Résumé:

Due to the loose structure of amorphous SiO2 presenting with many voids, small atomic species have the possibility to penetrate in this compound. It has been recently shown[1] that helium can be "dissolved" into silica glass, strongly modifying its properties, whereas molecular hydrogen cannot. For example its compressibility is strongly reduced when using helium as a pressure transmitting medium in comparison with that measured with the standard methanol-ethanol mixture[1]. It is well known that a-SiO2 presents a sound velocity pressure dependence anomaly with a deep minimum situated around 2 GPa coincident with a maximum in the hypersound attenuation[2, 3]. In Ref. 3, it was shown that the anomalous hardening is of dynamical origin and therefore that this coincidence is not intrinsic to the material. In the present study, using high resolution Brillouin scattering (confocal Fabry-Pérot) as well as standard Sandercock type tandem Fabry-Pérot interferometer, we measured the longitudinal and transverse sound velocities, and the sound attenuation in a-SiO2 in a diamond anvil cell using helium as a pressure transmitting medium up to 6 GPa, and compare the obtained results with those obtained using methanol-ethanol mixture. We show that the sound velocity minimum is shifted toward low pressure, the minimum depth being strongly reduced, whereas the maximum of the attenuation is shifted toward higher pressure. These results will be discussed in the same framework as Ref. 3. On the other hand, it is well known that the thermomechanical properties of a-SiO2 exhibit anomalous behaviors. In Ref. 3, it was shown that the onset of the anomalous temperature hardening of the elastic modulus strongly depends on pressure suggesting a dynamical origin of the structural changes that produce the hardening. Here we study the anomalous softening of the elastic modulus in function of pressure. We have measured the longitudinal and transverse acoustic modes by Brillouin scattering in the range 0 – 6 GPa, using a diamond anvil cell with helium as a pressure transmitting medium. We found that due to the penetration of helium into silica, the anomalous elastic behavior disappears almost totally. This should allow to shed light on the microscopic origin of the thermomechanical anomalies of silica. [1]. T. Sato, N. Funamori, T. Yagi, Nature Comm. 2, 245 (2011) [2]. S. Rau, S. Baessler, G. Kasper, G. Weiss, S. Hunklinger, Ann. Physik 4, 91 (1995) [3]. S. Ayrinhac, B. Rufflé, M. Foret, H. Tran, S. Clément, R. Vialla, R. Vacher, J. C. Chervin, P. Munsch, A. Polian, Phys. Rev. B 84, 024201 (2011)