Paul Estevenon

Thibault Kaczmarek

Mohamed Ruwaid Rafiuddin

Éléonore Welcomme

Stephanie Szenknect

Adel Mesbah

Philippe Moisy

Christophe Poinssot

Nicolas Dacheux

Eleonore Welcomme

Soft Hydrothermal Synthesis of Hafnon, HfSiO 4

Keywords: hafnium silicate, hafnon, wet chemistry route, hydrothermal synthesis, zircon structure type

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INTRODUCTION

Hafnon, HfSiO 4 , is one of the end-members belonging to the zircon-type silicate group (tetragonal, I41/amd) like ZrSiO 4 , CeSiO 4 , ThSiO 4 , PaSiO 4 , USiO 4 , NpSiO 4 , PuSiO 4 and AmSiO 4 . [START_REF] Curtis | Some Properties of Hafnium Oxide, Hafnium Silicate, Calcium Hafnate, and Hafnium Carbide[END_REF][START_REF] Keller | Untersuchungen über die Germanate und Silikate des Typs ABO4 der vierwertigen Elemente Thorium bis Americium[END_REF][START_REF] Speer | The actinide orthosilicates[END_REF][START_REF] Skakle | The crystal structures of CeSiO4 and Ca2Ce8(SiO4)6O2[END_REF] As a consequence, hafnon is reported to form a solid solution with ZrSiO 4 . [START_REF] Ramakrishnan | Solid solubility in the system zircon-hafnon[END_REF][START_REF] Hoskin | Raman spectral shift in the isomorphous series (Zr1-xHfx)SiO4[END_REF][START_REF] Cota | Solution properties of the system ZrSiO4-HfSiO4: a computational and experimental study[END_REF] It is usually observed as a minor component in natural zircons [START_REF] Hoskin | The composition of zircon and igneous and metamorphic petrogenesis[END_REF][START_REF] Ballard | Relative oxidation states of magmas inferred from Ce(IV)/Ce(III) in zircon: Application to porphyrycopper deposits of northern Chile[END_REF][START_REF] Uher | Zircon in hercynian granitic pegmatites of the Western Carpathians, Slovakia[END_REF] and less frequently as the major phase in natural assemblies where zircon appears as a minor secondary phase. [START_REF] Correia Neves | High hafnium members of the zircon-hafnon series from the granite pegmatites of Zambézia, Mozambique[END_REF] Owing to its interesting thermal and electric properties (such as thermal shock resistance, low dilatation coefficient on a wide temperature range, high thermal conductivity, low relative permittivity and low dielectric loss), [START_REF] Kanno | Effect of dopants on the formation of hafnon via a sol-gel route[END_REF][START_REF] Varghese | Hafnium silicate: a new microwave dielectric ceramic with low thermal expansivity[END_REF] hafnon is often considered as promising high-temperature refractory material [START_REF] Kanno | Effect of dopants on the formation of hafnon via a sol-gel route[END_REF] for semiconductor devices [START_REF] Manoun | A high-pressure Raman spectroscopic study of hafnon, HfSiO4[END_REF] or microwave substrate for electronic data transmission. [START_REF] Varghese | Hafnium silicate: a new microwave dielectric ceramic with low thermal expansivity[END_REF] Moreover, as zircon-type ceramics (e.g. ZrSiO 4 and HfSiO 4 ) are isostructural with actinide silicates, An IV SiO 4 and due to its high chemical stability, they have been suggested as potential actinide-specific matrices for the immobilization of radionuclides associated to high level nuclear waste and more specifically of plutonium excess coming from dismantled nuclear weapons. [START_REF] Lutze | Radioactive waste forms for the future[END_REF][START_REF] Meldrum | A transient liquid-like phase in the displacement cascades of zircon, hafnon and thorite[END_REF][START_REF] Meldrum | Radiation effects in zircon, hafnon and thorite implications for Pu disposals[END_REF][START_REF] Lutze | Zircon: a host phase for the disposal of weapons plutonium[END_REF][START_REF] Ewing | Radiation effects in zircon[END_REF] In this context, (Hf,Pu)SiO 4 compounds were prepared by Burakov et al. using sol-gel method. [START_REF] Burakov | Synthesis and study of 239 Pudoped ceramics based on zircon (Zr,Pu)SiO4, and hafnon (Hf,Pu)SiO4[END_REF] However, these experiments suggest that plutonium content cannot exceed 7 wt.% in such materials.

