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Local structures play a crucial role in the structural polyamorphism and novel electronic properties of amorphous materials, but their accurate measurement at high pressure remains a formidable challenge. In this article, we use the local structure of network-forming GeO2 glass as an example, to present our recent approaches and advances in high-energy x-ray diffraction, high-pressure x-ray absorption fine structure, and ab initio first-principles density functional theory calculations and simulations. Although GeO2 glass is one of the best studied materials in the field of high pressure research due to its importance in glass theory and geophysical significance, there are still some long-standing puzzles, such as the existence of appreciable distinct fivefold [5]Ge coordination at low pressure and the sixfold-plus [6+]Ge coordination at ultrahigh pressure. Our work sheds light on the origin of pressure-induced polyamorphism of GeO2 glass, and the [5]Ge polyhedral units may be the dominant species in the densification mechanism of network-forming glasses from tetrahedral to octahedral amorphous structures.

Pressure-induced phase transitions of monoclinic H-Nb2O5 have been studied by in situ synchrotron x-ray diffraction, pair distribution function (PDF) analysis, and Raman and optical transmission spectroscopy. The initial monoclinic phase is found to transform into an orthorhombic phase at ~9 GPa and then change to an amorphous form above 21.4 GPa. The PDF data reveal that the amorphization is associated with disruptions of the long-range order of the NbO6 octahedra and the NbO7 pentagonal bipyramids, whereas the local edge-shares of octahedra and the local linkages of pentagonal bipyramids are largely preserved in their nearest neighbors. Upon compression, the transmittance of the sample in a region from visible to near infrared (450-1000 nm) starts to increase above 8.0 GPa and displays a dramatic enhancement above 22.2 GPa, indicating that the amorphous form has a high transmittance. The pressure-induced amorphous form is found to be recoverable under pressure release, and maintain high optical transmittance property at ambient conditions. The recoverable pressure induced amorphous material promises for applications in multifunctional materials.

The compressibility of nanocrystalline gold (n-Au, 20 nm) has been studied by x-ray total scattering using high-energy monochromatic x-rays in the diamond anvil cell under quasi-hydrostatic conditions up to 71 GPa. The bulk modulus, K0, of the n-Au obtained from fitting to a Vinet equation of state is ~196(3) GPa, which is about 17% higher than for the corresponding bulk materials (K0: 167 GPa). At low pressures (

The structural behavior of GeO2 glass has been investigated up to 64 GPa using results from x-ray absorption spectroscopy in a diamond anvil cell combined with previously reported density measurements. The difference between the nearest Ge-O distances of glassy and rutile-type GeO2 disappears at the Ge-O distance maximum at 20 GPa, indicating completion of the tetrahedral-octahedral transition in GeO2 glass. The mean-square displacement σ(2) of the Ge-O distance in the first Ge-O shell increases progressively to a maximum at 10 GPa, followed by a substantial reduction at higher pressures. The octahedral glass is, as expected, less dense and has a higher compressibility than the corresponding crystalline phase, but the differences in Ge-O distance and density between the glass and the crystals are gradually eliminated over the 20-40 GPa pressure range. Above 40 GPa, GeO2 forms a dense octahedral glass with a compressibility similar to that of the corresponding crystalline phase (α-PbO2 type). The EXAFS and XANES spectra show evidence for subtle changes in the dense glass continuing to occur at these high pressures. The Ge-O bond distance shows little change between 45-64 GPa, and this may reflect a balance between bond shortening and a gradual coordination number increase with compression. The density of the glass is similar to that of the α-PbO2-type phase, but the Ge-O distance is longer and is close to that in the higher-coordination pyrite-type phase which is stable above ∼60 GPa. The density data provide evidence for a possible discontinuity and change in compressibility at 40-45 GPa, but there are no major changes in the corresponding EXAFS spectra. A pyrite-type local structural model for the glass can provide a reasonable fitting to the XAFS spectra at 64 GPa.