Jérôme Laurencin
email: jerome.laurencin@cea.fr

Aurelien Flura

Giuseppe Sdanghi

Sébastien Fourcade

Vaibhav Vibhu

Jean-Paul Salvetat

Jean-Marc Bassat
email: jean-marc.bassat@icmcb.cnrs.fr

Aurélien Flura

Vibhu Vaibhav

oxygen over-stoichiometric air electrode materials

teaching and research institutions in France or abroad, or from public or private research centers.

Oxygen diffusion and surface exchange coefficients determined under high pressure: comparisons between oxygen deficient vs.

Introduction

Clean and sustainable energy sources are currently becoming essential. However, since the renewable energies are intermittent, efficient solutions for their storage are required, to match the fluctuations between the demand and the production. In this frame, hydrogen is considered as one of the most efficient energy vector [1]. High Temperature Steam Electrolysis (HTSE), based on the Solid Oxide Electrolysis Cells (SOEC) technology, is a promising and attractive technical solution for hydrogen production at high efficiency [2]. A technical issue is that hydrogen has to be stored and distributed at high pressure. Indeed, since the compression of liquid water consumes much less energy than gaseous hydrogen, it is advantageous to perform the steam electrolysis directly under pressure [3]. Moreover, the high-temperature co-electrolysis of steam and carbon dioxide enables to convert electricity into a syngas composed of carbon monoxide and hydrogen [2,4]. This gas mixture can be transformed in a second step in liquid or gaseous hydrocarbons with conventional catalytic processes working at high pressure between 10 and 100 bars [START_REF]Catalysis in C1 Chemistry[END_REF]. In this case, an integrated system working directly under pressure could also be beneficial in terms of efficiency and cost [3,[START_REF] Hansen | [END_REF]. In this context, it is mandatory to investigate the impact of high pressure on the electro-catalytic properties of the electrode materials that control the cell electrochemical response, and more particularly the oxygen electrode. Mixed Ionic Electronic Conductors (MIEC) are commonly used as oxygen electrodes in SOEC cells. In addition to a high electronic conductivity, this type of electrode material require both high oxygen diffusivity and surface exchange coefficient (D and k, respectively) at the operating temperatures. Two different kind of efficient candidates are generally considered: i) the oxygen deficient perovskites such as La 0.58 Sr 0.4 CoO 3-δ (LSC) and La 0.58 Sr 0.4 Fe 0.8 Co 0.2 O 3-δ (LSFC) [7], which are nowadays the standard SOEC materials and ii) the oxygen over-stoichiometric oxides, such as nickelates Ln 2 NiO 4+δ (Ln = La, Pr, Nd), which belong to the so-called Ruddlesden series Ln n+1 Ni n O 3n+1 (here with n=1). These more innovative compounds with the K 2 NiF 4 -type layered structure have shown promising oxygen electrode performances because of their large anionic bulk diffusion and surface exchange coefficients, combined with a good electrical conductivity and a thermal expansion coefficient matching with those of other cell components [8,9]. To date, the reported measurements of D and k in the literature have been performed at ambient pressure and even below [10]. However, to the best of our knowledge there is no available data measured at high pressure whatever the considered electrode material.

Scientific Approach

The objectives of the current work consist in: 1) performing isotopic oxygen exchanges on dense pellets with Ln 2 NiO 4+δ (Ln = La, Pr, Nd), LSC and LSFC compositions under high oxygen pressure, 2) checking that the resulting oxygen stoichiometry  was significantly changed after such exchanges and 3) evaluating the consequence of  variation on the tracer D* and k* coefficients.

Experiments/Calculations

Dense samples in pure materials with a density of at least 95% are required for the oxygen exchange experiments. In this study, pellets were prepared to fulfill these conditions, with a diameter of roughly 20 mm after sintering. Pure powders of the three lanthanide nickelates were produced using the modified Pechini method [11] from Pr 6 O 11 (Aldrich chem, 99.9%), La 2 O 3 and Nd 2 O 3 (Strem Chemical, 99.99%) and Ni(NO 3 ) 2 ,6H 2 O (Acros Organics, 99%) precursors. La 2 O 3 powder was initially heat-treated at 900 °C overnight to remove residual water before weighing. The final annealing was performed at 1200 °C for 12 h in air, leading to well crystallized phases. The purity of the powder was controlled by X-ray Diffraction (XRD) analysis. The materials were then sintered at 1350 °C for 4 h in order to get dense pellets. La 0.58 Sr 0.4 Fe 0.80 Co 0.2 O 3-δ (LSFC) and La 0.58 Sr 0.4 CoO 3-δ (LSC) powders were synthesized by a standard solid-state route using following precursors: La 2 O 3 (99.99%, Sigma Aldrich), SrCO 3 (99.9%, Sigma Aldrich), Fe 2 O 3 (99%, Sigma Aldrich) and Co 3 O 4 (99%, Alfa Aeser). The powders were weighed according to the compositions and then ball-milled for 4 h at 250 rpm using zirconia balls and isopropanol (98.8%, VWR). After drying at 80 °C overnight, the mixtures were then calcined in air at 1080 °C for 8 h. After getting the pure phase, the powders were again ball-milled for 4 h in order to get the average particle size ∼1μm. The dense pellets of LSFC and LSC were prepared by uniaxial pressing of the powders followed by a sintering at 1350 °C for 4 h. For the five studied compositions, the dense pellets were first mirror polished, using abrasives of decreasing grain diameter, then diamond paste to complete the work. Finally, cubes of roughly 4 mm side length were cut from the pellet, using a diamond cutting-disc (Isomet Buehler), in order to ensure that our samples are comparable.