From a general point of view, the synthesis of HfSiO 4 was reported by high temperature solid state chemistry, [START_REF] Curtis | Some Properties of Hafnium Oxide, Hafnium Silicate, Calcium Hafnate, and Hafnium Carbide[END_REF][START_REF] Ramakrishnan | Solid solubility in the system zircon-hafnon[END_REF][START_REF] Cota | Solution properties of the system ZrSiO4-HfSiO4: a computational and experimental study[END_REF][START_REF] Varghese | Hafnium silicate: a new microwave dielectric ceramic with low thermal expansivity[END_REF][START_REF] Salt | Synthesis and X-ray study of hafnium silicate[END_REF][START_REF] Vasquez | Nuclear quadrupole interaction in hafnon[END_REF][START_REF] Ueno | Corrosion and oxidation behavior of ASiO4 (A = Ti, Zr and Hf) and silicon nitride with an HfSiO4 environmental barrier coating[END_REF][START_REF] O'neill | Free energy of formation of zircon and hafnon[END_REF][START_REF] Chain | PAC study in the HfO2-SiO2 system[END_REF] sol-gel methods, [START_REF] Kanno | Effect of dopants on the formation of hafnon via a sol-gel route[END_REF][START_REF] Kanno | Thermodynamics and crystallographic discussion of the formation and dissociation of zircon[END_REF][START_REF] Jaeger | Perturbed angular correlation measurement of the electric field gradient at 181Ta in ZrSiO4 and HfSiO4[END_REF] chemical transport reaction, [START_REF] Manoun | A high-pressure Raman spectroscopic study of hafnon, HfSiO4[END_REF][START_REF] Speer | Crystal structure of synthetic hafnon, HfSiO4, comparison with zircon and the actinide orthosilicates[END_REF][START_REF] Fuhrmann | Bildung von HfSiO4-einkristallen durch chemische Transportreaktion[END_REF] physical vapor deposition [START_REF] Punchaipetch | Hafnium silicate formation by ultra-violet/ozone oxidation of hafnium silicide[END_REF] and hydrothermal synthesis. [START_REF] Caruba | Nouvelles synthèses hydrothermales du zircon : substitutions isomorphiques; relation morphologie-milieu de croissance[END_REF][START_REF] Mcneil | Hydrothermal synthesis of columbite-(Mn), Tantalite-(Mn), hafnon and zircon at 800-850°C and 200 MPa[END_REF] However, the only hydrothermal syntheses were reported by Caruba et al. under acid conditions at T = 800°C, 75 MPa, [START_REF] Caruba | Nouvelles synthèses hydrothermales du zircon : substitutions isomorphiques; relation morphologie-milieu de croissance[END_REF] and by McNeil et al. in very acid conditions, i.e. C HF = 6 mol•L -1 and C H2SO4 = 0.5 mol•L -1 at T = 850°C, 200 MPa. [START_REF] Mcneil | Hydrothermal synthesis of columbite-(Mn), Tantalite-(Mn), hafnon and zircon at 800-850°C and 200 MPa[END_REF] These conditions appear to be quite surprising taking into account the possibility to prepare isostructural silicates, e.g. ZrSiO 4 , [START_REF] Caruba | Nouvelles synthèses hydrothermales du zircon : substitutions isomorphiques; relation morphologie-milieu de croissance[END_REF][START_REF] Mumpton | Hydrothermal stability studies of the zircon-thorite group[END_REF][START_REF] Frondel | Hydrothermal synthesis of zircon, thorite and huttonite[END_REF][START_REF] Valero | Mécanismes de la synthèse hydrothermale du zircon[END_REF][START_REF] Valero | A new hydrothermal fluoro-zircon[END_REF][START_REF] Valero | Mechanism of hydrothermal synthesis of zircon in a fluoride medium[END_REF][START_REF] Valero | Hydrothermal synthesis of porous zircon in basic fluorinated medium[END_REF][START_REF] Valero | Influence des ions fluorures et de la silice amorphe sur la solubilité des gels de zircone et caracteriation des fluroro-complexes de zirconium en milieu moyennement acide[END_REF][START_REF] Valero | Rietveld refinement of two fluoro-hydroxy-zircons[END_REF][START_REF] Kido | Hydrothermal processing of zircon[END_REF][START_REF] Mosset | A new solution route to silicates Part 4. Submicronic zircon powders[END_REF][START_REF] Caruba | An experimental study of hydroxyl groups and water in synthetic and natural zircons: a model of the metamict state[END_REF][START_REF] Ilyushin | Phase relations in the Na2CO3-ZrO2-SiO2-H2O system at 0.1 and 0.05 GPa and 450[END_REF][START_REF] Ilyushin | KOH-ZrO2-SiO2-H2O hydrothermal system: formation of potassium zirconosilicates and crystallochemical correlations among them[END_REF][START_REF] Ilyushin | Hydrothermal synthesis of K2ZrSi6O15, K2ZrSi3O9, K2ZrSi2O7, and ZrSiO4 in the system KOH-ZrO2-SiO2-H2O[END_REF] CeSiO 4 , [START_REF] Skakle | The crystal structures of CeSiO4 and Ca2Ce8(SiO4)6O2[END_REF][START_REF] Dickson | Cerium(III, IV) in cement Implications for actinide (III, IV) immobilisation[END_REF][START_REF] Estevenon | Formation of CeSiO4 from cerium (III) silicate precursors[END_REF][START_REF] Estevenon | Preparation of CeSiO4 from aqueous precursors under soft hydrothermal conditions[END_REF] ThSiO 4 , [START_REF] Keller | Untersuchungen über die Germanate und Silikate des Typs ABO4 der vierwertigen Elemente Thorium bis Americium[END_REF][START_REF] Frondel | Hydrothermal synthesis of zircon, thorite and huttonite[END_REF][START_REF] Fuchs | X-ray studies of synthetic coffinite, thorite and uranothorites[END_REF][START_REF] Sinha | On the synthetic preparation and lattice structure of thorite[END_REF][START_REF] Clavier | From thorite to coffinite: A spectroscopic study of Th1-xUxSiO4 solid solutions[END_REF][START_REF] Clavier | Purification of uranothorite solid solutions from polyphase systems[END_REF][START_REF] Costin | Solutions solides d'uranothorite : de la préparation à la dissolution[END_REF][START_REF] Costin | How to explain the difficulties in the coffinite synthesis from the study of uranothorite?[END_REF][START_REF] Costin | Preparation and characterization of synthetic Th0.5U0.5SiO4 uranothorite[END_REF][START_REF] Szenknect | From uranothorites to coffinite: a solid solution route to the thermodynamic properties of USiO4[END_REF][START_REF] Bauer | The crystal structures of synthetic coffinite, USiO4, and uranothorite, UxTh1-xSiO4, analyzed by Powder Diffr[END_REF][START_REF] Labs | Secondary uranium phases of spent nuclear fuelcoffinite, USiO4, and studtite, UO4•4H2O -Synthesis, characterization, and investigations regarding phase stability[END_REF][START_REF] Labs | Synthesis of coffinite, USiO4, and structural investigations of UxTh(1-x)SiO4 solid solutions[END_REF][START_REF] Guo | Energetics of a uranothorite (Th1-xUxSiO4) solid solution[END_REF][START_REF] Mesbah | Incorporation of thorium in the zircon structure type through the Th1-xErx(SiO4)1-x(PO4)x thorite-xenotime solid solution[END_REF][START_REF] Knyazev | Synthesis and high-temperature X-ray diffraction study of thorium orthosilicate[END_REF][START_REF] Estevenon | Multiparametric study of the synthesis of ThSiO4 under hydrothermal conditions[END_REF][START_REF] Estevenon | Impact of carbonate ions on the synthesis of ThSiO4 under hydrothermal conditions[END_REF] USiO 4 , 2, 59, 60, 66-76 NpSiO 4 , 2 PuSiO 4 [START_REF] Keller | Untersuchungen über die Germanate und Silikate des Typs ABO4 der vierwertigen Elemente Thorium bis Americium[END_REF] and AmSiO 4 2 under soft hydrothermal conditions (i.e. for T ≤ 250°C). However, it may be also noticed that the formation of amorphous hafnon has been observed by alteration, in aqueous solution, of Hfbearing borosilicate glasses at 90°C and pH = 1. [START_REF] Calas | Incipient formation of zircon and hafnon during glass alteration at 90°C[END_REF] More specifically, we recently reported that modifications of the starting pH and elementary concentrations allowed to decrease the temperature of formation of ThSiO 4 and CeSiO 4 under hydrothermal conditions. [START_REF] Estevenon | Preparation of CeSiO4 from aqueous precursors under soft hydrothermal conditions[END_REF][START_REF] Estevenon | Multiparametric study of the synthesis of ThSiO4 under hydrothermal conditions[END_REF] Therefore, the aim of this study was to determine an efficient way of synthesis allowing the formation of HfSiO 4 under soft hydrothermal conditions.

MATERIALS AND METHODS

Syntheses

All the reagents used for the materials preparation were supplied by Sigma-Aldrich. Na 2 SiO 3 •5H 2 O (95%) and HfCl 4 (98%) were used as aqueous silicate and hafnium precursors, respectively. 1.5 and 1.0 mol•L -1 HCl solutions were prepared by dilution of Sigma Aldrich ACS grade mother solutions HCl (37%). 8 mol•L -1 NaOH solution was freshly prepared from Sigma Aldrich ACS grade NaOH (98 %) before the experiments.

The syntheses of HfSiO 4 were performed by adapting the protocol recently developed for ThSiO 4 . [START_REF] Estevenon | Multiparametric study of the synthesis of ThSiO4 under hydrothermal conditions[END_REF] Aqueous mixtures of hafnium at the oxidation state +IV and silicate were prepared by dissolving HfCl 4 and Na 2 SiO 3 •5H 2 O in 1.5 mol•L -1 hydrochloric acid (Table S1). At this stage, a silicate excess of 3 mol.% was considered to avoid the formation of hafnium dioxide or hydroxides during the synthesis. The pH was then adjusted to the final value with 8 mol•L -1 NaOH.

All the prepared mixtures were introduced in a 23 mL Teflon lined container. The container was placed in a Parr auto-clave in an oven to reach hydrothermal conditions during 1 to 20 days with a given temperature and under autogenous pressure. The final cooling to room temperature was done within one hour. Then, the precipitates were separated from the supernatant by centrifugation at 14 000 rpm for 12 min, washed twice with deionized water and once with ethanol. They were finally dried overnight at 60°C in an oven.

Characterization

PXRD data were collected on the resulting powders using the Bruker D8 advance diffractometer equipped with a lynxeye detector and working with Cu Kα radiation (λ = 1.54184 Å) in a reflection geometry (parallel beam). 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. The collected data were refined by the Rietveld method using the Fullprof_suite package. [START_REF] Frontera | FullProf as a new tool for flipping ratio analysis[END_REF] During the refinements, different profile and structure parameters were adjusted, such as the zero shift, unit-cell parameters, scale factor, and overall displacement factor. An anisotropic size and strain model was also used in order to consider the broadening effect.

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 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 × 50LMP objective, resulting in a spot area of ∼1 μm 2 and a power of 39 mW. For each spectrum, a dwell time of 1 to 30 s was used. Four scans were recorded for each analyzed area in order to minimize the measurement error.

FTIR spectra were recorded with a Perkin-Elmer FTIR Spectrum 100 device in the 300-4000 cm -1 range. 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 using a FEI Quanta 200 electronic microscope on small powder samples without prior preparation such as metallization. The electronic microscope was equipped either with an Everhart-Thornley Detector (ETD) or a Back-Scattered Electron Detector (BSED), under high vacuum conditions with a low accelerating voltage (8 kV). These conditions were chosen in order to create a beam deceleration effect that led to high resolution images.

Thermogravimetric analyses were performed to determine the hydration content of the samples prepared at the end of the syntheses. All of these analyses were done between room temperature and 1000°C under air atmosphere thanks to a SETSYS evolution analyzer. These measurements were coupled with mass spectroscopy analyses on the residual gaz.

RESULTS AND DISCUSSION

Effect of pH of the starting solution

The effect of pH of the starting mixture on the nature of the resulting precipitate was followed between C HCl = 1.5 mol•L -1 and pH = 8 considering a starting mixture with a hafnium concentration of 0.21 mol•L -1 and a Si:Hf molar ratio of 1.03. Hydrothermal treatments were performed at 250°C during 24 hours.

