Structural stability and lattice dynamics of SiO2 cristobalite

Details Structural stability and lattice dynamics of SiO2 cristobalite Structural stability and lattice dynamics of SiO2 cristobalite

2008-06-23

arxiv.org/abs/0806.3737v2

Phys. Rev. B 78, 054117 (2008)

Physics

Condensed Matter

Materials Science

15 pages, 9 figures and 7 tables. Supplementary video: ftp://ftp.aip.org/epaps/phys_rev_b/E-PRBMDO-78-097829 or http://physi

Scientific paper

10.1103/PhysRevB.78.054117

Among the phases of SiO2 are alpha and beta cristobalites, which have a long and somewhat controversial history of proposed structural assignments and phase-transition mechanisms. Recently, Zhang and Scott found new indications that the higher-temperature beta phase has space group I-42d and, by assuming a group-subgroup relationship between phases, they argued that the lower-temperature alpha phase should have lower symmetry than that of the widely-accepted P4_12_12 space group. With this motivation, we use first-principles calculations to investigate the energy, structure, and local stability of P4_12_12 and I-42d structures. We also compute the frequencies of the zone-center phonon modes in both structures, as well as certain zone-boundary modes in the I-42d structure, and compare with experiment. We then argue that the various P4_12_12 and I-42d enantiomorphs can be grouped into three clusters, each of which is identified with a three-dimensional manifold of structures of P2_12_12_1 symmetry in which the P4_12_12 and I-42d appear as higher-symmetry special cases. We find that there are relatively high energy barriers between manifolds, but low barriers within a manifold. Exploring the energy landscape within one of these manifolds, we find a minimal-energy path connecting P4_12_12 and I-42d structures with a surprisingly low barrier of ~5 meV per formula unit. Possible implications for the phase-transition mechanism are discussed.

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