The isotopic oxygen exchange setup was developed for an operation under total pressure up to 40 bars (Fig. 1) and temperature up to 1100°C. It was more extensively described in [12]. The first line of the setup is dedicated to pumping: using a turbomolecular pump a vacuum of roughly 10 -6 mbar is obtained. The marked 18 O gas (with purity close to 98%) is routed through a second line, from a gas (air) cylinder provided by Euriso-top company with the following specifications: 1L, P = 20 bars. Another line is dedicated to 16 O from the laboratory. A fourth line is dedicated to cryopumping of the marked 18 O at the end of the exchange, which is achieved by immersing an stainless steel rod (one meter long) into liquid helium. The gas may then be used for another experiment, provided that the ratio 18 O/ 16 O is still high. Two manometers allows measuring respectively the low and high pressures. The sample holder is a closed cylinder made of magnesium doped zirconia: it was designed to withstand both the high pressure and high temperature in steady state operation and also the stresses induced by quick thermal quenching. It was thus able to withstand, through all our experiments, rapid cooling from high operating temperature (700°C -500°C) down to room temperature. This thermal quenching was achieved by removing the sample holder from the sliding furnace mounted on rails. In a first step, the samples were equilibrated at the target temperature (700 °C for instance) in 16 O 2 for a period at least ten times longer than the one chosen for the oxygen exchange experiment, then quenched down to room temperature. This treatment creates the baseline in 18 O/ 16 O inside the sample that will be used as reference later during the TOF-SIMS analyses. Besides, it fixes the chemistry in the bulk of the materials: the oxygen over-stoichiometry of the lanthanide nickelate and the oxygen under-stoichiometry in the perovskites. Next, the 16 O 2 gas was pumped and replaced with the desired pressure of 18 O 2 at room temperature. The samples were quickly heated to reach the target temperature, and the oxygen exchange experiments carried out for a determined time. Typically, the duration times used for the oxygen exchange steps varies from about 90 min. at 700°C till about 20h at 500°C. Samples were finally quenched down to room temperature, and 18 O 2 recovered using liquid helium. Finally, the samples were taken out of the zirconia cylinder to be prepared for the Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS) analyses.

Our first goal was to determine the delta (δ) value characterizing the oxygen under/over-stoichiometric of the oxides, after each thermal treatment performed under pressure followed by a thermal quenching. For this purpose, Thermogravimetric Analysis (TGA) experiments were carried out using a TA Instrument ® TGA-5500 device. First, the dense bare samples (after exchange experiments) were crushed into powder using mortar-pestle. Then the powders were heated under air up to 150 °C/1h, then cooled down to room temperature with a slow rate (2 °C.min -1 ), with the aim to remove any traces of water. A second cycle was performed under Ar -5% H 2 flux with a very slow heating rate of (0.5 °C.min -1 ) up to 1000 °C. The full decomposition of the material under reducing atmosphere leads to the determination of the oxygen stoichiometry after cycling the sample down to room temperature. The decomposition products (Ln 2 O 3 , SrO, metallic Ni, Fe and Co) were verified by XRD after the thermal cycle. However, despite several attempts, the complete reduction of LSFC under Ar -5% H 2 flux was never reached (the XRD analyses evidenced the formation of (La,Sr) 2 (Fe,Co)O 4 + Fe + Co) making the calculation of  not possible in this case.

Prior to the TOF-SIMS analyses, the cubes were cut in half with the utmost care using the diamond cutting-disc. This cut is necessary to reveal the gradient in 18 O. The new face created was then mirror polished. The resulting two samples were pressed altogether and fixed inside a stainless-steel ring with Wood's metal alloy. The particularity of this preparation is that one of the samples exposes the surface of the cube that was in direct contact with 18 O 2 , while the other sample exposes the bulk of the cube that contains the gradient of 18 O. The benefit of this setup is to measure with precision the ratio 18 O/ 16 O at the surface of the sample, averaged over a large area. This value is used as the origin of the gradient measured on the other sample.

SIMS experiments were performed using a TOF SIMS 5 (IONTOF, Münster, Germany).

