Dense yttria phase eclipsing the A-type sesquioxide structure: high-pressure experiments and ab initio calculations.

In situ X-ray diffraction experiments and ab initio calculations elucidated the high-pressure phase transition properties of yttrium sesquioxides. The C-, B-, and A-type sesquioxides structure sequence observed in the room-temperature compression does not coincide with the high-pressure phase sequence of yttrium sesquioxides at high temperature. A reconstructive-type transformation taking place at high temperature yields the Gd(2)S(3) structure around 8 GPa with a drastic change in cation-oxygen coordinations. Ab initio structural optimization suggests that a displacive-type transformation from B- to A-type sesquioxides structure metastably occurs under pressure at room temperature. The calculated density of states indicates that the transition to the Gd(2)S(3) structure causes a significant decrease in the band gap. The Gd(2)S(3) phase was also found to be partially recovered at ambient pressure. We briefly discuss the quenchability of the Gd(2)S(3) structure in sesquioxides on the basis of the enthalpy differences between the ambient phase and the recovered products.

High-pressure phase transition to the Gd(2)S(3) structure in Sc(2)O(3): a new trend in dense structures in sesquioxides.

In situ X-ray diffraction experiments using a laser-heated diamond anvil cell revealed a novel dense phase with the Gd(2)S(3) structure stabilizing in Sc(2)O(3) at pressures over 19 GPa. Although no phase transformation was induced during room-temperature compression up to 31 GPa, the C rare earth sesquioxide structure transformed into the B rare earth sesquioxide structure at 10 GPa after laser annealing and subsequently into the Gd(2)S(3) structure at 19 GPa. Neither the A rare earth sesquioxide structure nor the U(2)S(3) structure was found in Sc(2)O(3). Static density functional lattice energy calculations demonstrated that the C structure prefers Gd(2)S(3) over U(2)S(3) as the post phase. Sc(2)O(3) is the second sesquioxide, after In(2)O(3), to crystallize into a Gd(2)S(3) structure at high pressures and high temperatures.

The electrical conductivity of post-perovskite in Earth's D'' layer.

Recent discovery of a phase transition from perovskite to post-perovskite suggests that the physical properties of Earth's lowermost mantle, called the D'' layer, may be different from those of the overlying mantle. We report that the electrical conductivity of (Mg0.9Fe0.1)SiO3 post-perovskite is >10(2) siemens per meter and does not vary greatly with temperature at the conditions of the D'' layer. A post-perovskite layer above the core-mantle boundary would, by electromagnetic coupling, enhance the exchange of angular momentum between the fluid core and the solid mantle, which can explain the observed changes in the length of a day on decadal time scales. Heterogeneity in the conductivity of the lowermost mantle is likely to depend on changes in chemistry of the boundary region, not fluctuations in temperature.

Equation of state of the postperovskite phase synthesized from a natural (Mg,Fe)SiO3 orthopyroxene.

Using the laser-heated diamond anvil cell, we investigate the stability and equation of state of the postperovskite (ppv, CaIrO(3)-type) phase synthesized from a natural pyroxene composition with 9 mol.% FeSiO(3). Our measured pressure-volume data from 12-106 GPa for the ppv phase yield a bulk modulus of 219(5) GPa and a zero-pressure volume of 164.9(6) A(3) when K'(0) = 4. The bulk modulus of ppv is 575(15) GPa at a pressure of 100 GPa. The transition pressure is lowered by the presence of Fe. Our x-ray diffraction data indicate the ppv phase can be formed at P > 109(4) GPa and 2,400(400) K, corresponding to approximately 400-550 km above the core-mantle boundary. Direct comparison of volumes of coexisting perovskite and CaIrO(3)-type phases at 80-106 GPa demonstrates that the ppv phase has a smaller volume than perovskite by 1.1(2)%. Using measured volumes together with the bulk modulus calculated from equation of state fits, we find that the bulk sound velocity decreases by 2.3(2.1)% across this transition at 120 GPa. Upon decompression without further heating, it was found that the ppv phase could still be observed at pressures as low at 12 GPa, and evidence for at least partial persistence to ambient conditions is also reported.

Quantitative Rietveld texture analysis of CaSiO(3) perovskite deformed in a diamond anvil cell.

The Rietveld method is used to extract quantitative texture information from a single synchrotron diffraction image of a CaSiO(3) perovskite sample deformed in axial compression in a diamond anvil cell. The image used for analysis was taken in radial geometry at 49 GPa and room temperature. We obtain a preferred orientation of {100} lattice planes oriented perpendicular to the compression direction and this is compatible with [Formula: see text] slip.

The pyrite-type high-pressure form of silica.

Silica (SiO2) exhibits extensive polymorphism at elevated pressures. X-ray diffraction measurements showed that a high-pressure form with a pyrite-type structure, denser than other known silica phases, is stable above 268 giga-pascals and 1800 kelvin. The silicon coordination number increases from 6 in the alpha-PbO2-type phase to 6+2 in the pyrite-type phase, leading to a large increase in density by about 5% at the phase transition.

Crystal structure of a high-pressure/high-temperature phase of alumina by in situ X-ray diffraction.

Alumina (alpha-Al(2)O(3)) has been widely used as a pressure calibrant in static high-pressure experiments and as a window material in dynamic shock-wave experiments; it is also a model material in ceramic science. So understanding its high-pressure stability and physical properties is crucial for interpreting such experimental data, and for testing theoretical calculations. Here we report an in situ X-ray diffraction study of alumina (doped with Cr(3+)) up to 136 GPa and 2,350 K. We observe a phase transformation that occurs above 96 GPa and at high temperatures. Rietveld full-profile refinements show that the high-pressure phase has the Rh(2)O(3) (II) (Pbcn) structure, consistent with theoretical predictions. This phase is structurally related to corundum, but the AlO(6) polyhedra are highly distorted, with the interatomic bond lengths ranging from 1.690 to 1.847 A at 113 GPa. Ruby luminescence spectra from Cr(3+) impurities within the quenched samples under ambient conditions show significant red shifts and broadening, consistent with the different local environments of chromium atoms in the high-pressure structure inferred from diffraction. Our results suggest that the ruby pressure scale needs to be re-examined in the high-pressure phase, and that shock-wave experiments using sapphire windows need to be re-evaluated.

Post-perovskite phase transition in MgSiO3.

In situ x-ray diffraction measurements of MgSiO3 were performed at high pressure and temperature similar to the conditions at Earth's core-mantle boundary. Results demonstrate that MgSiO3 perovskite transforms to a new high-pressure form with stacked SiO6-octahedral sheet structure above 125 gigapascals and 2500 kelvin (2700-kilometer depth near the base of the mantle) with an increase in density of 1.0 to 1.2%. The origin of the D" seismic discontinuity may be attributed to this post-perovskite phase transition. The new phase may have large elastic anisotropy and develop preferred orientation with platy crystal shape in the shear flow that can cause strong seismic anisotropy below the D" discontinuity.