ABINITIO STRUCTURAL AND THERMOELASTIC PROPERTIES OF ORTHORHOMBIC MGSIO3 PEROVSKITE

We present results of an ab initio study of the structural and thermoelastic properties of orthorhombic (Pbnm) MgSiO//3 perovskite as a function of temperature and pressure. Self-consistent free energies are computed using quasiharmonic lattice dynamics with interatomic potentials derived from an electron-gas formulation. At zero pressure the calculated bulk modulus and thermal expansion are in excellent agreement with the available data. We find that for high-temperature isotherms, a finite-strain decompression of the high-pressure equation of state of perovskite substantially overestimates its zero-pressure density and bulk modulus; hence, constraints on the chemical composition of the lower mantle, based on a comparison of the decompressed seismological properties with zero-pressure laboratory data may significantly overestimate the proportion of perovskite in the lower mantle.