Samples were analysed in spectroscopy mode by scanning a primary 30 keV Bi + beam over 500x500 µm 2 area, interlaced with a 1 keV Cs + sputter beam scanned over 600x600 µm 2 to eliminate surface impurities and enhance production of negative secondary ions. Area of interest was adjusted using the integrated optical camera and sample stage controls so that a sample edge was located vertically on the right or left of the raster field of view. Data acquisition was started in the 3D mode, and the following procedure was used to determine the stopping condition: once the secondary ions of interest (e.g. 18 O and 16 O) peaks were selected on the spectra subpanel, images of ion intensity with definition 256 x 256 pixels were automatically generated by the instrument software (SurfaceLab 7) in the image stockpile subpanel. The images of 18 O and 16 O intensities are then combined to generate an image of the isotopic ratio of interest I( 18 O)/(I( 18 O)+I( 16 O)), and performed an average line scan thereof, using the Xarea feature of SurfaceLab linescan editor. Using this procedure we could follow the gradient line-scan progress in real-time so that data acquisition could be stopped, typically after 5 to 10 min, when the gradient was stabilized at a good S/N ratio. Doing so an isotopic ratio profile averaged over a volume of approx. 500X500X0.1 µm 3 is generated, which minimizes artifacts from impurities and inhomogeneities. The data were finally exported as ASCII file for further analysis. In some cases, 18 O gradient spanned over 500 µm so that a second adjacent area was analysed similarly and stitched to the previous one. The experimental oxygen profiles (evolution of I( 18 O)/I( 18 O)+I( 16 O) vs. depth) were then fitted using the Crank solution to the equation of the second Fick's law of gas diffusion in solids [START_REF] Crank | The mathematics of Diffusion[END_REF][START_REF] Bassat | [END_REF], which leads determining the D* and k* coefficients (* being relative to the diffusion of the 18 O tracer).

Results and discussion

After the oxygen exchange steps, the samples were quenched to stop the oxygen diffusion in order to keep the same oxygen composition that was reached in the material during the investigated experimental conditions. Our first goal was to determine the corresponding oxygen stoichiometry values and, for this purpose, we used TGAs experiments performed under reducing atmosphere (Ar/5%H 2 flow) on powders (crushed pellets). Regarding nickelates with oxygen over-stoichiometry, the decomposition of the oxide occurs in two steps: 1) At intermediate temperature a first plateau is evidenced, which corresponds to the stabilization of the oxide only containing Ni +II , i.e. a composition Ln 2 NiO 4.0 (Ln=La, Pr, Nd). The measured weight loss between room temperature and this temperature corresponds exactly to the disappearance of the oxygen over-stoichiometry ; 2) in a second step, at higher temperature the full decomposition of the oxide is observed, leading as checked by XRD to the formation of Ln 2 O 3 plus metallic Ni. Measuring the weight loss recorded between room temperature and the full reduction is then a second way to determine . On the contrary, regarding LSC only the full reduction is observed on the TGA curves. Table 1 gathers the results obtained for each exchanged sample, for 500 < T°C < 700 and pO 2 = 6.5 bars (total pressure around 30 bars). Again, no experimental data are available for LSFC. For the nickelates, each reported  value is an average between the two calculations detailed above.

Table 1. Oxygen over / under -stoichiometry's  values determined by TGAs experiments performed under reducing atmosphere, on the samples previously exchanged under 18 O atmosphere, at a given temperature, under pO 2 = 6. As expected, whatever the temperature used for the oxygen exchange, all the nickelates exhibit a significant increase of the oxygen over-stoichiometry after the thermal treatment performed under high oxygen pressure, compared to the value obtained after the chemical synthesis. Indeed, the compositions determined at room temperature after the synthesis performed in air were: La [15] and the joined references). On another side, the oxygen under-stoichiometry of La 0.58 Sr 0.4 CoO 3-  is reduced compared to the composition determined after the synthesis in air. Indeed, it is equal to 0.02 after synthesis whereas the samples after the thermal treatment performed under pressure become almost oxygen stoichiometric (table 1). This experimental result was also expected. Finally, at this stage, our conclusion is that the studied compositions are significantly over-oxidized after the thermal treatments performed under 18 O pressure.

The evolutions of the D* and k* coefficients are plotted as a function of temperature in Fig. 2, 3 and 4 for the over-oxidized nickelates, i.e. La 2 NiO 4+ , Pr 2 NiO 4+ and Nd 2 NiO 4+ , respectively. Fig. 2 -Evolutions as a function of temperature of the k* (red dots) and D* (black dots) coefficients for over-oxidized La 2 NiO 4+ samples (table 1). The red and black dashed lines give respectively the corresponding k* and D* data for isotopic exchanges performed under pO 2 = 0.2 bar (from [8])

The k* plot of La 2 NiO 4+ was quite peculiar, and we suspected that an experimental artifact for the measurements at 650°C and 700°C. Indeed, the exchange rate largely increases with increasing the temperature, in such a way that the surface of our sample can be saturated with 18 O if the exchange time is too long. In this case, the measured value of k* becomes inaccurate. To check this assumption, we reduced the exchange time at 700 °C from 80 min down to 42 min (red and green dot, respectively): we measured a k* value that is close to the linear regression plot of the k* values obtained at 500 °C, 550 °C and 600 °C. 1). The red and black dashed lines give respectively the corresponding k* and D* data for isotopic exchanges performed under pO 2 = 0.2 bar (from [8]).

