Brillouin scattering from amorphous materials at high pressures

In situ high pressure and high temperature Raman studies of (1-x)SiO(2)xGeO(2) glasses

The structure of glasses in the binary system SiO(2)-GeO(2) has been studied by Raman spectroscopy. Our results are consistent with mixing of SiO(2) and GeO(2) tetrahedra. The changes induced by temperature and by pressure on the structure are monitored by in situ measurements on the same mixed glasses. Anomalous temperature dependences are observed not only for SiO(2) glass and GeO(2) glass but also for mixed glasses. Particular attention is focused on the pressure densification mechanism in mixed glasses. Via the pressure dependence of the width of the main Raman band, we show that the compression mechanism in mixed glasses is intermediate between that of the end members.

A new EXAFS investigation of local structural changes in amorphous and crystalline GeO(2) at high pressure

Structural transformations at high pressure in amorphous and quartz-like crystalline GeO(2) have been investigated by using a Paris-Edinburgh press coupled to EXAFS spectroscopy. From both the germanium absorption edge position and the Ge-O distance evolution, new detailed information has been obtained about the pressure behavior of the short range order. Crystalline GeO(2) undergoes a transformation from four- to six-fold coordination at about 8.5 GPa, but at least the whole 6-12 GPa pressure range should be considered as the transition region. On the other hand, amorphous GeO(2) is characterized by a much more gradual structural change and the full octahedral state is not reached at 13 GPa as commonly believed. Furthermore, no support to the recently claimed fully pentahedral intermediate state can be given. EXAFS signals of glassy GeO(2) beyond the first Ge-O shell qualitatively confirm the continuous breakdown of the intermediate range order up to 10 GPa.

Nature of the structural transformations in B2O3 glass under high pressure

We study high-pressure polyamorphism of B2O3 glass using x-ray diffraction up to 10 GPa in the 300-700 K temperature range, in situ volumetric measurements up to 9 GPa, and first-principles simulations. Under pressure, glass undergoes two-stage transformations including a gradual increase of the first B-O (O-B) coordination numbers above 5 GPa. The fraction of boron atoms in the fourfold-coordinated state at P<10 GPa is smaller than was assumed from inelastic x-ray scattering spectroscopy data, but is considerably larger than was previously suggested by the classical molecular dynamics simulations. The observed transformations under both compression and decompression are broad in hydrostatic conditions. On the basis of ab initio results, we also predict one more transformation to a superdense phase, in which B atoms are sixfold coordinated.

Density measurements of noncrystalline materials at high pressure with diamond anvil cell

We describe an x-ray absorption method for in situ density measurement of non-crystalline materials in the diamond anvil cell using a monochromatic synchrotron x-ray microbeam. Sample thickness, which is indispensable in the absorption method, can be determined precisely by extrapolating the thickness profile of the gasket obtained by x-ray absorption and diffraction measurements. Diamond deformation across the sample chamber becomes noticeable at high pressures above 10 GPa, which can be monitored with a precision better than 1%, as demonstrated by measurements on crystalline Ag. We have applied the developed method to measure densities of the classic network-forming GeO(2) glass in octahedral form at pressures up to 56 GPa. The fit to the pressure-volume data with the Birch-Murnaghan equation from 13 to 56 GPa gives parameters of V(0)=23.2+/-0.4 cm(3)mol and K=35.8+/-3.0 GPa, assuming that K(')=4. This method could be applicable for in situ determination of the density of liquids and other noncrystalline materials using a diamond anvil cell up to ultrahigh pressures.

Structure of B2O3 Glass at High Pressure: A 11B Solid-State NMR Study

We report spectroscopic evidence for the pressure-induced structural changes in B2O3 glass quenched from melts at pressures up to 6 GPa using solid-state NMR. While all borons are tri-coordinated at 1 atm, the fraction of tetra-coordinated boron increases with pressure, being about 5% and 27% in the B2O3 glass quenched from melts at 2 and 6 GPa, respectively. The fraction of boroxol ring species increases with pressure up to 2 GPa and apparently decreases with further compression up to 6 GPa. Two densification mechanisms are proposed to explain the variation of boron species with pressure.

Spectroscopic evidence of polymorphism in vitreous B2O3

Brillouin and Raman spectroscopy were performed on B2O3 glass compressed to 57 GPa at 273 K. Upon compression the sound velocities increase smoothly and the boroxol ring Raman mode vanishes by 11 GPa. Upon decompression the sound velocities follow a different path and at 3 GPa a discontinuity of 3 km/s in V(p) and 2 km/s in V(s) returns the velocities to the values seen on compression. After the transition, the boroxol ring Raman mode reappears. A second pressure cycle produces the same behavior, suggesting the 3 GPa transition occurs between vitreous polymorphs with different boron coordination.