Brillouin spectroscopy is used to investigate the elastic properties of XAlSiO4 aluminosilicate glasses where X =Li, Na, K, Mg0.5, Ca0.5, Sr0.5, Ba0.5, and Zn0.5. The Brillouin frequency shifts obtained in two different scattering geometries allow the calculation of the refractive index, the two sound velocities and Poisson's ratio.Measurements of the mass density give in turn the elastic moduli and the Debye temperature.We find that the elastic properties scale with the atomic density of the glassy network or the charge-balancing cation field strengthwhile they negatively correlate with the glass transition temperature. Further, Poisson's ratio depends on the nature of the non-framework cations in this glass series.

We present accurate measurement of the VV- and VH-polarized Raman spectra of vitreous SiO2 pressurized in the range 0 – 8 GPa in argon or helium atmosphere by means of diamond anvil cell. A fine quantitative analysis of the behavior of the Raman modes in function of pressure is achieved with the help of an old central-force model by Sen and Thorpe describing the dynamics of covalently bonded networks. The model predicts the effect of changes in force constants and bond angles on the vibrational frequencies of the coupled SiO4 tetrahedral network. We show that the pressure shift of the Raman bands cannot be understood without encompassing the variations of both parameters. Investigating two different pressurizing fluids including He which is adsorbed into the silica network, allows revealing the role of each parameter in the compression mechanism of the glass.

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.