Silica exists under a number of different forms, either crystalline or vitreous. The density of silica glass is about 5% lower than that of cristobalite and 20% lower than that of quartz, two crystals of the same chemical composition. Further, vitreous silica can be permanently densified under high pressure and high temperature by more than 20%, keeping an amorphous structure. What is the origin of these differences at the microscopic scale ? We will first review our knowledge of the structure of silica glass : the basic element of the structure is a SiO4 tetrahedron as in crystals, but the angles between tetrahedra are irregular. Then, results of experiments for silica glass under compression will be presented. Under pressures in the GPa range, the results depend drastically on the fluid used as a pressure-transmitting medium for the compression : while the volume of the silica sample changes dramatically in argon, only a very small decrease in size is observed under helium. This shows that helium penetrates inside silica, the open and flexible structure of the glass allowing gas atoms to distend the network, the same way a porous material deforms upon fluid adsorption. Furthermore, a fine quantitative analysis of the behavior of the Raman modes in function of pressure shows that changes in force constants between atoms and bond angles between tetrahedra in the network occur simultaneously.

Under a high pressure of helium, the volume change of amorphous silica is much smaller than expected from its elastic properties. This is due to helium insertion in the free volume of the glass network. Here, we report spectroscopic experiments using either helium or neon as penetrating pressurizing media and molecular simulation that indicate a relationship between the amount of gas adsorbed and the strain of the network. A generalized poromechanical approach, describing the elastic properties of microporous materials upon adsorption, is shown to successfully describe the physics of deformation of such silica glasses in which the free volume exists only at the sub-nanometer scale.

We report picosecond acoustic measurements of longitudinal sound dispersion and attenuation in an amorphous SiO2 layer at temperatures from 20 to 300 K over frequencies ranging from about 40 to 200 GHz. The sample is a radio frequency cathodic sputtered silica layer grown on a sapphire substrate with an aluminum filmtransducer deposited on top. Acoustic attenuation is evaluated from the simultaneous analysis of three successive echoes using transfer matrix calculation. Results are found to follow rather well a model combining coupling to thermally activated relaxations of structural defects and interactions with thermal vibrations. This leads to a nontrivial variation of the attenuation coefficient with frequency and temperature. The number density of relaxing defects in the SiO2 layer is found to be slightly higher than that in bulk v-SiO2. In contrast, similar anharmonic contribution to acoustic absorption is observed in both systems. The velocity variations are also measured and are compared to the dynamical velocity changes deduced from the sound attenuation.

High-pressure polarized Raman spectra of vitreous silica are measured up to 8 GPa in a diamond-anvil cell at room temperature. The combined use of either a non-penetrating pressurizing medium, argon, or a penetrating one, helium, allows to separate density from stress effects on the Raman frequencies. In the framework of a simple central force model, the results emphasize the distinct role played by the shrinkage of the inter-tetrahedral angle Si-O-Si and the force-constant stiffening during the compression. The polarization analysis further reveals the existence of an additional isotropic component in the high frequency wing of the Boson peak. The pressure dependence of the genuine Boson peak frequency is found to be much weaker than previously reported and even goes through a minimum around 2 GPa in a remarkable coincidence with the anomalous compressibility maximum of silica.

We investigate vitreous silica pressurized under the noble gases, He, Ne and Ar by both spectroscopic techniques and numerical simulations. We find an unexpectedhuge solubility of He and Ne at high pressure together with a large decrease of the static compressibility. We show that this relates to the open and flexible structure of the glass allowing gas atoms to distend the network, the same way a porous material deform upon fluid adsorption. This behavior can be rationalized using generalized poromechanical constitutive equations. Direct optical microscopy observations highlight the swelling of the silica network upon fluid adsorption. Very sharp diffusion front of the fluid into the silica the sample is further observed indicating non trivial kinetics of the diffusing atoms. In the Ne case, adsorption-desorption kinetics canbe tracked by Brillouin spectroscopy.