We have developed an efficient and reliable methodology for crystal structure prediction, merging ab initio total-energy calculations and a specifically devised evolutionary algorithm. This method allows one to predict the most stable crystal structure and a number of low-energy metastable structures for a given compound at any P-T conditions without requiring any experimental input. Extremely high (nearly 100%) success rate has been observed in a few tens of tests done so far, including ionic, covalent, metallic, and molecular structures with up to 40 atoms in the unit cell. We have been able to resolve some important problems in high-pressure crystallography and report a number of new high-pressure crystal structures (stable phases: epsilon-oxygen, new phase of sulphur, new metastable phases of carbon, sulphur and nitrogen, stable and metastable phases of CaCO3). Physical reasons for the success of this methodology are discussed.

Crystal structure prediction using ab initio evolutionary techniques: principles and applications.

We have developed an efficient and reliable methodology for crystal structure prediction, merging ab initio total-energy calculations and a specifically devised evolutionary algorithm. This method allows one to predict the most stable crystal structure and a number of low-energy metastable structures for a given compound at any P-T conditions without requiring any experimental input. Extremely high success rate has been observed in a few tens of tests done so far, including ionic, covalent, metallic, and molecular structures with up to 40 atoms in the unit cell. We have been able to resolve some important problems in high-pressure crystallography and report a number of new high-pressure crystal structures. Physical reasons for the success of this methodology are discussed.

Crystal structure prediction using ab initio evolutionary techniques: principles and applications

Gallium oxide (Ga2O3) is emerging as a viable candidate for certain classes of power electronics, solar blind UV photodetectors, solar cells, and sensors with capabilities beyond existing technologies due to its large bandgap. It is usually reported that there are five different polymorphs of Ga2O3, namely, the monoclinic (β-Ga2O3), rhombohedral (α), defective spinel (γ), cubic (δ), or orthorhombic (e) structures. Of these, the β-polymorph is the stable form under normal conditions and has been the most widely studied and utilized. Since melt growth techniques can be used to grow bulk crystals of β-GaO3, the cost of producing larger area, uniform substrates is potentially lower compared to the vapor growth techniques used to manufacture bulk crystals of GaN and SiC. The performance of technologically important high voltage rectifiers and enhancement-mode Metal-Oxide Field Effect Transistors benefit from the larger critical electric field of β-Ga2O3 relative to either SiC or GaN. However, the absence of clear demonstrations of p-type doping in Ga2O3, which may be a fundamental issue resulting from the band structure, makes it very difficult to simultaneously achieve low turn-on voltages and ultra-high breakdown. The purpose of this review is to summarize recent advances in the growth, processing, and device performance of the most widely studied polymorph, β-Ga2O3. The role of defects and impurities on the transport and optical properties of bulk, epitaxial, and nanostructures material, the difficulty in p-type doping, and the development of processing techniques like etching, contact formation, dielectrics for gate formation, and passivation are discussed. Areas where continued development is needed to fully exploit the properties of Ga2O3 are identified.

A review of Ga2O3 materials, processing, and devices

USPEX—Evolutionary crystal structure prediction

Oxygen thermobarometry measurements on spinel peridotite rocks indicate that the oxygen fugacity at the top of the upper mantle falls within ±2 log units of the fayalite-magnetite-quartz (FMQ) oxygen buffer. Measurements on garnet peridotites from cratonic lithosphere reveal a general decrease in fo2 with depth, which appears to result principally from the effect of pressure on the controlling Fe3+/Fe2+ equilibria. Modeling of experimental data indicates that at approximately 8 GPa, mantle fo2 will be 5 log units below FMQ and at a level where Ni-Fe metal becomes stable. Fe-Ni alloy and an Fe2O3-garnet component will be formed as a result of the disproportionation of FeO, which is experimentally demonstrated through observations of high Fe3+/ΣFe ratios in minerals in equilibrium with metallic Fe. In the lower mantle, the favorable coupled substitution of Al and Fe3+ into (Fe,Mg)SiO3 perovskite results in very high perovskite Fe3+/ΣFe ratios in equilibrium with metallic Fe. As a result, the lower mantle sh...

The Redox State of Earth's Mantle

In situ x-ray diffraction measurements of MgSiO3 were performed at high pressure and temperature similar to the conditions at Earth9s 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.

/pdf/post-perovskite-phase-transition-in-mgsio3-376siybbwr.pdf

Post-Perovskite Phase Transition in MgSiO3

Phase transition and density of subducted MORB crust in the lower mantle

In situ x-ray diffraction measurements of MgSiO 3 were performed at high pressure and temperature similar to the conditions at Earth’s core-mantle boundary. Results demonstrate that MgSiO 3 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.

https://www.ipgp.jussieu.fr/~fournier/Enseignement/tue42g/Projet/murakami_etal2004.pdf

Post-Perovskite Phase Transition

[1] Phase relations of a natural mantle composition were determined up to 126 GPa and 2450 K by in-situ x-ray diffraction measurements in a laser-heated diamond-anvil cell (LHDAC). MgSiO3-rich perovskite (MgPv) transforms to a post-perovskite phase (MgPP) at about 113 GPa and 2500 K (400-km above the core-mantle boundary) and the lowermost mantle consists of MgPP, (Mg, Fe)O magnesiowustite (Mw), and CaSiO3-rich perovskite (CaPv). Chemical analyses on recovered samples using transmission electron microscope (TEM) show that the distribution of iron significantly changes at the post-perovskite phase transition. A strong enrichment of iron in Mw leads to the unique geophysical and geochemical properties of the lowermost mantle.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2004GL021956

Post‐perovskite phase transition and mineral chemistry in the pyrolitic lowermost mantle

Angle dispersive powder X-ray diffraction experiments using a flat imaging plate (IP) are one of the most popular methods in high-pressure material science. In order to support such experiments, we developed two software, IPAnalyzer and PDIndexer. IPAnalyzer can convert a two-dimensional Debye-ring pattern to one-dimensional (Bragg-Brentano) geometry. IPAnalyzer can also calibrate experimental parameters (wave length, camera length, and so on) automatically. PDIndexer can display the converted pattern(s) and diffraction peaks calculated for any crystals.

Nagayoshi Sata

Papers

Post-Perovskite Phase Transition in MgSiO3

Phase transition and density of subducted MORB crust in the lower mantle

Post-Perovskite Phase Transition

Post‐perovskite phase transition and mineral chemistry in the pyrolitic lowermost mantle

Development of a Software Suite on X-ray Diffraction Experiments