Phase stability of CaSiO3 perovskite at high pressure and temperature: Insights from ab initio molecular dynamics

We report the dynamics of the structure of CaSiO3 perovskite from ab initio molecular dynamics (AIMD) calculations at high pressure (P up to 130 GPa) and high temperature (T up to 5000 K). Our calculations indicate three separate stability fields: orthorhombic, tetragonal and cubic, with the tetragonal phase dominating the pressure and temperature region between room temperature and 4000 K. These regions are defined by the stress symmetry of the AIMD calculation. The boundary between the orthorhombic and the tetragonal structures is found to have a positive Clapyron slope and is close to room temperature at low pressure. The boundary is marked by the transition from stable, constant octahedral tilts, to dynamically varying tilts that change sign with time. The calculated atom positions indicate that the orientation of the octahedra can be noted as a−a−c+ in the orthorhombic phase (T = 150 K). The magnitude of the octahedra rotation varies little over the entire P–T range at high T (1000 K and above) while, at elevated temperature, the rotation angles of the octahedra oscillate positively and negatively with time. The tetragonal structure is probably due to a shortened SiO bond distance along one axis. Calculated X-ray diffraction patterns indicate small super-lattice reflections that could result from the octahedral rotations throughout the P, T region investigated. The small spontaneous strain of the tetragonal phase relative to the aristotype, cubic phase, throughout conditions appropriate to the lower mantle, creates the possibility for seismic energy absorption (low Q) in the deep Earth.