Whatever the considered nickelate, both D* and k* coefficients are improved after oxygen exchanges performed under pressure compared to the corresponding values obtained after oxygen exchange performed under pO 2 = 0.2 atm. Considering the Nernst-Einstein relation, it can supposed that the ionic conductivity of such MIEC materials will be probably improved when operating under pressure (the measurements are in progress). It is of course a very interesting result leading to carefully consider these innovative oxygen electrodes for the SOEC application under pressure. Some years ago Chroneos et al. [16], also Parfitt et al. [17] have performed Molecular Dynamics (MD) simulations regarding the oxygen diffusion in La 2 NiO 4+ and Pr 2 NiO 4+ , respectively. The dominant mechanism of oxygen transport occurs via the network of apical oxygen sites connected by interstitial ion sites along a two-dimensional network (the so-called interstitialcy mechanism [16]). In the article by Parfitt et al., the oxygen diffusivities were calculated for a range of hyper-stoichiometries, and an excellent agreement was found between the absolute calculated values and those observed experimentally. Initially (the first calculated point being  = 0), the diffusivity rises very quickly as a function of , but rapidly levels off. Between  = 0.05 and  = 0.25 the diffusivity still increases but smoothly. The increase of the formation energy of oxygen interstitials, as well as a stiffening of the lattice cause by the additional oxygen interstitial pinning the NiO 6 sub-lattice, which reduces the passage of the oxygen ions, were involved. Our results are in good agreement with such calculations, indeed when increasing  for each studied nickelate by means of oxygen pressure, we observed a limited increase of the diffusion coefficient, while a larger one could be expected in first approximation. The largest increase is observed when the initial oxygen overstoichiometry is the lowest one, for La 2 NiO 4+ ( = 0.16 at room temperature after annealing under air at normal pressure).

On the opposite, within this work, we also evidenced that the kinetic constant of oxygen exchange k* increases significantly with the pressure. This dependency is consistent with the theoretical evolution of the chemical kinetic constant k chem (which is directly related to k*). Indeed, the reaction of oxygen insertion into the material can be written as follows:

(1) where is the oxygen interstial and the empty site. The kinetic rate of this reaction is given by:

(2) where and denote respectively the forward and backward kinetic constants of the reaction. The molar fraction and are expressed through the concentrations in the material: and with . Assuming that the oxygen partial pressure in the gas phase is a constant imposed by the experimental conditions ( , it can be easily shown that:

The kinetic rate of the oxygen exchange can be also expressed as [18]. The chemical constant is thus given by: (4) Therefore, the chemical constant must increase with the square root of the oxygen partial pressure. This theoretical dependence is thus consistent with the experimental increase of the kinetic constant k* with the pressure [18]. 1). The red and black dashed lines give respectively the corresponding k* and D* data for isotopic exchanges performed under pO 2 = 0.2 bar (from [19]).

In both cases, at 650°C, we used two different exchange times (26h (red dot) and 2h (green dot)). Once again, as observed on La 2 NiO 4+ , reducing the exchange time leads to an increase of the k* coefficient. On the opposite to what was observed for the oxygen over-stoichiometric nickelates, D* and k* coefficients for the oxygen deficient perovskites are found to decrease with increasing from pO 2 = 0.2 bar to pO 2 = 6.5 bar. This trend is especially pronounced for LSC, for which we have determined that the oxide became almost oxygen stoichiometric after the thermal treatment performed under pressure. Our hypothesis is that the oxygen vacancy concentration is too low to ensure the oxygen diffusion mechanism properly when the material is used under pressure. Regarding k*, the kinetic constant should increase as the square root of the oxygen partial pressure as predicted by eq. ( 4). For instance, this dependency was well retrieved by Katsuki et al. [18] for La 0.6 Sr 0.4 Co 0.8 Fe 0.2 O 3- over a range of low oxygen partial pressures changing from 10 -4 atm to 1 atm at 800°C. This statement is also consistent with the results published by Elshof et al. [20]. Nevertheless, in our case, the kinetic constant for the investigated under-stoichiometric materials follows an opposite trend when increasing the oxygen partial pressure from 0.2 atm to 6.5 atm. This behavior remains still unclear. In ref. [18], the authors mentioned that 'further research seems necessary for a detail explanation of why k chem decreases with reducing pO 2 '. However, it seems that Eq. ( 4) could explain this evolution.

