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
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.
PACS: 63.20.-e – Phonons in crystal lattices / 71.15.Mb – Density functional theory, local density approximation / 91.35.-x – Earth's interior structure and properties
© EDP Sciences, Società Italiana di Fisica, Springer-Verlag, 2000