Ab initio study of phonons and structural stabilities of the perovskite-type MgSi{O_3} |
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Authors: | K Parlinski Y Kawazoe |
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Institution: | (1) Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Sendai 980-8577, Japan, JP;(2) Institute of Nuclear Physics, ul. Radzikowskiego 152, 31-342 Cracow, Poland, PL |
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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.
Received 15 October 1999 and Received in final form 14 January 2000 |
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Keywords: | 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 |
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