All the samples prepared were characterized by PXRD (Figure 1). From these data, HfSiO 4 (zircon-type structure, space group I4 1 /amd) was formed using a starting mixture whose acidity was below pH = 1.6. For higher pH values, the precipitation of monoclinic HfO 2 (space group P2 1 /c) was obtained due to hafnium hydrolysis leading to the precipitation of the hafnium hydroxides then hafnium oxide by ageing. In these conditions, the unit cell parameters of HfSiO 4 reached a = 6.596(2) Å and c = 5.958(3) Å (i.e. V = 259.2(1) Å 3 ; values calculated by averaging the data obtained on the samples (1) to (4)syntheses leading to HfSiO 4 as single phase). The a and c lattice parameters were slightly higher and smaller, respectively, than the reference lattice parameters obtained by high temperature methods (a = 6.5725 (7) Å and c = 5.9632(4) Å, i.e. V = 257.60(7) Å 3 28 ). According to the recent results already reported for ThSiO 4 , these variations were attributed to the insertion of hydroxide groups in the HfSiO 4 structure [START_REF] Estevenon | Multiparametric study of the synthesis of ThSiO4 under hydrothermal conditions[END_REF][START_REF] Frondel | Hydroxyl substitution in thorite and zircon[END_REF] , which may be correlated to the non-ideal crystallization in the soft hydrothermal conditions considered.

Rietveld refinements performed on the PXRD data did not allow to observe any significant variation of the HfSiO 4 unitcell parameters (Table 1) nor of the crystallite size (Figure S1 and Table 1 and Table S2) according to the initial pH of the reactive media.

Characterization by Raman and IR spectroscopies (Figure 2) revealed the presence of symmetric and antisymmetric stretching modes of SiO 4 at 979 cm -1 and 1015 cm -1 , respectively by Raman (868 cm -1 and 1054 cm -1 , respectively by IR spectroscopy). The symmetric bending modes were observed at 451 cm -1 . On the contrary, the antisymmetric mode was not observed due to its low intensity (both symmetric and antisymmetric modes were observed at 433 cm -1 and 627 cm -1 , respectively by IR spectroscopy) (Table 2).

It worth noting that the positions of the HfSiO 4 's ν 1 and ν 3 vibration bands observed by infrared spectroscopy are significantly different to the ones reported in the literature (Table 2). However, the values obtained here are in great agreement with the ones reported in the literature for the other zircon type silicate phases (Table S3) and especially for ZrSiO 4 (ν 1 = 866 cm -1 and ν 3 = 1049 cm -1 ). [START_REF] Dawson | The vibrational spectrum of zircon (ZrSiO4)[END_REF] Moreover, the position of the ν 3 vibration bands at around 1050 cm -1 should be taken with care because this position correspond to the ν 4 mode of SiO 2 .

Additionally, the presence of small amounts of hydroxide groups or water was suggested from the observation of broad bands around 3400 cm -1 and 1630 cm -1 in the IR spectra, which was consistent with the hypothesis proposed regarding the insertion of hydroxide groups in the HfSiO 4 lattice already suggested from PXRD analyses. (2), pH = 0.5 (3), pH = 1.0 (4), pH = 1.6 (5), pH = 2.0 (6), pH = 3.0 (7) and pH = 5.0 (8). Bragg positions of the characteristic peaks of hafnon were extracted from 28 . The presence of HfO

   
Intensity (a.u.)

Wavenumber (cm -1 )

    3  3 (SiO 4 )  1 (SiO 4 )  4 (SiO 4 )  2 (SiO 4 )

Table 2. Assignment of the vibration bands (expressed in cm -1

) associated to silicate groups observed in the Raman and IR spectra of HfSiO 4 .

Raman Spectroscopy Infrared spectroscopy ν

Effect of the starting hafnium and silicate concentrations

Since the concentration of the reactants were crucial parameter for the synthesis of ThSiO 4 and CeSiO 4 , [START_REF] Estevenon | Preparation of CeSiO4 from aqueous precursors under soft hydrothermal conditions[END_REF][START_REF] Estevenon | Multiparametric study of the synthesis of ThSiO4 under hydrothermal conditions[END_REF][START_REF] Estevenon | Impact of carbonate ions on the synthesis of ThSiO4 under hydrothermal conditions[END_REF] it was suspected that the concentration of the hafnium and silicate present in the starting mixture could affect the saturation indexes in solution, and thus the formation of HfSiO 4 . In this frame, several syntheses were performed under hydrothermal conditions with different concentrations. The impact of the concentrations of both reactants was followed between 8.4 × 10 -3 mol•L -1 and 1.0 mol•L -1 , keeping constant the Si:Hf molar ratio (1.03), the initial pH value (pH = 0.5) and the conditions of the hydrothermal treatment (T =250°C, t = 24 hours). As a result, pure HfSiO 4 was obtained for C Hf ≥ 0.21 mol•L -1 whereas the formation of HfO 2 was observed as a by-product for lower concentrations (Figure 3). In the less concentrated media, the silicate concentrations in solution were not sufficient to fully counterbalance the hydrolysis of hafnium. Therefore, the formation of hafnium silicate was favored for C Hf ≥ 0.21 mol•L -1 .

Moreover, the considered concentration significantly affected the crystallization state of HfSiO 4 . Specifically, decreasing the hafnium and silicate ions concentration in the reactive media induced the increase of the crystallite size of HfSiO 4 (Figure 4 and Table 1). This behavior is similar to what was observed previously for ThSiO 4 . [START_REF] Estevenon | Multiparametric study of the synthesis of ThSiO4 under hydrothermal conditions[END_REF] It may be attributed to the faster kinetics of nucleation in the more concentrated reactive media, leading to the formation of numerous nucleation centers, whose growth may be hindered by the limited amounts of reactants in the synthesis medium. Additionally, the Rietveld refinements allowed to observe the increase of the HfSiO 4 's lattice parameters and volume cell and the decrease of the crystallite size (Table 1 and Figure S3). Furthermore, the decrease of the crystallite size could be observed in each crystalline planes (Table S2), especially the low size value obtained in the (hkl) planes with l ≠ 0 at high concentration could explain the anisotropic effect which could be observed with the widening of the corresponding peaks.

Impact of the hydrothermal treatment (temperature and duration)

In order to identify the impact of the temperature of the hydrothermal treatment on the formation of HfSiO 4 , several experiments were performed at 150°C, 200°C and 250°C, keeping constant the concentration of the starting hafnium (0.21 mol•L -1 ), the molar Si:Hf ratio (1.03), the starting pH value (pH = 0.5) and the heating time (t = 24 hours). From PXRD analyses, HfSiO 4 was formed whatever the temperature of synthesis (Figure 5), which was also confirmed by IR spectroscopy (Figure S3). Additional experiments performed in the same conditions at 100°C did not lead to the formation of HfSiO 4 after 24 hours holding time.

However, working at the lower temperatures led to the formation of HfSiO 4 compounds exhibiting a very strong anisotropic effect (Figure 5). Indeed, the (hkl) reflections involving the c axis were strongly broadened and attenuated whereas the (hk0) reflections were finer and more intense when the temperature of the hydrothermal treatment decreased, which underlined the formation of platelet crystallites. Considering that the decreasing temperature was associated to slower kinetics of reaction, this phenomenon was assigned to the growth process of HfSiO 4 crystallites, involving first the formation of bidimensional crystallites and then crystal growth to form three-dimensional particles. Finally, varying the starting pH value at 150°C or 200°C confirmed that HfSiO 4 was always obtained for pH < 1.6 (Figure S4 and Figure S5), as already discussed for the experiments at 250°C. Moreover, the preferential growth observed in this study is in agreement with the results reported recently by Calas et al. for the alteration of Hf-bearing borosilicate glasses at 90°C and pH = 1 in aqueous solution, showing the (200) reflection of hafnon as the only observable diffraction peak in their conditions. [START_REF] Calas | Incipient formation of zircon and hafnon during glass alteration at 90°C[END_REF] The reflections (220), ( 400) and (420) are invisible due to their low intensity compared to the (200), while the (hkl) peaks with l ≠ 0 are invisible due to an anisotropic effect.

Increasing the temperature of the hydrothermal treatment led to the decrease of the unit-cell parameters of HfSiO 4 (Figure S8 and Table 1). No significant variation of the average crystallite size was observed (Figure S8). However, Rietveld refinements performed on the PXRD patterns allowed to confirm the qualitative results suspected on the anisotropy which could be explained by the crystallite growth along the (hkl) planes with l ≠ 0 (Figure 6 and Table S2) when the temperature of the hydrothermal treatment increased. Thermogravimetric analyses, coupled with mass spectroscopy, performed between room temperature and 1000°C under air atmosphere, with the samples dried overnight at 60°C, allowed to observe the loss of water in two steps. The first one around 100°C corresponded to the elimination of free water and the second one between 200°C and 850°C was probably associated to the elimination of water inserted in HfSiO 4 structure (as H 2 O or HO -group).

