Phase Transition and Thermal Expansion of MgSiO 3 Perovskite

First-Principles Thermodynamics of CsSnI3

CsSnI₃ is a promising ecofriendly solution for energy harvesting technologies. It exists at room temperature in either a black perovskite polymorph or a yellow 1D double-chain, which irreversibly deteriorates in the air. In this work, we unveil the relative thermodynamic stability between the two structures with a first-principles sampling of the CsSnI₃ finite-temperature phase diagram, discovering how it is driven by anomalously large quantum and anharmonic ionic fluctuations. Thanks to a comprehensive treatment of anharmonicity, the simulations deliver a remarkable agreement with known experimental data for the transition temperatures of the orthorhombic, rhombohedral, and cubic perovskite structures and the thermal expansion coefficient. We disclose how the perovskite polymorphs are the ground state above 270 K and discover an abnormal decrease in heat capacity upon heating in the cubic black perovskite. Our results also significantly downplay the Cs⁺ rattling modes' contribution to mechanical instability. The remarkable agreement with experiments validates our methodology, which can be systematically applied to all metal halides.

Incorporation mechanism of Fe and Al into bridgmanite in a subducting mid-ocean ridge basalt and its crystal chemistry

The compositional difference between subducting slabs and their surrounding lower-mantle can yield the difference in incorporation mechanism of Fe and Al into bridgmanite between both regions, which should cause heterogeneity in physical properties and rheology of the lower mantle. However, the precise cation-distribution has not been examined in bridgmanites with Fe- and Al-contents expected in a mid-ocean ridge basalt component of subducting slabs. Here we report on Mg_0.662Fe_0.338Si_0.662Al_0.338O₃ bridgmanite single-crystal characterized by a combination of single-crystal X-ray diffraction, synchrotron ⁵⁷Fe-Mössbauer spectroscopy and electron probe microanalysis. We find that the charge-coupled substitution ^AMg²⁺  + ^BSi⁴⁺  ↔ ^AFe³⁺(high-spin) + ^BAl³⁺ is predominant in the incorporation of Fe and Al into the practically eightfold-coordinated A-site and the sixfold-coordinated B-site in bridgmanite structure. The incorporation of both cations via this substitution enhances the structural distortion due to the tilting of BO₆ octahedra, yielding the unusual expansion of mean <A–O> bond-length due to flexibility of A–O bonds for the structural distortion, in contrast to mean <B–O> bond-length depending reasonably on the ionic radius effect. Moreover, we imply the phase-transition behavior and the elasticity of bridgmanite in slabs subducting into deeper parts of the lower mantle, in terms of the relative compressibility of AO₁₂ (practically AO₈) and BO₆ polyhedra.

Mineral phases of the Earth´s mantle

Our knowledge of the structure of the Earth´s interior has been obtained by analysing seismic waves that travel in the Earth, and the reference Earth global models used by geophysicists are essentially seismological. Depth profiles of the seismic waves velocities reveal that the deep Earth is divided in several shells, separated by velocity and density discontinuities. The main discontinuity located at a depth of 2900 km corresponds to the transition between the mantle and the core. The Earth´s mantle can be further divided into the upper mantle and the lower mantle, with a transition zone characterised by two prominent increases in velocities observed at 410- and 660-km depths. This article will be focused on the mineral phases of the Earth´s mantle. The interpretation of seismological models in terms of chemical composition and temperature relies on the knowledge of the nature, structure and elastic properties of the candidate materials. We will describe to what extent recent advances in experimental mineral physics and X-ray diffraction have yielded essential knowledge on the structure and high-pressure high-temperature behaviour of pertinent materials, and major improvements in our understanding of the chemical and mineralogical composition of the Earth´s mantle.

White beam synchrotron X-ray topography studies of twinning in GdFeO3-type perovskite crystals

The twin structure of NdGaO3 and YAlO3 : Dy crystals was examined using white beam synchrotron X-ray topography. The relation of the orthorhombic lattice basis vectors has been obtained for every domain pair in the crystals with GdFeO3-type structure. A new computer program has been used to generate Laue simulation patterns and to calculate the image shifts in topographs. The proposed approach allow for the analysis of both, sample images for specified reflections and Laue patterns as a whole. Topography was used to map domain patterns, to show misorientations and to detect symmetric aspects of ferroelastic twins. Information on twin laws and the twin distribution was obtained simultaneously.

IN SITU STUDIES OF THE PROPERTIES OF MATERIALS UNDER HIGH-PRESSURE AND TEMPERATURE CONDITIONS USING MULTI-ANVIL APPARATUS AND SYNCHROTRON X-RAYS

▪ Abstract  Increased access to multi-anvil high-pressure devices interfaced to synchrotron X-ray radiation sources has led to a new class of experiments. These new capabilities include (a) high-precision crystal structure determination and refinement from powder X-ray diffraction data; (b) the determination of kinetic parameters and structure from time-resolved diffraction data; (c) the determination of absolute pressures by the combined use of ultrasonic techniques at high pressures and temperatures with simultaneous monitoring of X-ray diffraction; and (d) the determination of the strength and rheological properties of materials through the monitoring of the relaxation of broadened diffraction peak widths in the presence of a well-characterized deviatoric stress field generated in the multi-anvil high-pressure apparatus.

Oxygen Diffusion in Perovskite: Implications for Electrical Conductivity in the Lower Mantle

Experimental determination of oxygen self-diffusion in CaTiO(3) perovskite, a structural analog of (Mg,Fe)SiO(3) perovskite, confirms a theoretical relation between diffusion constants and anion porosity. Oxygen diffusion rates in (Mg,Fe)SiO(3) perovskite calculated with this relation increase by about eight orders of magnitude through the lower mantle. Electrical conductivity values calculated from these diffusion rates are consistent with inferred conductivity values for the lower mantle. This result suggests that the dominant conductivity mechanism in the deep mantle is ionic.

Comparative Compressibilities of Silicate Spinels: Anomalous Behavior of (Mg,Fe)2SiO4

Compressibilities of five silicate spinels, including gamma-Mg(2)SiO(4), gamma-Fe(2)SiO(4), Ni(2)SiO(4) and two ferromagnesian compositions, were determined on crystals positioned in the same high-pressure mount. Subjection of all crystals simultaneously to the same pressure revealed differences in compressibility that resulted from compositional differences. Ferromagnesian silicate spinels showed an anomalous 13 percent increase in bulk modulus with increasing iron content, from Mg(2)SiO(4) (184 gigapascals) to Fe(2)SiO(4) (207 gigapascals). This result suggests that ferrous iron and magnesium, which behave similarly under crustal conditions, are chemically more distinct at high pressures characteristic of the transition zone and lower mantle.

Thermoelasticity of Silicate Perovskite and Magnesiowüstite and Stratification of the Earth's Mantle

Analyses of x-ray-diffraction measurements on (Mg,Fe)SiO(3) perovskite and (Mg,Fe)O magnesiowüstite at simultaneous high temperature and pressure are used to determine pressure-volume-temperature equations of state and thermoelastic properties of these lower mantle minerals. Detailed comparison with the seismically observed density and bulk sound velocity profiles of the lower mantle does not support models of this region that assume compositions identical to that of the upper mantle. The data are consistent with lower mantle compositions consisting of nearly pure perovskite (>85 percent), which would indicate that the Earth's mantle is compositionally, and by implication, dynamically stratified.