PSD of lift coefficient, transonic morphing prototype of A320 type the buffet amplitude as well as the overall PSD level related to the noise. This results to a constriction of the shear layers and to an attenuation of the predominant frequencies of the shear-layers vortices as shown in figure1. These numerical simulations are in strong synergy and contribute to the experimental studies carried out in the SMS project concerning the design of the morphing prototype in cruise phase. Furthermore, the numerical studies in the SMS project in-

Figure 2 :

 2 Figure 2: Reduced-Scale prototype using Shape Memory Alloys (SMA) clude low subsonic regime simulations for the morphing wing design concerning the take-off and landing phases. In this context, the Reduced-Scale (RS) A320 prototype is considered having a chord of 70 cm and consists of a single element wing operating in clean position (at angle of attack of 10 degrees), instrumented by Shape Memory

Figure 3 :

 3 Figure 3: Series of piezo-electric patches installed in the wing's trailing-edge

Figure 4 :

 4 Figure 4: Morphing high-lift LS prototype of the A320 wing near scale 1 in the S1 wind tunnel of IMFT

Figure 5 :

 5 Figure 5: Presentation of the wake turbulent structures and near trailing-edge separation (recirculation) by means of the OES model haviour around the RS prototype in static (non morphing) configuration and provide the near-trailing edge overview

Figure 6 :

 6 Figure 6: Q criterion (1500) colored by the velocity magnitude using the OES-DDES model

Figure 7 :

 7 Figure 7: Extracted spanwise velocity through a vertical plane z/C a long the span direction y/C

Figure 8 :

 8 Figure 8: Power Spectral Density of the vertical velocity component at x/C = 1.47, z/C = 0.04

Figure 9 :

 9 Figure 9: Q criterion (1000) colored by the velocity magnitude for the Large-Scale prototype at the take-off position using the OES-DDES turbulence model

Figure 11 :

 11 Figure 11: Streaklines illustrating the morphing effect: Top: static. Bottom: morphing with piezoactuators vibrating at 300 Hz with slight trailing-edge deformation showing the vortex breakdown of the coherent structures and the shear layers and wake thinning

Figure 12 :

 12 Figure 12: Mean lift coefficient at the take-off position as a function of different actuation frequencies and amplitudes compared to the static case

Table 1 :

 1 Relative gain (or loss if negative) of the averaged lift-to-drag ratio for different piezo-actuation frequencies compared to the static (non morphing) case

Table 2 :

 2

Relative gain (if negative in respect of drag reduction) or loss (if positive) of the averaged drag coefficient for different piezo-actuation frequencies comparing to the static case

http://www.cnrs.fr/insis/recherche/ actualites/2014/morphing.htm

https://www.youtube.com/watch?v=AoAaCCvn38M

Acknowledgement

This study is a contribution of N. Simiriotis, A. Marouf, J.B. Tô, Y. Bmegaptche Tekap, N. Bhardwaj, Y. Hoarau, J. Scheler, D. Harribey, C. Nadal, J.F. Rouchon. Our thanks are expressed to the IMFT services: "Signaux -Images" (S. Cazin, M. Marchall) for the PIV acquisitions, COSINUS team, (P. Elyakime) for the parallel post-processing of the experimental data and the Technical Constructions Service, "Atelier" of IMFT (R. Soeparno and his team), as well as to the Techical Services of the LAPLACE Laboratory in ENSEEIHT (Ecole Nationale Supérieure d'Electrotechnique, Electronique, Hydraulique et Télécommunications). This work was supported by the STAE Foundation "Sciences et Technologies pour l'aéronautique et l'Espace, http://www.fondationstae.net/, the European Commission funding of the project H2020 N°723402 Smart Morphing and Sensing for aeronautical configurations, www.smartwing.org/SMS/EU, the AIRBUS -ETCT (Emerging Technologies and Concepts Toulouse) project PJOR D14012929, the DGA, "Direction Générale de l'Armement", for having providing the PhD fellowship of J. Scheller and N. Simiriotis, l'ENS -"Ecole Normale Supérieure of Rennes" for the PhD fellowship of G. Jodin and the French Ministry of Education and Research for having provided the PhD fellowship of A. Marouf. The authors are grateful to the French Supercomputing Centres CALMIP, CINES, IDRIS and TGCC-CEA for the Computer allocation, as well as to the PRACE Supercomputing allocation N°2017174208 of the PRACE-FWING project, which provided the SMS project simulations with 15 million CPU hours in the TGCC-CEA Supercomputing Centre.