Résumé
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.