 = 200°C  = 150°C Intensity (a.u.) 2• (degrees) HfSiO 4  = 250°C
The characterizations performed on the TGA analyses residues allowed to confirm that the final phase obtained corresponded to HfSiO 4 as single crystalline phase, whatever the temperature of hydrothermal treatment considered. It worth noting that the HfSiO 4 lattice parameters evolved during the thermogravimetric analysis to reach parameters close to reference values: a = 6.5714(8) Å, c = 5.9685 (11) Å and V = 257.74(7) Å 3 (against a = 6.5725(7) Å, c = 5.9632(4) Å, i.e. V = 257.60(7) Å 3 28 ) (Table 1). Raman and infrared spectroscopy characterizations did not lead to significant differences compared to the spectra measured before the thermogravimetric analyses.

In order to evidence the impact of the duration of the hydrothermal treatment on the formation of HfSiO 4 , several experiments were also performed at 250°C, for pH = 0.5 and C Hf = 0.21 mol•L -1 with holding times spread from 1 day to 20 days. From PXRD analysis, it was clear that HfSiO 4 was always single phase and was not degraded by extending the holding time (Figure 7).

Moreover, extending the duration of the hydrothermal treatment did not lead to significant change of the HfSiO 4 unit cell parameters determined by Rietveld refinement (Table 1). The HfSiO 4 lattice parameters reached a = 6.5937(1) Å, c = 5.9605(1) Å and V = 259.14(1) Å 3 for 20 days of hydrothermal treatment (Figure 8). As already described, the lattice parameters obtained were slightly different to those obtained by high temperature methods (a = 6.5725(7) Å, c = 5.9632(4) Å and V = 257.60(7) Å 3 28 ), which might correspond to the insertion of hydroxide groups in the HfSiO 4 structure. [START_REF] Estevenon | Multiparametric study of the synthesis of ThSiO4 under hydrothermal conditions[END_REF][START_REF] Frondel | Hydroxyl substitution in thorite and zircon[END_REF] The extension of the heating time tended to reduce the anisotropic effect evidenced by PXRD. Therefore, initial HfSiO 4 platelets crystallites may evolve to form 3D particles through crystal growth process.

The determination of the size of the HfSiO 4 crystallites performed by Rietveld refinement indicated that increasing the duration of the hydrothermal treatment promoted their growth (Figure 9).

The direct comparison by SEM of the samples prepared at 250°C clearly confirmed the growth of HfSiO 4 grains from ~ 60 nm after 1 day to ~ 200 nm after 7 days and ~ 450 nm after 20 days (Figure 10). It also exhibited the evolution of the grain morphology with the clear identification of a square based bipyramid morphology characteristic of zircon-type materials, for the hydrothermal treatments extended for 7 days and more. [START_REF] Caruba | Crystal growth of synthetic zircon round natural seeds[END_REF] This evolution of morphology may be correlated to the crystalline growth of the HfSiO 4 grains. Moreover, HfSiO 4 particles also appeared to be very flattened and may be correlated to the anisotropic effect previously observed. However, it is worth noting that a size discrepancy was observed between the crystallite size determined by XRD method (Figure 9) and grain size measured thanks to SEM. Therefore, it can be concluded that these grains observed are polycrystalline, This behavior was already observed for other silicate based compounds including ThSiO 4 . [START_REF] Estevenon | Multiparametric study of the synthesis of ThSiO4 under hydrothermal conditions[END_REF] Additionally, SEM-EDX characterization allowed to confirm the sample composition, without any metallic insertion in HfSiO 4 (Figure S13).

CONCLUSION

The multiparametric study of HfSiO 4 synthesis allowed determining an appropriate set of hydrothermal conditions to prepare single phase samples, which are listed below:

-acidic reactive media (typically with pH ≤ 1.6, fixed to pH = 0.5 in this study); -hafnium and silicate concentrations over 0.21 mol•L -1 with a molar Si/Hf ratio of 1.03 in order to avoid the formation of hafnium dioxide in the final mixtures;

-hydrothermal treatment for 24 hours at T ≥ 150°C. Increasing the temperature and the duration of the hydrothermal treatment promoted the formation and then the growth of the square based bipyramid feature.

The formation of HfO 2 in the place of HfSiO 4 at high pH and low reactant concentration was easily explained by the competition between the formation of hafnium hydroxide and hafnium silicate. This behavior and more generally the conditions associated to the synthesis of HfSiO 4 are very similar to those of thorite (ThSiO 4 ) in ligand free reactive media.

Moreover, it worth noting that such kind of synthesis of HfSiO 4 in soft hydrothermal conditions or hydrothermally assisted synthesis could be considered as a potential option to increase the solubility of tetravalent actinides in zircon-type materials or to prepare hafnon based materials doped by divalent or trivalent elements.

ASSOCIATED CONTENT Supporting Information

The Supporting Information is available free of charge on the ACS Publications website. Table S1 reporting the parameters considered for all the syntheses of HfSiO 4 under hydrothermal conditions. Table S2 gathering the crystallite size along different (hkl) plan determined by Rietveld refinement for HfSiO 4 samples. Table S3 reporting the assignment of the bands associated to silicate groups in zircon type silicates.

Figure 1 .

 1 Figure 1. PXRD patterns recorded for samples prepared under hydrothermal conditions (24 hours, T = 250°C) with hafnium and silicate concentrations of 0.21 mol•L -1 and for various initial pH values: C HCl = 1.5 mol•L -1 (1), C HCl = 1.0 mol•L -1

2 is

 2 pointed out by empty circles in the PXRD patterns.

Figure 2 .

 2 Figure 2. Raman (a) and IR (b) spectra recorded for HfSiO 4 samples prepared under hydrothermal conditions during 24 hours at T = 250°C starting from hafnium and silicate concentrations of 0.21 mol•L -1and with pH = 0.5 (3).

°°Figure 3 .Figure 4 .

 34 Figure 4. Variation of the HfSiO 4 crystallite size determined by Rietveld refinement as a function of the starting hafnium and silicate concentrations, obtained for samples under hydrothermal conditions (250°C, 24 hours) and with pH = 0.5.

Figure 5 .Figure 6 .

 56 Figure 5. PXRD pattern and corresponding Miller index obtained for samples prepared under hydrothermal conditions with starting silicon and hafnium concentrations of 0.21 mol•L -1 and pH = 0.5, after hydrothermal treatment performed for 24 hours at T = 150°C (12), at T = 200°C (13) and at T = 250°C (3). The characteristic XRD lines of HfSiO 4 were extracted from 28 .

Figure 7 . 15 Figure 8 .Figure 9 .

 71589 Figure 7. PXRD patterns recorded for samples prepared under hydrothermal conditions (T = 250°C) starting with pH = 0.5 and C Hf = 0.21 mol•L -1 and for various heating times: 1 day (3), 7 days (14) and 20 days (15). The XRD lines characteristic of HfSiO 4 were extracted from 28 .

Figure 10 .

 10 Figure 10. SEM micrographs recorded for HfSiO 4 samples prepared under hydrothermal conditions at T = 250°C, starting with pH = 0.5 and C Hf = 0.21 mol•L -1 , and for heating times of 1 day (3) (a), 7 days (14) (b) and 20 days (15) (c).

Figure

  S1 representing the variation of the HfSiO 4 unit cell parameters and volume and of the crystallite size as a function of the starting pH.

  Figure S2 representing the IR spectra obtained for HfSiO 4 samples prepared considering various initial pH values. Figure S3 representing the variation of the HfSiO 4 unit cell parameters and volume as a function of the reactant concentration.

Figure

  S4 representing the IR spectra obtained for HfSiO 4 samples prepared with various hafnium and silicon concentrations.

  Figure S5 representing the IR spectra obtained for HfSiO 4 samples prepared at various temperatures of hydrothermal treatment.

  Figure S6 and Figure S7 representing the PXRD patterns obtained for HfSiO 4 samples prepared at 150°C and at 200°C, respectively, with various starting pH values. Figure S8 representing the variation of the HfSiO 4 unit cell parameters and volume and of the crystallite size versus the temperature of the hydrothermal treatment.

