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)

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).

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 T0 . We find evidence for an increase of T0 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).

The Mossbauer spectra of an albite glass and melt doped with Fe-57 (bulk composition NaFe0.04AlSi3O8) were measured in situ to 1100 degrees C. A specially designed furnace was used where a disk of the sample was enclosed in a capsule of graphite. The furnace was purged with argon to avoid oxidation of the graphite, and shields of boron nitride (BN) were used to protect both the gamma-ray source and the detector from thermal radiation. Room-temperature spectra show contributions from paramagnetic Fe2+, paramagnetic Fe3+, and magnetic Fe3+. The signal of Fe2+ disappears around 400 degrees C, and the signal of Fe3+ disappears at about 1100 degrees C. This behavior cannot be due to changes in the oxidation state of iron because in situ visible and near infrared spectra of the same sample show that Fe2+ persists to at least 1000 degrees C and the quenched samples do not show any change in oxidation state. Rather, the disappearance of Fe2+ and Fe3+ in the spectra is likely clue to the structural relaxation time around these cations becoming comparable to the Mossbauer timescale (lifetime of excited state approximate to 10(-8) s). We show that the temperature at which the Fe3+ contribution disappears corresponds to the expected glass transition temperature of a "ferrialbite" NaFeSi3O8 melt for a timescale of 10(-8) s. Our data therefore confirm that viscous flow is due to the rearrangement of the chemical bonds between oxygen anions and network-forming cations. Furthermore, our data show that Mossbauer spectroscopy can be used to study the speciation and relaxation time around iron in silicate melts even at temperatures above the macroscopic glass transition temperature as defined from viscosimetry and calorimetry.