The tetragonal to orthorhombic ferroelastic phase transition between rutile- and ${\text{CaCl}}_{2}$-type ${\text{SiO}}_{2}$ at high pressures is studied using first-principles calculations and the Landau free-energy expansion. The phase transition is systematically investigated in terms of characteristic phonon modes with ${\text{B}}_{1g}$ and ${\text{A}}_{g}$ symmetries, shear moduli, transverse-acoustic mode, rotation angle of the ${\text{SiO}}_{6}$ octahedra, spontaneous symmetry-breaking and volume strains, and enthalpy. The results show that these physical behaviors at the transition are well described using the Landau free-energy expansion parametrized by the first-principles calculations.

/pdf/first-principles-calculations-of-the-ferroelastic-transition-4b3qx4ec16.pdf

First-principles calculations of the ferroelastic transition between rutile-type and CaCl2-type SiO2 at high pressures

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

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

MgSiO3 perovskite has been assumed to be the dominant component of the Earth's lower mantle, although this phase alone cannot explain the discontinuity in seismic velocities observed 200–300 km above the core–mantle boundary (the D″ discontinuity) or the polarization anisotropy observed in the lowermost mantle1. Experimental and theoretical studies that have attempted to attribute these phenomena to a phase transition in the perovskite phase have tended to simply confirm the stability of the perovskite phase2,3,4,5,6. However, recent in situ X-ray diffraction measurements have revealed7 a transition to a ‘post-perovskite’ phase above 125 GPa and 2,500 K—conditions close to those at the D″ discontinuity. Here we show the results of first-principles calculations of the structure, stability and elasticity of both phases at zero temperature. We find that the post-perovskite phase becomes the stable phase above 98 GPa, and may be responsible for the observed seismic discontinuity and anisotropy in the lowermost mantle. Although our ground-state calculations of the unit cell do not include the effects of temperature and minor elements, they do provide a consistent explanation for a number of properties of the D″ layer.

The elasticity of the MgSiO3 post-perovskite phase in the Earth's lowermost 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

Determination of the shear-wave velocities for silicate perovskite and ferropericlase under the pressure and temperature conditions of the deep lower mantle indicates that perovskite constitutes much more of the lower mantle than predicted by the conventional mantle model and is consistent with the chondritic Earth model. Earth's upper mantle is apparently depleted of silicon when compared with meteorites, which are thought to represent the material from which the Earth formed. This 'missing silicon' problem has provoked intense debate: it suggests that the deficit might be balanced by silicon in the core, or that the upper mantle is not representative of the entire mantle and that the lower mantle is enriched in silicon. Murakami et al. provide evidence to support the latter case. They made laboratory measurements of sound-wave transmission through silicate perovskite and ferropericlase minerals at pressures and temperatures matching those of the lower mantle, and compared the resulting shear-wave values with seismic data that sample the lower mantle. They conclude that the mineralogical model that provides the best fit is one in which perovskite constitutes greater than 93% of the lower mantle. This suggests that the lower mantle is enriched in silicon in comparison with the upper mantle, consistent with the chondritic Earth model, and that there is limited mass transport between the upper and lower mantle. The determination of the chemical composition of Earth’s lower mantle is a long-standing challenge in earth science. Accurate knowledge of sound velocities in the lower-mantle minerals under relevant high-pressure, high-temperature conditions is essential in constraining the mineralogy and chemical composition using seismological observations1, but previous acoustic measurements were limited to a range of low pressures and temperatures. Here we determine the shear-wave velocities for silicate perovskite and ferropericlase under the pressure and temperature conditions of the deep lower mantle using Brillouin scattering spectroscopy2. The mineralogical model that provides the best fit to a global seismic velocity profile1 indicates that perovskite constitutes more than 93 per cent by volume of the lower mantle, which is a much higher proportion than that predicted by the conventional peridotitic mantle model. It suggests that the lower mantle is enriched in silicon relative to the upper mantle, which is consistent with the chondritic Earth model. Such chemical stratification implies layered-mantle convection with limited mass transport between the upper and the lower mantle.

/pdf/a-perovskitic-lower-mantle-inferred-from-high-pressure-high-26bvoix2cz.pdf

A perovskitic lower mantle inferred from high-pressure, high-temperature sound velocity data

[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

Motohiko Murakami

Papers

Post-Perovskite Phase Transition in MgSiO3

The elasticity of the MgSiO3 post-perovskite phase in the Earth's lowermost mantle.

Post-Perovskite Phase Transition

A perovskitic lower mantle inferred from high-pressure, high-temperature sound velocity data

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