AB-INITIO STUDIES OF STRUCTURAL INSTABILITIES IN MAGNESIUM-SILICATE PEROVSKITE

Density-functional simulations are used to calculate structural proper ties and high-symmetry phonons of the hypothetical cubic phase, the st able orthorhombic phase and an intermediate tetragonal phase of magnes ium silicate perovskite. We show that the structure of the stable phas e is well described by freezing a small number of phonons into the cub ic phase. We use the frequencies of these modes to estimate transition temperatures for cubic --> tetragonal and tetragonal --> orthorhombic phase transitions. Unstable modes are investigated further to find th at the coupling with the strain suggests that phonons give a better re presentation than rigid unit modes. The phonons of an intermediate tet ragonal phase were found to be stable except for two rotational modes. The eigenvectors of the most unstable mode of each of the cubic and t etragonal phases account for all the positional parameters of the orth orhombic phase. The phase boundary for the orthorhombic-tetragonal tra nsition intersects possible mantle geotherms, suggesting that the tetr agonal phase may be present in the lower mantle.