Structural stability and lattice dynamics of SiO2 cristobalite

Among the phases of SiO2 are α and β 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 β phase has space group I 4̄ 2d and, by assuming a group-subgroup relationship between phases, they argued that the lower-temperature α phase should have lower symmetry than that of the widely accepted P 41 21 2 space group. With this motivation, we use first-principles calculations to investigate the energy, structure, and local stability of P 41 21 2 and I 4̄ 2d 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 4̄ 2d structure, and compare with experiment. We then argue that the various P 41 21 2 and I 4̄ 2d enantiomorphs can be grouped into three clusters, each of which is identified with a three-dimensional manifold of structures of P 21 21 21 symmetry in which the P 41 21 2 and I 4̄ 2d 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 P 41 21 2 and I 4̄ 2d structures with a surprisingly low barrier of ∼5 meV per formula unit. Possible implications for the phase-transition mechanism are discussed.