Conclusions

To increase the efficientcy at the system level, it is envisaged to operate the solid oxide electrolyser at high pressure. In this context, the goal of this work was to determine the oxygen diffusion and surface exchange coefficients (which characterize the O 2-ionic conductivity) of two standard under-stoichiometric oxygen electrode materials (La 0.58 Sr 0.4 CoO 3-δ (LSC) and La 0.58 Sr 0.4 Fe 0.8 Co 0.2 O 3-δ (LSFC)) compared to three overstoichiometric nickelates (La 2 NiO 4+ , Pr 2 NiO 4+ and Nd 2 NiO 4+ ). These latter materials have been developed for several years as alternative oxygen electrodes to the more conventional oxygen deficient perovskites. After a preliminary isotopic exchange step performed under high oxygen pressure, the measurements were performaed using the IEDP method was used. For this purpose, an original set-up has been developed. As expected, the five materials became over oxidized after the oxygen isotopic exchanges performed in the temperature range 500 < T°C < 700 under p(O 2 )  6.5 bars compared to the as-prepared ones (under atmospheric condition at p(O 2 )  0.2 bars).

The measured evolutions of D* and k* coefficients are thus representative of materials in a very different oxidation state. It means first that the oxygen vacancy amounts of LSC and LSFC are largely reduced after the thermal treatments performed under pressure compared to that performed under pO 2 = 0.2 atm. Correspondingly, both the D* and k* coefficients values have been found significantly lowered when increasing the pressure up to 6.5 bar. On the contrary, the oxygen over-stoichiometry of the nickelates is enlarged after the thermal treatment under pressure. Correspondingly, D* and k* coefficients of the oxygen over-stoichiometric nickelates are increased after oxygen exchanges performed under pressure. However, whatever the studied composition, the impact of pressure is less pronounced for D* compared to k*.

These statements would thus indicate that the electrochemical properties of such nickelates with MIEC properties are improved for a SOEC operation under pressure. They could thus constitute standard electrode materials for operations performed under oxygen pressure instead of Co-based perovskites. Indeed the interstitial amount of the nickelates structure can house a large range of oxygen corresponding to the overoxidation of the electrodes, while on the contrary, the amount of oxygen vacancy in Co-based perovskites becomes too low in the same operating conditions, limiting the ionic transport properties of the corresponding electrodes. Nevertheless, the long-term stability of these materials when submitted to high pressure and polarization still needs to be studied.

Fig. 1 -

 1 Fig.1 -Original isotopic exchange setup for experiments up to P = 40 bars and T=1100°C

Fig. 3 -Fig. 4 -

 34 Fig.3 -Evolutions as a function of temperature of the k* (red dots) and D* (black dots) coefficients for over-oxidized Pr 2 NiO 4+ samples (table 1). The red and black dashed lines give respectively the corresponding k* and D* data for isotopic exchanges performed under pO 2 = 0.2 bar (from [8])

Fig. 5 Fig. 5 -

 55 Fig. 5 and 6 plot the evolutions of the D* and k* coefficients vs. T, on the over-oxidized perovskites, i.e. La 0.58 Sr 0.4 CoO 3-δ (LSC) and La 0.58 Sr 0.4 Fe 0.8 Co 0.2 O 3-δ (LSFC), respectively.

Fig. 6 -

 6 Fig.6 -Evolutions as a function of temperature of the k* (red dots) and D* (black dots) coefficients for La 0.58 Sr 0.4 CoO 3-δ (LSC, table1). The red and black dashed lines give respectively the corresponding k* and D* data for isotopic exchanges performed under pO 2 = 0.2 bar (from[19]).

  2 NiO 4.16 , Pr 2 NiO 4.22 and Nd 2 NiO 4.21 (i.e.  = 0.16, 0.22 and 0.21, respectively). It can noted that here is a large agreement in the literature regarding these values (see, for instance,

Acknowledgments

A.F. thanks the CNRS French grouping GDR HysPàc for supporting this research work via an internal project. CEA is acknowledged for its valuable technical aid, including the supply of the pressurized air bottle enriched in 18 O.


Daniel T Bowron

David A Keen

Mathieu Kint

Coralie Weigel

Benoit Ruffle

Leszek Konczewicz

Sylvie Contreras

Benoit Coasne

Gaston Garbarino

Mickael Beaudhuin

Julien Haines

Jérôme Rouquette

Atomic-Spring-like Effect in Glassy Silica-Helium Composites

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Finding new ways to accumulate and restore mechanical energy at the nanoscale is highly desirable, particularly when a solution is found for the archetype glassy material: amorphous silica SiO2, a-silica, which exhibits both fundamental and technological interest. An atomic-spring could be used in subnanoscale devices and sensors to measure "nano" forces and would offer an outstanding energy dissipation mechanism. The asilica-He composite does not, at first instance, appear as an appropriate candidate for such an issue as a-silica is not defined as a porous material and only 0.0084 mol/cm 3 /GPa of He were found to be inserted in a-silica at pressures up to 0.13 GPa 9 . However, a-SiO2 exhibits a corner-sharing SiO4 tetrahedra local structure which forms voids of n-fold rings (with n ≥ 2, in a-SiO2 mostly n = 4, 5, 6, 7, 8) [START_REF] Keen | Local structures of amorphous and crystalline phases of silica, SiO(2), by neutron total scattering[END_REF][START_REF] Elliott | Medium-range structural order in covalent amorphous solids[END_REF][START_REF] King | Ring configurations in a random network model of vitreous silica[END_REF][START_REF] Sarnthein | Structural and electronic-properties of liquid and amorphous SIO2 -An ab-initio molecular-dynamics study[END_REF][START_REF] Tucker | Application of the reverse Monte Carlo method to crystalline materials[END_REF] , the diameter of which is large enough for He (rHe=1.3 Å) to be inserted under pressure in the most common six-membered units [START_REF] Pasquarello | Identification of Raman defect lines as signatures of ring structures in vitreous silica[END_REF] , (r6-fold ring =1.5 Å). This scenario was confirmed and up to 1 mole of He could be inserted experimentally [START_REF] Sato | Helium penetrates into silica glass and reduces its compressibility[END_REF][START_REF] Shen | Effect of helium on structure and compression behavior of SiO2 glass[END_REF][16] (0.53 mole theoretically) [START_REF] Coasne | Poroelastic Theory Applied to the Adsorption-Induced Deformation of Vitreous Silica[END_REF] in a-silica under pressure.