  Figure S9 representing the thermogravimetric analyses for HfSiO 4 samples prepared at 150°C and 250°C.Figure S10 representing the IR spectra obtained for HfSiO 4 samples prepared for various heating times at 250°C. Figure S11 representing the variation of the HfSiO 4 unit-cell parameters and volume and of the crystallite size versus the heating time of the hydrothermal treatment. Figure S12 representing the SEM micrographs recorded for HfSiO 4 samples prepared at 150°C and 250°C. Figure S13 representing the EDX analysis of HfSiO 4 .

Table 1 .

 1 Lattice parameters and crystallite size determined by Rietveld refinement for HfSiO 4 samples.

			Unit cell parameters
	Label				Crystallite size (nm)
		a (Å)	c (Å)	V (Å 3 )
	(1)	6.5946(1)	5.9605(1)	259.22(1)	20 ± 11
	(2)	6.5935(1)	5.9594(1)	259.08(1)	36 ± 13
	(3)	6.5956(1)	5.9561(1)	259.11(1)	24 ± 7
	(4)	6.5988(1)	5.9544(1)	259.27(1)	22 ± 8
	(5)	6.5869(1)	5.9575(1)	258.47(1)	48 ± 4
	(6)	6.5945(2)	5.9583(2)	259.11(1)	27 ± 2
	(7)	6.5912(4)	5.9555(4)	258.73(3)	15 ± 1
	(8)	6.594(2)	5.947(3)	258.6(2)	ND
	(9)	6.5939(1)	5.9542(1)	258.89(1)	381 ± 237
	(10)	6.5913(1)	5.9602(1)	258.94(1)	88 ± 14
	(11)	6.5995(1)	5.9575(1)	259.47(1)	17 ± 8
	(12)	6.6447(3)	5.9801(9)	264.03(4)	16 ± 14
	(13)	6.6217(1)	5.9511(2)	260.94(1)	16 ± 9
	(14)	6.5961(1)	5.9611(1)	259.36(1)	31 ± 6
	(15)	6.5935(1)	5.9606(1)	259.13(1)	80 ± 18
	(3) after TGA	6.5714(8)	5.9685(11)	257.74(7)	ND
	ND : Not Determined

AUTHOR INFORMATION

Author Contributions

The manuscript was written through contributions of all authors.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENT

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

FOR TABLE OF CONTENTS USE ONLY

Soft hydrothermal synthesis of hafnon, HfSiO 4

Paul Estevenon †, ‡, §, ∥ , Thibault Kaczmarek †, ‡ , Mohamed Ruwaid Rafiuddin ‡ , Eleonore Welcomme † , Stephanie Szenknect ‡ , Adel Mesbah ‡ , Philippe Moisy † , Christophe Poinssot † , Nicolas Dacheux *, ‡ † CEA, DEN, DMRC, Univ Montpellier, Marcoule, France. ‡ ICSM, Univ Montpellier, CNRS, CEA, ENSCM, Marcoule, France. § The European Synchrotron, CS40220, 38043 Grenoble Cedex 9, France.

∥ Helmholtz Zentrum Dresden-Rossendorf (HZDR), Institute of Resource Ecology, P.O. Box 510119, 01314, Dresden, Germany.

The conditions allowing the preparation of single phase HfSiO 4 has been determined through a multiparametric study by varying the concentrations of the reactants, the pH of the reactive media or the temperature and duration of the hydrothermal treatment. HfSiO 4 was prepared through relatively soft conditions (pH ≤ 1.6, C Si ≈ C Hf ≥ 0.21 mol•L -1 , T ≥ 150°C for t ≥ 24 hours).


Paul Estevenon

Jérémy Causse

Stephanie Szenknect

Éléonore Welcomme

Adel Mesbah

Philippe Moisy

Christophe Poinssot

Nicolas Dacheux

Jeremy Causse

Eleonore Welcomme

In situ study of the synthesis of thorite (ThSiO 4 ) under environmental representative conditions

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.

a. ICSM, Univ Montpellier, CEA, CNRS, ENSCM, Bagnols-sur-Cèze, France. b. CEA, DES, ISEC, DMRC, Univ Montpellier, Marcoule, France.

† Present addresses:

The European Synchrotron, CS40220, 38043 Grenoble Cedex 9, France. Helmholtz Zentrum Dresden-Rossendorf (HZDR), Institute of Resource Ecology, P.O. Box 510119, 01314, Dresden, Germany. Electronic Supplementary Information (ESI) available: Figure S1 to Figure S6; Table S1 to Table S3. See DOI: 10.1039/x0xx00000x

Introduction

The thorium chemistry may be considered as indicative of the reactivity of the actinide elements at the oxidation state +IV. Due to the abundance of silicate species in the environment, identifying the actinide-silicate interactions is a key point to evaluate the actinides behaviour in environment. On the one hand, thorium, uranium and neptunium are known to form nanometric particles in silicate reactive media [1][2][3][START_REF] Riglet-Martial | Competition carbonate-silicate sur la complexation des actinides en milieu argileux riche en silice[END_REF][START_REF] Zänker | [END_REF][6][7][8][9] . The structure, size and stability of these intrinsic actinideoxyhydroxy-silicate colloids have been investigated 1,3,[START_REF] Zänker | [END_REF]9 . The prevailing size of the colloids is less than 20 nm, they appear as virtually amorphous or poorly structured and exhibit low isoelectric point (pH IEP ≤ 4.6)

3,6,8

, corresponding to a negative surface charge. Thus, An

IV oxyhydroxy-silicate colloids may be able to stabilize significant amounts of An IV in a waterborne form and may enhance their mobility in soils and aquifers. On the other hand, the zircon-type structure minerals thorite (ThSiO 4 ) and coffinite (USiO 4 ) are among the most abundant natural sources of thorium and uranium in ore deposits.

Although they could play an important role in the formation of ore deposits, to the best of our knowledge, no specific link between the colloidal species and the corresponding crystalline phases (i.e. thorite and coffinite) has been highlighted until now.

The thorite formation has been extensively studied using high temperature dry chemistry [10][11][12][13][14][START_REF] Keller | Karlsruhe Kernforschungszentrum report[END_REF] and hydrothermal synthesis methods 13-31 . However, even though thermodynamic measurements established that ThSiO 4 was more stable than a ThO 2 and SiO 2 mixture at 298 K 23,27,32 , hydrothermal conditions or elevated temperatures have always been used to synthesize thorite. To the best of our knowledge, the lowest temperature reported is 150°C in ligand rich reactive media (fluoride or carbonate ions) [START_REF] Frondel | [END_REF]30 and 160°C in ligand free reactive media (pH = 1 and C Th = 4.2  10 -2 mol.L -1 )

31

. Nevertheless, a recent study has demonstrated that increasing the reactant concentration and modifying the pH of the reactive media may enable the formation of the silicate phase on a wider range of conditions 31 . Small and Wide-Angle X-rays Scattering (SWAXS) allowed us to follow the evolution of the size and shape of colloids, to assess the nature of their interactions (electrostatic repulsion, aggregation…) and to identify the nature of crystalline phases formed in solution. This technique was already used to investigate the first stages of formation of various crystalline phases such as zeolite, titanium dioxide, microporous aluminophosphonate, molybdate catalysts, tungstite, zirconia or MOFs. For example, Panzarella et al. showed that microwave synthesis of NaY zeolite started first with the formation of amorphous nanoparticles 33 . Then, these colloids tended to aggregate throughout the synthesis, to finally produce crystalline NaY zeolite. Fan et al. studied LTA zeolite formation and concluded that homogeneous nucleation took place in a first step to produce amorphous nanoparticles of about 5-15 nm, then a growth step occurred, followed by the crystallization of this species 34 . While in-situ SWAXS technique has been mainly used to study inorganic materials, some authors used it to depict the crystallisation process of Metal Organic Frameworks (MOFs . They combined SAXS and EXAFS to propose a core-shell model of the colloids with a core made of UO 2 and a shell made of uranium silicate. However, no direct link between the existence of these colloids and the formation of the crystalline phase, coffinite was made. Thermodynamic data available for thorite indicated that this phase was more stable at 298 K than the mixture of the binary oxides. Thus, hydrothermal conditions were not required to precipitate thorite from a thorium and silicate solution. However, elevated temperature increased the kinetics of crystallization of thorite 30,31 . The present paper aims to evidence the formation of Th-based colloids in silicate solutions at low temperatures and under environmental conditions. The relevant synthesis conditions were identified thanks to the multiparametric study of ThSiO 4 reported in a recent article

31

. The impact of the temperature and of the pH of the synthesis medium on the kinetics of crystallisation of thorite was investigated using in-situ SWAXS technique. This technique, associated to X-rays diffraction, Raman and IRspectroscopies allowed to establish the link between the presence of thorium silicate colloids in synthesis solutions and the formation of the crystalline thorite.

