Ab initio molecular dynamics study of $\mathrm{Ca}\mathrm{Si}{\mathrm{O}}_{3}$ perovskite at $P\text{\ensuremath{-}}T$ conditions of Earth's lower mantle

Ab initio molecular dynamics study of $Ca Si O_{3}$ perovskite at $P − T$ conditions of Earth’s lower mantle

Donat J. Adams and Artem R. Oganov

Phys. Rev. B 73, 184106 – Published 4 May 2006

Abstract

First-principles molecular dynamics calculations were performed in order to investigate the structure and properties of what is thought to be the third most abundant phase in the Earth’s lower mantle, $Ca Si O_{3}$ perovskite. The commonly assumed cubic structure was found to be stable at high temperatures $(T > 1000 - 2000 K)$ and unstable at low temperatures at all pressures. For this structure we investigate the thermal equation of state and the Grüneisen parameter. We predict that the ground state of $Ca Si O_{3}$ perovskite is tetragonal (space group $I 4 ∕ m c m$ ). At room temperature an orthorhombic structure (space group $I m m a$ ) is possible, which explains puzzling experimental X-ray powder diffraction patterns. We consider the structure relation between the $I m m a$ and the $I 4 ∕ m c m$ structures and show that the $I m m a$ structure can be obtained by a counterintuitive symmetry-lowering transition on increasing temperature.