Interestingly, He as a pressurization fluid can be considered as a penetrating medium, whereas Ar atoms (rAr=1.88 Å), cannot be inserted so that pressurization using this fluid results in the normal densification of the material under compression. So far, the mechanism of helium insertion has never been determined, so to gain insight we performed an in situ structural investigation of helium insertion in a-silica at the beamline ID27 of the European Synchrotron Radiation Facility (ESRF) based on a high-pressure pair distribution function study (HP-PDF). High-quality PDFs require high-energy photons and access to large scattering angles to measure data to high values of momentum transfer (Q = 4π sin θ/λ) [START_REF] Keen | Local structures of amorphous and crystalline phases of silica, SiO(2), by neutron total scattering[END_REF][START_REF] Tucker | Application of the reverse Monte Carlo method to crystalline materials[END_REF][START_REF] Tucker | Refinement of the Si-O-Si bond angle distribution in vitreous silica[END_REF] . In addition, in order to obtain reliable HP-PDF, Bragg and Compton scattering from the diamond anvils have to be removed. These surprising experimental results were made possible by using a large area detector and by subtracting the scattered intensity of the 3 µm-spot size-61 keV beam due to helium from the a-silica signal at the same focal point 19 for each pressure point (Extended data, Fig. 1). The raw detector image data are shown in Extended data, Fig. 2, whilst reduced and corrected data at Patm and 8.8 GPa used for structural modelling are shown in Figure 1. The X-ray structure factor of a-silica obtained at Patm in the diamond cell up to Q = 22.5 Å -1 along with the structure factor calculated from an empirical potential structure refinement (EPSR) model 20,[START_REF] Bowron | An experimentally consistent atomistic structural model of silica glass[END_REF] are shown on Figure 1. Upon increasing pressure, the first sharp peak of the X-ray structure factor of a-silica decreases in intensity and broadens up to the highest pressure reached, Figure 1b (see also Extended Data Fig. 3). The comparison of the partial radial distribution functions obtained from the EPSR model with those simulated by molecular dynamics using the CHIK-potential [START_REF] Carre | New fitting scheme to obtain effective potential from Car-Parrinello molecular-dynamics simulations: Application to silica[END_REF] show very good agreement and are shown in Extended Data Fig. 4. This consistency between the two modelling approaches enhances our confidence that the local structure of a-silica derived by our atomistic structure refinement of the experimental scattering data is robust. In the EPSR structural model, the helium inserted values NHe P at each pressure point were fixed to those determined using a poromechanical model relying on atom-scale Grand Canonical Monte Carlo simulations [START_REF] Coasne | Poroelastic Theory Applied to the Adsorption-Induced Deformation of Vitreous Silica[END_REF] , see Extended data Fig. 5. The partial radial distribution functions for Si-Si, Si-O, O-He obtained from the EPSR model are shown on Figure 2ac, respectively. These data provide a means of understanding the structural modifications induced by the helium insertion at high pressure. The pressure induced changes consist of: i) a broadening of the Si-Si distance distribution (inter-tetrahedral distances) between the first and second coordination shells (Figure 2a), which additionally results in the appearance of a new contribution, observed above 3 Å, in the Si-O RDF (Figure 2b), and ii) a decrease in the O-He distance distribution with increasing pressure, which becomes sharper as a result of helium confinement (Figure 2c). Based on the EPSR model, access to bond angle distributions was possible and the Si-O-Si distributions are shown in Fig. 2d. This representation gives the probability of a given bond angle, P(θSi-O-Si), and is corrected for solid angle sampling, where the number of angles available at an angle θ is proportional to sin θ [START_REF] Kroon | Non-linearity of hydrogen bonds in molecular crystals[END_REF][START_REF] Baur | Straight si-o-si bridging bonds do exist in silicates and silicon dioxide polymorphs[END_REF] ; the estimation of the mean bond angle values can be obtained as the sum of these probabilities P(θSi-O-Si) is equal to 1, so the mean is simply the point where Σ(P(θSi-O-Si)) is 50% (Extended data Fig. 6). At Patm the Si-O-Si angle was found to have a mean value of 164° that is in agreement with literature reports from first principles calculations [START_REF] Umari | Concentration of Small Ring Structures in Vitreous Silica from a First-Principles Analysis of the Raman Spectrum[END_REF][START_REF] Giacomazzi | Medium-range structure of vitreous ${¥text{SiO}}_{2}$ obtained through first-principles investigation of vibrational spectra[END_REF] , PDF [START_REF] Tucker | Refinement of the Si-O-Si bond angle distribution in vitreous silica[END_REF][START_REF] Kohara | Intermediate-range order in vitreous SiO2and GeO2[END_REF] and NMR studies [START_REF] Mauri | Si-O-Si bond-angle distribution in vitreous silica from first-principles ${}^{29}¥mathrm{Si}$ NMR analysis[END_REF] . As already observed in Figure 2a with the Si-Si distance distribution, increasing pressure results in a change from a unito a bi-modal Si-O-Si bond angle distribution with the mean value decreasing from 164° to 161° at the highest pressure reached.

