Prediction of Phase Transition in CaSiO$_3$ Perovskite and Implications for Lower Mantle Structure

TL;DR

This study uses first principles calculations to show that CaSiO3 perovskite in the Earth's lower mantle is likely distorted rather than cubic, with a phase transition at high temperatures that may explain seismic reflections.

Contribution

It predicts a distorted ground state structure for CaSiO3 perovskite and identifies a temperature-induced phase transition relevant to lower mantle properties.

Findings

Cubic structure is dynamically unstable at all pressures.

CaSiO3 perovskite has a lower symmetry distorted phase.

A phase transition occurs at high temperature within the lower mantle.

Abstract

First principles linear response calculations are used to investigate the lattice dynamics of what is thought to be the third most abundant phase in the lower mantle, CaSiO_3 perovskite. The commonly assumed cubic structure (Pm3m) is found to be dynamically unstable at all pressures, exhibiting unstable modes along the Brillouin zone edge from the M-point to the R-point. Based on these results, we predict that the ground state structure of CaSiO_3 perovskite is a distorted phase with lower than cubic symmetry. We predict that a phase transition occurs in CaSiO_3 perovskite within the earth's lower mantle from the low temperature distorted phase to the cubic phase at high temperature. The predicted phase transition provides a possible explanation of some of the seismological observations of reflective features within the lower mantle.