We have studied in situ the kinetics of the amorphous-amorphous coordination transformation in ${\mathrm{GeO}}_{2}$ glass and of the initial stage of transformation in vitreous ${\mathrm{SiO}}_{2}$ up to 9 GPa by volume vs time measurements using the precise strain-gauge technique. The volume vs time dependence at all pressure points was found to obey the logarithmic law, which distinguishes the observed transformation kinetics from that of the ordinary transitions in crystals. The first evidence of anelastic behavior was found from the quasistatic equation of state at $P\ensuremath{\approx}6--7$ GPa for ${\mathrm{SiO}}_{2}$ glass and at 2--3 GPa for ${\mathrm{GeO}}_{2}$ glass, i.e., at lower pressures than previously believed.

Logarithmic Kinetics of the Amorphous-Amorphous Transformations in SiO 2 and GeO 2 Glasses under High Pressure

We outline the current state of experimental study and basic ideas for describing phase transitions in topologically disordered condensed matter, such as liquids and amorphous solids. Reviewing briefly the study of molten elementary substances under pressure, we pay primary attention to the results for liquid Se, S, and P and also to those substances that have not been represented in previous publications, mainly the liquid oxides B2O3 and GeO2. The experimental data reveal the possibility of rather sharp transformations in relatively simple liquids that are smoothed at high temperatures. Comparing the transitions in amorphous solids and in liquids, one should emphasize the metastable and non-ergodic nature of amorphous substances and the existence of static local atomic stresses fluctuating in thermally frozen amorphous networks. In particular, the kinetic study of amorphous–amorphous transformations (AATs) in SiO2 and GeO2 glasses and amorphous H2O ice under pressure highlights a number of anomalous features that distinguish the AATs from ordinary first-order transitions and from transformations in liquids. The recent in situ study of the volume changes in glassy silica a-SiO2 upon compression at high temperatures provides a new conclusion as regards the existence of two pressure-induced AATs in a-SiO2 with different microscopic mechanisms of structural rearrangements. We also perform the analysis of two possible kinetic scenarios for AATs, including sharp and diffuse transitions. The key relation determining the transformation scenario is the relationship between the radius of structural correlations in amorphous solid and the size of the critical nucleus of the growing disordered modification. The comparative analysis emphasizes the main difference between the transformations in liquids and amorphous solids that consists in the fact that the transitions in liquids are mainly determined by thermodynamic relationships, whereas the transitions in amorphous solids take place far away from equilibrium and are governed by the corresponding kinetics.

https://iopscience.iop.org/article/10.1088/0953-8984/15/36/301/pdf

High-pressure phase transformations in liquids and amorphous solids

Publisher Summary This chapter discusses the optical properties of semiconductors under pressure. The effect of a large hydrostatic pressure on semiconductor optical properties can be best illustrated with a simple experiment—namely, the optical inspection with a microscope of light transmitted through a piece of gallium arsenide (GaAs) as it is being pressurized in a diamond anvil cell (DAC). Absorption (transmittance) measurements often require high resolution and the coverage of a wide spectral range. These measurements are therefore done with a conventional scanning-type spectrometer. The chapter discusses the effects of high pressure on fundamental semiconductor band-structure parameters as they are largely determined by the magnitude of band gaps and the characteristics of the electronic states at valence- and conduction-band extrema.

Chapter 4 Optical Properties of Semiconductors under Pressure

We propose a new scheme to parameterize effective potentials that can be used to simulate atomic systems such as oxide glasses. As input data for the optimization, we use the radial distribution functions of the liquid and the vibrational density of state of the glass, both obtained from ab initio simulations, as well as experimental data on the pressure dependence of the density of the glass. For the case of silica, we find that this new scheme facilitates finding pair potentials that are significantly more accurate than the previous ones even if the functional form is the same, thus demonstrating that even simple two-body potentials can be superior to more complex three-body potentials. We have tested the new potential by calculating the pressure dependence of the elastic moduli and found a good agreement with the corresponding experimental data.

https://hal.archives-ouvertes.fr/hal-01806569/document

New optimization scheme to obtain interaction potentials for oxide glasses

Brillouin scattering allows the determination of acoustic velocities and adiabatic elastic moduli in matter. These data are crucial in many areas of science, such as fundamental physics, geosciences and technology, especially when measured as a function of the density (pressure). In this paper, we present a review of the work performed on Brillouin scattering under high pressure in diamond anvil cells, emphasizing the most recent results. Copyright © 2003 John Wiley & Sons, Ltd.

Brillouin scattering at high pressure: an overview

The photoluminescence spectrum and absorption spectrum as well as resonant Raman scattering tuned by pressure in heavily doped CdS have been studied. The pressure coefficients of shallow levels (${I}_{2}$ and ${I}_{1}$) and deep levels (${G}_{5}$ and R) are measured. A new optical-absorption method under pressure was developed and the shift of band gap with pressure of CdS in the direct-band-gap wurtzite phase and indirect-band-gap rock-salt phase was measured. From the experimental results of resonant Raman scattering tuned by pressure we conclude that at about 27 kbar CdS undergoes a complete phase transition from the wurtzite phase to the rock-salt phase. After the phase transition there is no two-phase mixture, and the material still has semiconductor characteristics.

Spectroscopic investigations of CdS at high pressure.

Brillouin and Raman Scattering Spectra in SiO2 and GeO2 glasses have been measured in a diamond anvil cell up to pressures of 14 GPa. The elastic properties and equation of state for each glass type were obtained from the Brillouin scattering measurements with respect to pressure. Both elastic constants and compressibility of SiO2 and GeO2 showed anomalous behavior with respect to pressure. This anomalous behavior is reconciled with a model based on the pressure dependent bending of the oxygen angles in both glass types. The Raman measurements corroborate the conclusions from the Brillouin scattering results, namely that the SiO2 and GeO2 bond angles are changing with pressure or the oxygen angle distribution is changed without bond breaking.

Thomas G. Bilodeau

Papers

Spectroscopic investigations of CdS at high pressure.

Brillouin and raman scattering from glasses under high pressure