In order to understand the change in the local structure of a-silica due to helium insertion characterized by HP-PDF, it is particularly interesting to note the structural consequences of compression/gas insertion in microporous silica. In siliceous zeolites, compression behaviour is generally associated with changes in inter-tetrahedral bridging angles in the SiO2 framework resulting in a strong increase in pore ellipticity [START_REF] Thibaud | High-Pressure Phase Transition, Pore Collapse, and Amorphization in the Siliceous 1D Zeolite, TON[END_REF] ; such a mechanism can be blocked by gas insertion, which will fill pores and prevent pore collapse [START_REF] Colligan | Synchrotron X-ray powder diffraction and computational investigation of purely siliceous zeolite Y under pressure[END_REF][START_REF] Arletti | Pressure-induced penetration of guest molecules in high-silica zeolites: the case of mordenite[END_REF][START_REF] Thibaud | Saturation, of the Siliceous Zeolite TON with Neon at High Pressure[END_REF] . As previously mentioned, the local structure of a-silica is defined by corner-sharing SiO4 tetrahedra forming n-fold rings. During He-insertion, He-atoms (rHe=1.3 Å) are small enough: i) to be inserted under pressure in the most common six-membered rings [START_REF] Pasquarello | Identification of Raman defect lines as signatures of ring structures in vitreous silica[END_REF] (r6-fold ring =1.5 Å) and prevent collapse of the framework and ii) to induce a change from uni-to bi-modal distribution in the inter-tetrahedra distance, which could be considered as a local distortion of the ring as determined by this HP-PDF study. Compression behaviour of asilica in He therefore induces a splitting in the Si-Si distance distribution (Fig. 2a) associated with the appearance of a new contribution in the Si-O pair correlations at approximately 3 Å (Fig. 2b) and induces a bimodal Si-O-Si bond angle distribution (Fig. 2d). Figure 3a schematically represents the mechanism of He-insertion in a six-membered rings, in which insertion of He-atoms acts as an atomic-spring preventing ring collapse and inducing a local distortion under compression. Note that the He-insertion induced appearance of a local distortion could not be predicted in the poromechanical model as such a microscopic change in the local structure would require density functional theory calculations using a similar a-silica cell dimension as those of the models used here, which is not currently achievable [START_REF] Deringer | Origins of structural and electronic transitions in disordered silicon[END_REF] .

In order to characterize He-insertion in a-silica, which is found to exhibit an atomicspring effect as described above, Sato et al. previously reported microscopic observation of the system to estimate the equation of state of a-silica in a penetrating (He) and nonpenetrating medium (methanol-ethanol) [START_REF] Sato | Helium penetrates into silica glass and reduces its compressibility[END_REF] . Later, Weigel et al. [START_REF] Weigel | Vitreous Silica Distends in Helium Gas: Acoustic Versus Static Compressibilities[END_REF] used the Clausius-Mossotti relationship to estimate the helium inserted values NHe P based on the measurement of the refractive index in a-silica, studied by Brillouin spectroscopy 34 . Due to the relationship between refractive index, dielectric permittivity and complex impedance, spring glass properties of the a-silica-He composite can then be determined by impedance spectroscopy (Extended data Fig. 7). Figure 3b shows the pressure dependence of the relative impedance data (Z/Z0) obtained for a-silica in a penetrating (He), i.e. atomic-spring, and non-penetrating medium (gasoline F) up to 1.6 GPa during compression/decompression. It is interesting to note first that both data exhibit an equation of state-like profile with Z/Z0 values higher in He than those in gasoline. Data are perfectly reproducible under decompression and this is consistent with a reversible mechanism of He insertion/ removal along with the atomic-spring effect. Additionally, relative impedance values for a penetrating/non penetrating medium at the highest pressure reached, i.e. ~ 0.957/0.932, are in qualitative agreement with those reported for V/V0 (~ 0.975/0.95); notably, the impedance difference value is similar to that of the volume difference between the two sets of data in a penetrating/non penetrating medium (~2.5%). Impedance spectroscopy measurements probe an absence of change in the electron density of state in a-silica within this pressure-range, which depends therefore only on the volume; this is why one can understand such similarities between Z/Z0 and measurement are probably more accurate than those estimated by microscopic observation due to a greater sensitivity and resolution.