Materials and methods

Sample preparation

All of the reagents used for the syntheses were of analytical grade and were supplied by Sigma-Aldrich. Th(NO 3 ) 4 •4-5H 2 O is known to be a highly hygroscopic salt. Therefore, in order to accurate control the thorium concentration, a stock solution was prepared by dissolving thorium nitrate salt in 1 mol•L -1 HNO 3 . The obtained solution was titrated by inductively coupled plasma atomic emission spectroscopy (ICP-AES) and the thorium concentration obtained reached C Th = 0.96 ± 0.01 mol•L -1 . In such acidic conditions, thorium was also protected against hydrolysis. The synthesis of ThSiO 4 was performed using the protocol we recently developed for preparation of the thorium silicate in carbonate free reactive media

31

. An aqueous silicate solution was prepared by dissolving Na 2 SiO 3 in water and a precise volume of the thorium stock solution was added to this solution to obtain a reacting mixture with C Si ≈ C Th = 0.21 mol•L . In order to obtain thorium oxide-free samples, a silicate excess of 3 mol.% in the reacting mixture was maintained. The pH was then adjusted to the chosen final value with 8 mol•L -1

NaOH.

In order to perform in-situ SWAXS measurements, a small volume of the solution was taken and placed in glass capillary tubes (1 mm diameter). These capillaries were then sealed thanks to an epoxy resin which was reticulated during 15 minutes under a UV lamp ( = 365 nm). This hermetic sealing was performed to ensure the confinement of the sample and to avoid sample dehydration. The prepared ex-situ mixtures were introduced in 5 mL Savillex PFA hermetic vials and kept at room temperature or placed in an oven at temperatures equal to 40°C, 60°C or 90°C for a variable period of time (ranged between 50 and 257 days), while SWAXS capillaries were also kept at fixed temperature (40°C or 60°C) thanks to a specific oven. The synthesis conditions of the ThSiO 4 samples are summarized in Table 1, in which in-situ experiments refer to SWAXS samples. At the end of the heat treatment, samples were cooled down to room temperature. Then, the precipitates were separated from the supernatant by centrifugation for 12 min at 14000 rpm, washed twice with deionized water and once with ethanol, and finally dried overnight at 60°C in an oven.

Methods and characterization

PXRD data were collected on the resulting powders using 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). Adapted sample holders were used in order to avoid any potential radioactive contamination. 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. Instrumental function was extracted thanks to measurements on pure silicon metal sample. The collected data were refined by the Rietveld method using the Fullprof_suite package

36

. During the refinements, different profile and structure parameters, such as the zero shift, unitcell parameters, scale factor, and overall displacement factor were allowed to vary. However, the size of the crystallite has been determined thanks to the Scherrer equation. This method has been preferred to the Rietveld method because Scherrer equation has also been used to evaluate the size of the coherent-domain of ThSiO 4 on the diffraction peaks obtained by SWAXS measurements. Raman spectra were recorded with a Horiba-Jobin Yvon Aramis spectrometer equipped with an edge filter and a Nd:YAG laser (532 nm) that delivers 60 mW at the sample surface. The laser beam was then focused on a sample using an Olympus BX 41 microscope with a 50LMP objective, resulting in a spot area of ∼1 μm 2 and a power of 39 mW. Four scans were recorded for each analysed area and a dwell time of 1 to 30 s was used for each spectrum. FTIR spectra were recorded in the 300 -4000 cm -1 range using a Perkin-Elmer FTIR Spectrum 100 instrument. Powdered samples were deposited on the surface of an ATR crystal without any prior preparation. The spectra collected under such operating conditions exhibited a resolution of 1 cm -1

. Four scans were performed to average the measurement error. TEM observations were conducted on small powder samples deposed on a copper grid using a JEOL 1200 EX II electronic microscope, equipped with a QUEMESA (EMSIS) camera. The accelerating voltage of the electron beam was fixed at 120 kV. Thermogravimetric analyses were performed on the ThSiO 4 samples prepared, in order to determine their hydration content, thanks to a SETSYS evolution ATD/ATG analyser. All of these analyses were performed between room temperature and 1000°C under air. In-situ SWAXS experiments were conducted using a Guinier-Mering setup with a 2D image plate detector. The X-ray source was a molybdenum anode, which delivered a high-energy monochromatic beam (λ = 0.71 Å, E = 17.4 keV), providing structural information over scattering vectors q ranging from 0.25 to 30 nm -1 . Helium flowed between the sample and the image plate to prevent air absorption. A specific oven was used to set temperature at 40 and 60°C. The sample acquisition time was 1800s. Data corrections and radial averaging were performed by standard procedures. The image azimuthal average was determined by FIT2D software from ESRF (France) 37,38 . At the beginning of the experiment, spectra were acquired each 90 minutes, then the waiting time between two measurements was gradually increased. Spectra were recorded throughout the experiment, from the beginning of the heating up to 3 months of heat treatment.

Results and discussion

Characterization of low temperature synthesized ThSiO 4

Based on the protocol reported for the hydrothermal synthesis of ThSiO 4 31 , several attempts to prepare this phase were conducted at C Th = 0.21 mol•L -1 and C Si = C Th + 3 mol.%, at 90°C

for 50 days and pH ranged between 1 and 8. If, the treatment performed at pH = 1 did not lead to the precipitation of any solid phase, the experiment at pH ≥ 3 led to the formation of the thorite as single crystallized phase according to the PXRD data (Figure S1). The non-precipitation of ThSiO 4 species at pH = 1 is in good agreement with the results we obtained at 120°C and 150°C in our previous study 31 . Moreover, Raman and infrared spectroscopies allowed to confirm the formation of thorite as the final phase obtained without any evidence of the formation of another solid phase (Figure S1). Similar experiments have been performed for pH = 3 and pH = 6 at room temperature, 40°C, 60°C and 90°C for various holding times. Based on the PXRD data, the formation of crystalline thorite has been evidenced at 60°C for pH = 3 (Figure 1) and at 40°C at pH = 6 (Figure S2). At pH = 3, broad diffraction peaks corresponding to the ThSiO 4 pattern have been observed for the experiments performed at room temperature and 40°C. This behaviour might correspond to the formation of nanocrystalline or poorly crystallized silicate phase. Moreover, the shape of the XRD patterns obtained for the sample synthesized at pH = 6 at room temperature or 40°C suggests also the formation on nanometric or poorly crystallized ThO 2 .

Additionally, the spectroscopic characterization of these samples allowed to confirm the presence of thorite in the final phase (Figure 1 and Figure S2). Indeed, at pH = 3, the formation of thorite has been identified by infrared spectroscopy whatever the temperature considered, especially thanks to the characteristic ν 2 and ν 4 bands of the silicate entities. Wavenumber (cm -1 ) However, it may be noticed that these bands are shifted for the syntheses performed at low temperature (from ν 2 = 581 cm -1 to ν 2 = 561 cm -1 and from ν 4 = 444 cm -1 to ν 4 = 440 cm -1

 1  3  4  2  HNO 3
). This shift probably corresponds to a very strong modification of silicate groups geometry at low temperature compared to the ideal ThSiO 4 structure. Moreover, samples characterization by Raman spectroscopy are also consistent with the formation of ThSiO 4 , which may be identified thanks to the characteristic vibration bands associated to the symmetric and antisymmetric stretching modes (at 892 cm -1 and 918 cm -1

, respectively), although these bands appeared to be very broadened and attenuated at low temperature. The spectra obtained for the sample synthesized at pH = 6 were consistent with the formation of ThSiO 4 above 40°C (Figure S2).

