Eur. Phys. J. B 16, 49-58 (2000)
https://doi.org/10.1007/s100510070248

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

¹ Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Sendai 980-8577, Japan
² Institute of Nuclear Physics, ul. Radzikowskiego 152, 31-342 Cracow, Poland

Corresponding author: ^a krzysztof.parlinski@ifj.edu.pl

Received: 15 October 1999
Revised: 14 January 2000
Published online: 15 July 2000

Abstract

Using the local-density approximation, calculating the Hellmann-Feynman forces, applying the direct method and deriving the phonon dispersion relations, the stability of the perovskite-like structures of MgSiO₃ at T=0 have been studied. The cubic Pmm phase shows a dispersion-less soft phonon branch spreading from the R to M points of the cubic Brillouin zone. This soft branch persists up to high pressures of 150 GPa. The low-symmetry phases I4/mcm and Imma, P4/mbm can be considered as a result of the soft mode condensation at the M and R points, respectively. These phases prove to be unstable at T=0. The experimentally observed Pmnb phase is a consequence of the intersection of Imma and P4/mbm space groups. Thus, it can be 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.