## Abstract

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.

Original language | English (US) |
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Article number | 054117 |

Journal | Physical Review B - Condensed Matter and Materials Physics |

Volume | 78 |

Issue number | 5 |

DOIs | |

State | Published - Aug 21 2008 |

## All Science Journal Classification (ASJC) codes

- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics