Ab initio study of phonons and structural stabilities of the perovskite-type MgSiO3

Using the local-density approximation, calculating the Hellmann-Feynman for ces, applying the direct method and deriving the phonon dispersion relation s, the stability of the perovskite-like structures of MgSiO3 at T = 0 have been studied. The cubic Pm (3) over bar m phase shows a dispersion-less sof t phonon branch spreading from the R to M points of the cubic Brillouin zon e. This soft branch persists up to high pressures of 150 GPa. The low-symme try phases I4/mcm and Imma, P4/mbm can be considered as a result of the sof t mode condensation at the hi and R points, respectively. These phases prov e to be unstable at T = 0. The experimentally observed Pmnb phase is a cons equence of the intersection of Imma and P4/mbm space groups. Thus, it can b e regarded as a simultaneous: condensation of two soft modes: one at the M and a second at the R high-symmetry points of the cubic Brillouin zone. The phonon dispersion relations of Pmnb show that this phase is stable and its optical phonons appear above 4.0 THz only.