Rietveld refinement of the samples synthesized at pH = 3 at 60°C and 90°C allowed to observe a very strong modification of the ThSiO 4 lattice parameters. Indeed, the a lattice parameter has been determined to be significantly higher than the reference parameter reported in the literature for thorite single-crystal prepared at 1100°C (at 60°C a = 7.1674 (18) ). Consequently, the volume of the lattice was significantly higher for the synthesis performed at low temperature compared to the reference unit cell volume (at 60°C V = 324.06(15) Å ). This behaviour was probably due to the insertion of water or hydroxide groups in the thorite lattice according to the results reported by Frondel et al. 2 and those we obtained recently forThSiO 4 synthesis performed under hydrothermal conditions

31

. In order to confirm this hypothesis, thermogravimetric analyses have been conducted up to 1000°C from ThSiO 4 sample prepared at pH = 3 at 60°C and at 90°C. This analysis allowed to measure a mass loss of 9.85 % and 8.65 % for the samples synthesized at 60°C and 90°C, respectively (Figure S3). These mass losses were principally due to water elimination. It is worth noting that the mass loss observed for the syntheses conducted at 60°C led to the release of 1.97 and 1.71 water molecules per ThSiO 4 unit for the samples prepared at 60°C and 90°C, respectively. This content matched with the one reported for environmental actinide silicate samples [41][START_REF] Janeczek | Materials Research Society, Symposium Proceedings[END_REF][START_REF] Frondel | United State Departement of the Interior Geological Survey[END_REF] . From PXRD analysis of the samples heated at 1000°C, the ThSiO Moreover, the formation of a small amount of ThO 2 (around 10 wt.%) has been observed, probably due to the crystallization of nanometric or very poorly crystallized thorium dioxide or to the degradation of a part of the hydrated thorium silicate when heating. Transmission Electron Microscopy observations have been conducted for various samples (Figure 2 and Figure S5). These experiments did not lead to ThSiO 4 with a well-defined morphology, but to aggregates composed of small particles of around 10 nm in size. Whatever the temperature of the treatment, difference in the morphology was observed between the samples prepared at pH = 3 and at pH = 6; the first ones being more dispersed than the latest ones.

In-situ study of the ThSiO 4 formation

In order to obtain more information on the mechanism of formation of ThSiO 4 , in-situ SWAXS experiments were conducted at pH = 3 and pH = 6 at room temperature, 40°C and 60°C. No experiments were conducted at 90°C due to the brittleness in temperature of the glass capillary tubes used.

Complementary experiments were conducted with pH = 1 and pH = 8 at 60°C. However, neither of these experiments actually led to the formation of ThSiO 4 . Indeed, as previously reported, at pH = 1, no solid phase was found while at pH = 8, the formation of ThSiO 4 seemed to have been prevented by the precipitation of nanometric or poorly crystallized thorium dioxide particles. Additionally, it is worth noting that increasing the pH of the reactive media over 3.5 led to the formation of a gel in the reactive media, which conducted to the opacification of the reactive media (from an uncoloured and transparent solution to a white and opaque solution). This precipitation corresponds to the precipitation of particles, which are too big to allow the determination of particle size and shape by SWAXS method. Therefore, the results presented here will mainly correspond to the experiments performed at pH = 3 while the results obtained at pH = 6 are available in supporting information. SWAXS kinetics curves for three different temperatures at pH = 3 are presented in Figure 3. The global examination of these curves indicated first that the systems obtained at pH = 3 exhibited species under the colloidal form for short reaction times. For those samples the SWAXS curves at low q values showed a quasi-constant variation of the scattered intensity, which characterized the presence of small unaggregated objects. This constant regime at low q values was no longer observable when the objects started to aggregate. This change in the curve made then possible to set three different steps in the thorite formation. . ), experiment at t0 is fitted with a cylinder form factor model, P(q). Table 2. Life-times of unaggregated colloids (STEP I); aggregates (STEP II) and for the appearance of ThSiO4 crystals (STEP III) for experiments performed at pH = 3, RT, 40°C and 60°C.

RT pH=3 60°C pH=3 40°C pH=3

Colloids (STEP I)

Aggregates (STEP II)

ThSiO4 crystals (STEP III)

pH3 -RT 0 -152h 152h -30d >> 90d

pH3 -40°C 0 -44.7h 44.7h -24d 24d pH3 -60°C 0 -4.5h 4.5h -54.2h 54.2h

The three steps observed for the system prepared at pH = 3 and 60°C are presented in Figure 4. In the first step (STEP I), small unaggregated objects, describable by geometric model such as cylinder, remained in suspension until they started to aggregate (STEP II). As shown on the blue data points series, this aggregation led to the alteration of scattered intensity at low q values. Then, after a maturation time, crystallites appeared in solution, large enough to give clear diffraction peaks of the thorite crystal (lattice planes (1,0,1) ; (2,0,0) ;

(1,1,1) ; (2,1,1) ; (1,1,2) ; (2,2,0)) at large q values (STEP III). It was therefore possible to assess three different characteristic times for each system (Table 2 and Table S1 of supporting information for the samples obtained at pH = 3 and pH = 6, respectively) from the SAXS curves reported in Figure 3 and Figure S6. In Table 2, the time needed to move from unaggregated colloids (STEP I) to aggregated ones (STEP II) decreases when the temperature increases, i.e. from 152 hours at room temperature (20°C) to 44.7 hours at 40°C and to 4.5 hours at 60°C. While constant scattered intensity regime at low q values for very short lifetime (STEP I) was observed at pH = 3, it was clearly not the case at pH = 6 (Figure S6). At pH = 6, the colloids were aggregated from the very beginning of the reaction. Even at room temperature, the scattered intensity at low q values showed a drastic increase explained by the presence of big objects of several hundreds of nanometers in solution. The presence of these big scattering objects was attributed to the colloids low surface-charge at pH = 6. Indeed, the isoelectric point of thorium silicate sets at pH = 4.6

3

. In these conditions, electrostatic repulsion do not stabilize colloids and aggregation occurs at the early stage of reaction. The time needed for the observation of diffraction peaks characteristics of ThSiO 4 crystals is also presented in Table 2. This characteristic time was obtained when crystallites are large enough to give diffraction peaks in the high q region of SWAXS curves. The thorite peaks appeared for q values between 10 nm -1 and 30 nm -1

, in the focus at right bottom of each curve of Figure 3 (Figure S6 at pH = 6). As expected, the time needed to detect ThSiO 4 crystals decreased when temperature increased. At pH = 3 and RT, ThSiO 4 crystals were not detected, even after one month of reaction whereas ThSiO 4 diffraction peaks appeared for high q values after 24 days and 54.2 hours at 40°C and 60°C, respectively. It is worth noting that at room temperature and for pH = 3, crystals of ThSiO 4 was evidenced by PXRD after 257 days of reaction (Figure 1a). At pH = 6 (Table S1), only STEP II was observed at RT during 30 days while ThSiO 4 diffraction peaks appeared for high q values after 24 days and 35.7 hours at 40°C and 60°C, respectively. Thus, and as expected, at pH = 3 and 6, the kinetics of crystallization of ThSiO 4 increased with temperature. At geological timescale for example, it is thus reasonable to suggest that those samples should be finally crystallized under the thorite form even at temperature as low as room temperature.

From colloidal state to aggregated clusters

The SWAXS theory claims that the curves relative to scattering objects dispersed in a medium can be described by the following general equation:

(

Where I(q) is the scattered intensity; A is a prefactor depending on scattering objects volume fraction and the molecular volume of objects; P(q) is the scattering objects form factor; and S(q) is the structure factor describing interaction between scattering objects. When the colloids are diluted enough, without any aggregation in solution, S(q) equals to 1 and the SWAXS curves can be described with only form factor P(q). This means that for low reaction times, in the STEP I, the SWAXS curves can be fitted with a simple model not taking into account any structure factor. Therefore, this simplified description was only possible for experiments performed at pH = 3 due to aggregation phenomenon occurring in the gel state at pH = 6. Several form factor models were tested but only cylinder shape exhibited a good correlation with SWAXS curves (see the detailed expression of the form factor model in the supporting information). An example of a fitted curve is presented in Figure 4 (red curve). Therefore, fitting experimental data points at each reaction time allowed to analyze the evolution of cylinder length L during STEP I until those cylinders started to aggregate (Figure 5). For all the considered temperatures, the cylinder length increased while no noticeable change of the radius was observed (Table S2). The growth rate, given by the slope of L(t), also increased with the temperature. This u.)

result is in agreement with faster kinetics of reaction at high temperature. It is noticeable that regardless of the temperature, colloids started to aggregate as soon as the cylinder length reached 10 nm. It is assumed that at the early stage of the reaction, 1D-growth of the nuclei occured giving birth to elongated cylinders. Then, at a given time that corresponds to the end of STEP I, these isolated cylinders started to agglomerate leading to larger objects with heterogeneous sizes (STEP II). Another explanation could be that the growth mechanism changed from 1D to 3D. In that case, the extremities of the cylinders would be no more energetically favourable than the walls. This change of reaction site has been already observed for silicates. For example, it is known that silicic acid produced linear structures of silica at low pH values while denser structures were observed for higher pH values due to a change in the localization of condensation reaction in the colloids 44 .

