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Among the phases of SiO2 are alpha and beta cristobalites, which have a longand somewhat controversial history of proposed structural assignments andphase-transition mechanisms. Recently, Zhang and Scott found new indicationsthat the higher-temperature beta phase has space group I-42d and, by assuming agroup-subgroup relationship between phases, they argued that thelower-temperature alpha phase should have lower symmetry than that of thewidely-accepted P4_12_12 space group. With this motivation, we usefirst-principles calculations to investigate the energy, structure, and localstability of P4_12_12 and I-42d structures. We also compute the frequencies ofthe zone-center phonon modes in both structures, as well as certainzone-boundary modes in the I-42d structure, and compare with experiment. Wethen argue that the various P4_12_12 and I-42d enantiomorphs can be groupedinto three clusters, each of which is identified with a three-dimensionalmanifold of structures of P2_12_12_1 symmetry in which the P4_12_12 and I-42dappear as higher-symmetry special cases. We find that there are relatively highenergy barriers between manifolds, but low barriers within a manifold.Exploring the energy landscape within one of these manifolds, we find aminimal-energy path connecting P4_12_12 and I-42d structures with asurprisingly low barrier of ~5 meV per formula unit. Possible implications forthe phase-transition mechanism are discussed.

Structural stability and lattice dynamics of SiO2 cristobalite