Sound velocities of vitreous silica are measured under He compression in the pressure range of 0-6 GPa by Brillouin light scattering. It is found that the well-known anomalous maximum in the pressure dependence of the compressibility is suppressed by He incorporation into the silica network. This shows that the elastic anomaly relates to the collapse of the largest interstitial voids in the structure. The huge difference between the static and the acoustic compressibilities indicates that the amount of incorporated helium still increases at 6 GPa.

Sound velocities of vitreous silica are measured under He compression in the pressure range 0-6 GPa by Brillouin light scattering. It is found that the well-known anomalous maximum in the pressure dependence of the compressibility is suppressed by He incorporation into the silica network. This shows that the elastic anomaly relates to the collapse of the largest interstitial voids in the structure. The huge difference between the static and the acoustic compressibilities indicates that the amount of incorporated helium still increases at 6 GPa.

Sound velocities of vitreous silica are measured under He compression in the pressure range 0-6~GPa by Brillouin light scattering. It is found that the well-known anomalous maximum in the pressure dependence of the compressibility is suppressed by He incorporation into the silica network. This shows that the elastic anomaly relates to the collapse of the largest interstitial voids in the structure. The huge difference between the static and the acoustic compressibilities indicates that the amount of incorporated helium still increases at 6~GPa.

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[1,2] that helium can be "dissolved" into silica glass, strongly modifying its properties, whereas molecular hydrogen cannot [2]. 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[1,2]. It is well known that the thermomechanical properties of a-SiO2 exhibit anomalous behaviors. In Ref. 4, 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 shedding light on the microscopic origin of the thermomechanical anomalies of silica. [1] T. Sato, N. Funamori, T. Yagi, Nature Comm. 2, 245 (2011) [2] G. Shen, Q. Mei, V.B. Prakapenka, P Lazor, S. Sinogeikin, Y. Meng, C. Park, Proc. Nat. Acad. Sci. 108, 6004 (2011) [3] S. Rau, S. Baessler, G. Kasper, G. Weiss, S. Hunklinger, Ann. Physik 4, 91 (1995) [4] 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)

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)

New high-resolution Brillouin scattering experiments are performed under hydrostatic pressures ranging from 0 to 10 GPa in vitreous silica. The hypersound-attenuation coefficient exhibits a sharp maximum located at 2 GPa, which appears to coincide with the well-known minimum in sound velocity. Two main processes contribute to sound damping: the thermal relaxation of local defects and the anharmonic interactions with thermal vibrations. By analyzing the temperature dependence of the velocity and subtracting the changes produced by the above mechanisms, the unrelaxed velocity is found. It is constant at low temperatures and increases anomalously above a temperature onset. We show that the onset strongly depends on pressure, which is in agreement with a dynamical origin of the structural changes that produce the hardening. Reference: 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)

We have performed new Brillouin scattering measurements of the velocity and attenuation of hypersonic waves as a function of pressure P in vitreous silica [1]. The hypersoundattenuation coefficient exhibits a sharp maximum located at 2 GPa which appears to coincide with the well-known minimum in sound velocity. The results are analyzed together with previous literature data. Internal friction measurements are described satisfactorily using a model including dissipative effects of anharmonicity and thermally activated relaxation of structural defects. From this model, the associated velocity changes were calculated, and subtracted from the data to obtain the bare velocities (the so-called unrelaxed velocities) as a function of T at different P. v¥ (T) is almost constant at low T, then increases linearly above a temperature onset o T . We find evidence for an increase of o T with pressure P [1]. This is in agreement with numerical simulations in which the observed hardening is related to changes of the ring conformations with T [2,3]. [1] S. Ayrinhac, B. Rufflé, M. Foret, H. Tran, S. Clément, R. Vialla, R. Vacher, J.-C. Chervin, P. Munsch, A. Polian, Phys. Rev. B84, 024201 (2011). [2] L. Huang and J. Kieffer, Phys. Rev. B69, 224203 (2004). [3] Y. Liang, C.~R. Miranda, and S. Scandolo, Phys. Rev. B75, 024205 (2007).