Emergence and growth of nanocrystalline domains

If STEP I corresponds to the stage of 1D-growth of colloids and STEP II to an aggregation period, the STEP III refers to the appearance of thorite nanocrystalline domains. The inserts in the curves presented in Figure 3 show the SWAXS large q range where thorite diffraction peaks were detected. Thorite crystals were not observed at room temperature for the entire duration of the in-situ SWAXS experiment at pH = 3. However, as already mentioned, PXRD pattern obtained from ex-situ synthesis at RT showed a very broad and diffuse signal after 257 days of reaction, which was the sign of the presence of nanocrystalline thorite. Thanks to ex-situ PXRD and in-situ SWAXS measurements, it was possible to determine the size of coherent domain in thorite crystals using the Debye-Scherrer equation

45

. The variation of the size of the coherent domains in thorite crystals with reaction time is reported in Figure 6. Data were treated integrating peak at 20° (PXRD) and 13 nm -1 (SAXS) corresponding to lattice plane (1,0,1). First of all, the results obtained from SWAXS (full markers) and from PXRD patterns (open markers) are consistent. At RT, diffraction peaks in the large q range are not observed on SWAXS curves, even after 90 days of experiments. From the results obtained at 60°C, it appeared that the size of ThSiO 4 crystals increased mainly between 54.2 and 700 hours of reaction, from 4 ± 1 nm to 7.5 ± 0.5 nm. After this first increase, the values reached a plateau until the end of these in-situ measurements (3 months). Figure 6. Evolution of the size of the coherent domains in thorite crystals synthesized at pH = 3 determined from in-situ SAXS and ex-situ PXRD measurements. Dashed lines correspond to crystallite mean size at large lifetime (257 days for RT and pH=3, the data point obtained with PXRD is not represented to ease the reading of the figure).

The dotted blue line in Figure 6 stands for the size of coherent domains in ThSiO 4 crystals obtained from PXRD pattern of the synthesis performed at RT after 257 days, i.e. 2.5 nm. After stabilization, the size of the coherent domains in ThSiO 4 crystals reached 4 ± 1 nm and 7.5 ± 0.5 nm at 40°C and 60°C, respectively. From these results, it appeared that increasing the temperature improved the crystallinity of thorite. It is worth noting that the size of the coherent domains obtained at large reaction time were lower than the maximum length of the cylinders initially formed in the reactive media. This observation indicates that the objects formed at low temperatures (i.e. ≤ 60 °C) presented a large degree of disorder, which was reduced to some extent by increasing the temperature of the synthesis. Thermogravimetric analyses showed that the amount of water molecules or hydroxide groups in ThSiO 4 decreased with an increase of the temperature of synthesis. Thus, it could be suspected that a part of the observed disorder in the thorite crystals formed at low temperature (i.e ≤ 60°C) was caused by insertion of water molecules or hydroxide groups in the lattice. The variation of the rate of crystallization with respect to temperature was more difficult to establish due to the lack of values at 40°C and RT.

Conclusions

For the first time, thorite ThSiO 4 was obtained from direct precipitation in weakly acid silicate solution at room temperature. This result confirmed the thermodynamic stability at 298 K with respect to the mixture of the binary oxides. In addition, this study allowed following of the early RT stages of thorite formation. By decreasing the temperature of synthesis in comparison with classical hydrothermal conditions, the kinetic of formation of thorite was considerably reduced. In-situ SWAXS monitoring of the first steps of crystallization was thus made possible. This in-situ monitoring allowed characterising of the kinetics of thorite nucleation and growth as a function of the pH value and temperature of the reactive medium. At the early stages of the reaction, SWAXS measurements showed that thorium was maintained in solution in the presence of silicate species under a waterborne form during a long period of time. These Th-silicates colloids adopted a cylinder shape of less than 10 nm length, for a radius around 0.5 nm, before agglomeration or 3D-growth occurred. The duration of this first step increased when decreasing the temperature and pH value. After a given maturation time, during which SAXS signal was no longer describable with simple form factor model due to particles aggregation, the formation of ThSiO 4 crystals was systematically observed by WAXS and PXRD. This result led to the conclusion that colloidal species were either embryo, or intermediate for the crystallisation of thorite. The duration of maturation time was found to increase by decreasing the temperature. The resulting thorite nanocrystals of about 10 nm in diameter were poorly structured. However, the crystallinity of thorite was improved by increasing the temperature. Based on thermogravimetric analyses, it is found that the amount of water molecules or hydroxides groups inserted in the ThSiO 4 crystal structure and responsible for lattice distorsion, decreased by increasing the temperature. This behaviour led to less disordered material. Treating these compounds at high temperature (i.e. 1000°C) allowed recovering of the lattice parameters of thorite mineral, while no or very low amount of ThO 2 was observed in the mixtures.

These results have potential implications in the paragenesis of Th minerals deposits, but also in the behaviour of Th and, by analogy, of An IV in the environment. Under environmental conditions, concentration of Th based aqueous species is usually several orders of magnitude lower than silicates concentrations due to the very low solubility of Th phases. However, the formation of oxyhydroxide colloids is known to increase the concentration of waterborne Th by several orders of magnitude, even for near neutral pH values. In the presence of a large excess of silicate ions, Th-silicate or Thhydroxosilicate colloids could exist. The Th-silicate colloids evidenced in this work have, at low temperature and at near neutral pH, a long-term stability and a morphology in favor of a higher mobility in groundwaters. If these species are formed in more diluted media, this could affect the spreading of Th and An IV in permeable environmental media, which would allowed colloidal transport.

-

1

 1

Figure 1 .

 1 Figure 1. PXRD patterns (a), Raman (b) and infrared spectra (c) obtained for samples prepared with starting silicate and thorium(IV) concentrations of 0.21 mol•L -1 in nitric reactive media at initial pH = 3 at 90°C for 50 days (1), 60°C for 51 days (2), 40°C for 94 days (3) and at room temperature for 257 days (4). Characteristic XRD lines of ThO2 and ThSiO4 were extracted from references 40

3 ;

 3 at 90°C, V = 323.42(7) Å 3 ; reference V = 321.48(3) Å 3 39

3 )Figure 2 .

 32 Figure 2. Transmission Electron Microscopy micrographs recorded for the syntheses performed at CTh = 0.21 mol•L-1, pH = 3 at 60°C for 51 days (a), at 40°C for 94 days (b) and at room temperature for 257 days (c).

Figure 3 .-

 3 Figure 3.In-situ SWAXS measurements kinetics at room temperature (RT), 40°C and 60°C for pH = 3. The inset graphics refer to a focus in the q range from 10nm -

Figure 4 .

 4 Figure 4. In-situ SWAXS measurements kinetic of thorite formation at pH = 3 at 60°C. Description of STEP I, II and III. At low q values (< 4nm -1

Figure 5 .

 5 Figure 5. Evolution of the cylinders length at low reaction times (STEP I) for SWAXS at pH = 3, at RT, 40°C and 60°C.

  -situ SWAXS is relevant to study the first moments of a crystallisation process. However, this technique has never been used to describe the crystallization of actinidesilicate phases. To the best of our knowledge, only Neill et al. have used SAXS technique in order to determine the size of uranium silicate colloids and to investigate their stability with regard to pH and concentration 1

	Therefore, in
		). For example, Stavitski et al. have
	used SWAXS to differentiate the mechanisms of nucleation
	/crystal growth for two different MOFs (MIL-53 and MIL-101)
	obtained from the same molecular precursors but in various
	solvents	35 .

Table 1 .

 1 Synthesis parameters for the ThSiO4 samples obtained by ex-situ

	experiments, corresponding time of in-situ SWAXS experiments performed
	under the same conditions. Labels are then used in Figures 1, S1, S2 and S4 for
	easier reading. CSi = CTh + 3 mol.%.

Label pHini CThi (mol•L -1 ) Temperature (°C) Ex-Situ exp. heat treatment (days) In-Situ exp. heat treatment (days)

	(1)			90	50
	(2) (3)	3	0.21	60 40	51 94	90 90
	(4)			RT	257	30
	(5) (6)	8 6	0.21	90 90	51
	(7)			60	101	90
	(8)	6	0.21	40	94	90
	(9)			RT	257	30

Acknowledgements

The authors would like to acknowledge Bruno Corso (ICSM) for his technical support with PXRD and SWAXS measurements.

Notes and references

Conflicts of interest

There are no conflicts to declare.