Ab initio molecular dynamics study of CaSiO3 perovskite at P-T conditions of Earth's lower mantle

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, CaSiO3 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 Gruneisen parameter. We predict that the ground state of CaSiO3 perovskite is tetragonal (space group I4/mcm). At room temperature an orthorhombic structure (space group Imma) is possible, which explains puzzling experimental X-ray powder diffraction patterns. We consider the structure relation between the Imma and the I4/mcm structures and show that the Imma structure can be obtained by a counterintuitive symmetry-lowering transition on increasing temperature.