Abstract
We present an in situ study of vitreous silica v-SiO2 under hydrostatic noble gases pressure by Brillouin and Raman spectroscopy. Samples are pressurized in helium, neon and argon in a diamond anvil cell, in the range 0 - 8 GPa. Helium and neon atoms penetrate in the pressurized glass structure. We estimate that about one atom of helium per SiO4 tetrahedron and about half in the neon case are adsorbed at 6 GPa. Bulk and shear moduli as a function of fluid pressure are obtained from measurements of the longitudinal and transverse acoustic velocities. The behaviour anomaly of the bulk modulus of v-SiO2 (minimum at 2 GPa) is suppressed by He and Ne adsorption, structural rearrangements associated with this anomaly being prevented by the occupation of interstitial sites by helium and neon atoms. In the presence of helium and neon, the V(P) equation of state does not allow to recover bulk modulus because silica behaves like an open porous medium. Monte-Carlo simulation of adsorption isotherms combined with the generalized theory of poromechanics allows to describe volume deformations and adsorbed fluid amount in agreement with experimental results. In the neon case, adsorption-desorption kinetics is observed by Brillouin spectroscopy. Measurements of VV and VH Raman spectra of v-SiO2 are made as function of fluid pressure. The reduction of Si–O–Si angles distribution is prevented by the insertion of helium.