Pressure-induced structural transformation in radiation-amorphized zircon.

We study the response of a radiation-amorphized material to high pressure. We have used zircon ZrSiO4 amorphized by natural radiation over geologic times, and have measured its volume under high pressure, using the precise strain-gauge technique. On pressure increase, we observe apparent softening of the material, starting from 4 GPa. Using molecular dynamics simulation, we associate this softening with the amorphous-amorphous transformation accompanied by the increase of local coordination numbers. We observe permanent densification of the quenched sample and a nontrivial “pressure window” at high temperature. These features point to a new class of amorphous materials that show a response to pressure which is distinctly different from that of crystals

Logarithmic relaxation in radiation-amorphized zircon

We study relaxation under pressure of a radiation-amorphized material. The volume of zircon, ZrSiO4, amorphized by natural radiation over geologic times, was measured under high pressure using the precise strain-gauge technique. The volume shows logarithmic relaxation as a function of time. We compare the measured relaxation with the logarithmic relaxation of other different systems, and discuss the commonality of behavior in these systems in terms of local relaxation events that relax stress on a microscopic scale. We propose that logarithmic relaxation arises as the result of the elastic feed-forward interaction mechanism between local relaxation events

Poroelastic Theory Applied to the Adsorption-Induced Deformation of Amorphous Silica

International audienceUnder a high pressure of helium, the volume change of amorphous silica is much smaller than expected from its elastic properties. This is due to helium insertion in the free volume of the glass network. Here, we report spectroscopic experiments using either helium or neon as penetrating pressurizing media and molecular simulation that indicate a relationship between the amount of gas adsorbed and the strain of the network. A generalized poromechanical approach, describing the elastic properties of microporous materials upon adsorption, is shown to successfully describe the physics of deformation of such silica glasses in which the free volume exists only at the sub-nanometer scale

Phase transformations and the nature of the semiconductor-to-metal transition in bulk a-GaSb and a-(Ge₂)₁₋ₓ(GaSb)x semiconductors under high pressure

The pressure-induced transitions in bulk amorphous GaSb and Ge-GaSb solid solutions, prepared by a solid state amorphization of the high-pressure phases after decompression, were studied under pressure up to 9 GPa. According to x-ray diffraction, volume, and resistivity measurements the amorphous semiconductors reversibly transform to crystalline high-pressure metallic phases (GaSb II or solid solutions with GaSb II–like structure). For Ge-GaSb alloys the transition occurs abruptly in the range 4.5–6.5 GPa for various concentrations of the components. The a-GaSb samples gradually transform in a wide pressure range between 3.5 and 8.5 GPa. It is shown that such behavior is due to heterogeneity of microscopic structural characteristics of the network and to partial crystallization of zinc-blende GaSb. A semiconductor-to-metal transition in a-GaSb is observed at 3.5–4 GPa, and is driven by the percolation mechanism. Bulk moduli of amorphous compounds exhibit substantial softening above P∼1–2 GPa, which is accompanied by intensification of the irreversible resistivity relaxation with pressure. At room pressure the amorphous tetrahedral network of a-GaSb (B≊35 GPa) is more compressible than the crystalline lattice of GaSb (B≊55 GPa). Thermodynamics of the structural transformation in a-(Ge₂)₀․₂₇(GaSb)₀․₇₃ was studied by differential thermal analysis, and discussed in the framework of the nonequilibrium phase diagram of an amorphous solid.11 page(s

Nature of the Structural Transformations in B[sub 2]O[sub 3] 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