Below 10 GPa, hydrostatic pressures P are usually obtained in diamond-anvil cell with soft pressure transmitting medium such as methanol-ethanol mixture. When such experiments are performed in this P range for v-SiO2, the volumetric strain of the sample agrees with that expected from its bulk modulus, . It was revealed recently that v-SiO2 submitted to high pressures under He atmosphere exhibits surprisingly small relative volume changes leading to an apparent B greater than hundred GPa at ambient P, much higher than the well-known modulus GPa of silica. Using Brillouin light scattering we evidence that the elastic moduli are only weakly affected by the presence of He. The difference between the static and the acoustic measurements reveals that silica pressurized under He behaves as an open system allowing He to penetrate into the interstitial free volume of the network. In that case, the static measurement underestimates the compressibility by a term due to the gas charging the sample. The large difference found here shows that He continues penetrating v-SiO2 up to our highest investigated P, i.e. 6 GPa. Such a strong effect of the pressurizing medium on the static compressibility is usually observed in materials with open porosity, such as zeolites. It results from pore filling by molecules of the pressurizing fluid. An estimate of the amount of He entering v-SiO2 is derived from the measured refractive index, leading to the unexpected value of more than 1 mole of He per mole of SiO2 at 6 GPa [1]. Finally we present molecular simulations relating the amount of fluid adsorbed to the volumetric strain of the network. We show that a generalized poromechanical approach can be applied to silica glass in which the free volume exists only at the sub-nanometric length scale. In that picture, the small apparent compressibility of silica under high He or Ne pressure results from an adsorption-induced expansion [2].[1] C. Weigel, A. Polian, M. Kint, B. Rufflé, M. Foret, R. Vacher,, “Vitreous Silica Distends in Helium Gas: Acoustic Versus Static Compressibilies” Phys. Rev. Lett., 109, 245504 (2012).[2] B. Coasne, C. Weigel, A. Polian, M. Kint, J. Rouquette, J. Haines, M. Foret, R. Vacher, B. Rufflé, “The Startling Poroelastic Behavior of Vitreous Silica” submitted (2014).

When vitreous silica is submitted to high pressures under helium atmosphere, the change in volume observed is much smaller than expected from its elastic properties. 1 It results from helium penetration into the interstitial free volume of the glass network. We present here the results of concurrent spectroscopic experiments using either helium or neon and molecular simulations relating the amount of gas adsorbed to the strain of the network. We show that a generalized poromechanical approach, describing the elastic properties of microporous materials upon adsorption, can be applied success-fully to silica glass in which the free volume exists only at the sub-nanometer scale. In that picture, the adsorption-induced deformation accounts for the small apparent compressibility of silica observed in experiments.

Vitreous silica v-SiO2 has been extensively studied over large pressure (P) and temperature (T) ranges. Several anomalous physical properties were discovered. In particular, the compressibility $\chi$ does not show the P and T dependencies observed in most solids. Several models, supported by numerical simulations, have been proposed which rely on the intertetrahedral flexibility of the silica network allowing strong structural modifications with T and P. Recent publications revealed that v-SiO2 submitted to high P in a He atmosphere exhibits a surprisingly small change in volume. This was interpreted as a considerable reduction of compressibility, leading to an apparent bulk modulus B around 110 GPa near ambient P. However, another value of B = 36.5 GPa can be calculated from the longitudinal (vL) and transverse (vT) sound velocities and the density, all known at ambient P. Here we show that this large difference in compressibilities must relate to the open structure of v-SiO2 allowing He to penetrate the network in the static limit. In an open system, the static measurement underestimates the compressibility by a term due to the gas charging. We will present volume changes measurements and Brillouin light-scattering (BLS) measurements of the sound velocity for the longitudinal and transverse acoustic modes as a function of P up to 6 GPa in He atmosphere. It is found that the anomalous B minimum in the acoustic bulk modulus is to a large extent hindered by the penetration of helium atoms into the silica network. The acoustic bulk modulus is found almost less than half the static one demonstrating that He continues penetrating v-SiO2 up to our highest investigated P. An estimate of the amount of He entering v-SiO2 is derived from the measured refractive index, leading to the unexpected value of more than 1 mole of He per mole of SiO2 at 6 GPa.