The present study has allowed us to determine the structural mechanism linked to Heinsertion in a-silica based on an EPSR model, which was only possible due to the ability of obtaining accurate X-ray structure factors up to Q = 22.5 Å -1 . As previously mentioned, from a poromechanical perspective, up to 0.5 mole of He can be inserted in the most common six-membered rings, both preventing the collapse of the ring due to atomicspring effect and inducing a local distortion in the isotropic glass matrix. Such a composite can easily be achieved and characterized as shown by the present example of impedance spectroscopy measurements.

We expect that the present finding will stimulate further investigation in order to use such an effect in applications in subnanoscale devices and sensors.

Fig. 1│a -

 1│a Fig. 1│a-silica-He composite structure factor SX(Q) (black). a, at Patm, b, at 8.8 GPa; red and blue lines show calculated EPSR model and the difference between calculated and experimental data, respectively.

Fig. 2│Pressure dependence

  Fig. 2│Pressure dependence of the partial radial distribution functions from EPSR model of a-silica-He composite. a, gSi-Si(r), b, gSi-Si(r), c, gO-He(r); arrows either show the appearance of a bi-modal distribution or of a new contribution, and d, Pressure dependence of Si-O-Si bond angle distribution functions.

Fig. 3│Atomic -

 3│Atomic Fig. 3│Atomic-spring effect in a-silica. Mechanism of He-insertion in an example of a six-membered rings obtained a, at Patm and at b, 8.8 GPa from PDF data, in which the presence of He-atoms acts as an atomic-spring preventing ring collapse and inducing uninomodal (𝑑 ̅ )-to-bimodal (𝑑 𝑃 ̅̅̅ + δ, 𝑑 𝑃 ̅̅̅ -δ) distribution of the inter-tetrahedral distance; red, blue and white spheres represent Si and O and He atoms respectively c, Pressure dependence of the relative impedance measurements (obtained at 2 MHz) on a penetrating medium (He) and on a non-penetrating medium (Gasoline F)

Acknowledgements

The authors thank Olivier Cambon for his help on impedance measurements as well as Patrick Hermet for data extraction, Benoit Rufflé and Marie Forêt for stimulating discussions, the beamline ID27 from the ESRF.

Methods

High-Pressure Pair Distribution Function Study

The a-silica sample consists of a 54 µm thick Suprasil F300 platelet containing less than 1ppm OH (Heraeus Quartzglass, Germany), which was measured as a function of pressure up to 8.8 GPa using a diamond anvil cell at room temperature using helium as pressure transmitting medium and ruby sphere to measure pressure calibration (ref), Extended data Fig. 1.

X-ray energy of 61keV and a Perkin-Elmer flat pannel large area detector was used on ID27 at the ESRF.

Accurate structure factors of the a-silica-He composite were obtained by subtracting the intensity of the 3 µm-spot size-61 KeV beam from air/Helium to the a-silica signal at the same focal points for each pressure data allowing reliable diffraction patterns to be obtained up to a maximum scattering of Q ~ 23 Å -1 , Extended data Fig . 2.

Empirical Potential Structure Refinement (EPSR)

EPSR uses a simple Lennard-Jones + Coulomb pair-wise potential to restrain the model, as well as the data. The L-J parameters were chosen to mimic the Van der Waals radii and were fixed at the same value for all pressures. The coulomb charges were +2 (Si), -1 (O) and 0 (He).

The comparison of the obtained partial radial distribution functions obtained from the EPSR model with those simulated by molecular dynamics using the CHIK-potential [START_REF] Keen | Local structures of amorphous and crystalline phases of silica, SiO(2), by neutron total scattering[END_REF] show a perfect agreement, Extended data Fig. 3, which definitely validates our strategy to characterize the local structure of a-silica.

In the EPSR structural model, the helium inserted values NHe P at each pressure points were fixed to those determined using a poromechanical model [START_REF] Tucker | Refinement of the Si-O-Si bond angle distribution in vitreous silica[END_REF] , Extended data Fig . 4.

High-Pressure Impedance Spectroscopy Study

The Clausius-Mossotti relationship for a-silica-He composite can be written as: Impedance measurements as a function of frequency 100 kHZ-3 MHz were performed up to 1.6 GPa on a-silica (Suprasil F300) disk of (dimensions) metallized with gold on both faces, Extended data Fig. 7, with a Rhode Schwartz impedance analyzer on the third stage of a Unipress gas compressor using either a penetrating (He), i.e. atomic-spring, and a non-penetrating medium (gasoline F). The pressure was measured with a manganin gauge.

Author contributions

Additional information

Supplementary information is available in the online version of the paper.

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