A Rietveld structural analysis of stishovite, with angle-dispersive x-ray diffraction synchrotron source at the European Synchrotron Radiation Facility, confirmed a CaCl2 form of stishovite distortion at 54 ± 1 gigapascals but confirmed no further phase transformation up to 120 gigapascals. The deviatoric stress that is usually encountered at such pressures was relaxed after yttrium-aluminum-garnet–laser heating. A single Birch-Murnaghan equation of state fits volumes of stishovite and a CaCl2form, showing that the tetragonal distortion occurs without a substantial change in volume. At the 54-gigapascal transition, the pressure-induced lattice modifications were similar to those found in a Landau-type temperature-induced transition. It is proposed that, above the transition pressure, the critical temperature increases above 300 kelvin, so that the lower entropy form becomes stable.

https://science.sciencemag.org/content/sci/282/5389/720.full.pdf

Pressure-Induced Landau-Type Transition in Stishovite

Following the discovery of stishovite (the highest-pressure polymorph of silica known from natural samples), many attempts have been made to investigate the possible existence of denser phases of silica at higher pressures. Based on the crystal structures observed in chemical analogues of silica1,2,3, high-pressure experiments on silica4,5,6,7,8,9,10,11 and theoretical studies12,13,14,15,16, several possible post-stishovite phases have been suggested. But the likely stable phase of silica at pressures and temperatures representative of Earth's lower mantle remains uncertain. Here we report the results of an X-ray diffraction study of silica that has been heated to temperatures above ∼2,000 K and maintained at pressures between 68 and 85 GPa. We observe the occurrence of a new high-pressure phase which we identify with the aid of first-principles total-energy calculations. The structure of this phase (space group Pnc2) is intermediate between the α-PbO2 and ZrO2 structures, and is denser than other known silica phases.

Experimental and theoretical identification of a new high-pressure phase of silica

$\ensuremath{\alpha}$-quartz, which has been reported to undergo pressure-induced amorphization, was found to transform to a monoclinic, crystalline phase when compressed to 45 GPa at room temperature in a close to hydrostatic, helium pressure medium. The x-ray powder diffraction data obtained could be indexed based on a monoclinic cell, and the intensities are in agreement with a ${P2}_{1}/c$ model structure built up of $3\ifmmode\times\else\texttimes\fi{}2$ zigzag chains of ${\mathrm{SiO}}_{6}$ octahedra. This new polymorph of silica, which is metastable under ambient conditions, has been isolated for the first time and is one of several possible competing dense forms containing octahedrally coordinated silicon.

Crystalline post-quartz phase in silica at high pressure.

A Landau free energy expansion has been developed to describe the elastic constant variations of stishovite (SiO2) associated with the P42/mnm (rutile-type) ⇌ Pnnm (CaCl2-type) phase transition as a function of pressure. The transition appears to display classical second-order character, with an equilibrium transition pressure which is renormalized by coupling of the soft optic mode with spontaneous strain. Lattice parameter data from the literature show that the symmetry breaking strain is large, while the nonsymmetry breaking strains are small. These have been used to constrain the numerical values of the strain/order parameter coupling coefficients, which in turn have been used to constrain values for the Landau coefficients. When substituted into expressions for all the individual elastic constants, taking values for experimental and calculated values of the bare elastic constants from the literature, a clear view of the pattern of elastic constant variations is produced. Variations of P and S wave velocities derived from the calculated elastic constants for a second-order transition show a dip as the transition point is approached from high pressure and from low pressure but do not show a discontinuity. The velocity anomaly provides a signature for the presence of free silica in the lower mantle.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/1999JB900419

High‐pressure elasticity of stishovite and the P42/mnm ⇌ Pnnm phase transition

Because of its simple composition, vast availability in pure form and ease of processing, vitreous silica is often used as a model to study the physics of amorphous solids. Research in amorphous silica is also motivated by its ubiquity in modern technology, a prominent example being as bulk material in transmissive and diffractive optics for high-power laser applications such as inertial confinement fusion (ICF)1,2. In these applications, stability under high-fluence laser irradiation is a key requirement3, with optical breakdown occurring when the fluence of the beam is higher than the laser-induced damage threshold (LIDT) of the material3. The optical strength of polished fused silica transmissive optics is limited by their surface LIDT3. Surface optical breakdown is accompanied by densification4, formation of point defects5, cratering, material ejection, melting and cracking3. Through a combination of electron diffraction and infrared reflectance measurements we show here that synthetic vitreous silica transforms partially into a defective form of the high-pressure stishovite phase under high-intensity (GW cm−2) laser irradiation. This phase transformation offers one suitable mechanism by which laser-induced damage grows catastrophically once initiated, thereby dramatically shortening the service lifetime of optics used for high-power photonics.

/pdf/laser-driven-formation-of-a-high-pressure-phase-in-amorphous-e5rin2mgl5.pdf

Laser-driven formation of a high-pressure phase in amorphous silica

α-Quartz was compressed at room temperature in a diamond-anvil cell without a medium to maximum pressures of 31 to 213 GPa and was studied by energy-dispersive synchrotron X-ray diffraction Broad peaks observed in a previous high-pressure diffraction study of silica glass are evident in the present study of quartz compression, providing in situ confirmation of pressure-induced amorphization above 21 GPa The 21-GPa crystalline-crystalline (quartz 1–11) transformation previously observed on quasihydrostatic compression of quartz is found to also occur under the current nonhydrostatic conditions, at the identical pressure With nonhydrostatic compression, however, new sharp diffraction lines are observed at this pressure The measurements show the coexistence of at least one amorphous and two crystalline phases above 21 GPa and below 43 GPa The two crystalline phases are identified as quartz II and a new, high-pressure silica phase The high-pressure phases, both crystalline and amorphous, can be

Synchrotron X-ray diffraction of SiO2 to multimegabar pressures

/pdf/high-pressure-behavior-of-silica-and-its-analogs-4amhwambux.pdf

High-pressure behavior of silica and its analogs

We have compressed phosphorus oxynitride (PON) in a diamond-anvil cell to 70 GPa. The PON has a distorted cristobalite structure and is isoelectronic with SiO 2 . In situ micro-Raman spectroscopy and synchrotron X ray diffraction show that PON cristobalite undergoes a phase transformation in the range of 12 to 20 GPa. Based on the similarity of X ray and Raman spectra before and after the transition, the high-pressure phase also has a cristobalite-type structure. Remarkably, this open-framework structure remains stable to the maximum experimental pressures. Although there is evidence for a reversal transformation of the high-pressure phase near 17 GPa upon decompression, the crystalline phase quenched to ambient conditions is not identical to the starting material. No amorphization of crystalline PON is observed during the in situ compression and decompression measurements. However, X ray investigation of the decompressed sample at ambient pressure reveals a mixture of amorphous and crystalline PON components.

Compression of PON Cristobalite to 70 GPa

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Synchrotron X-ray diffraction of SiO2 to multimegabar pressures

High-pressure behavior of silica and its analogs

Compression of PON Cristobalite to 70 GPa