Structural stability and lattice dynamics of SiO2 cristobalite

TL;DR

This study uses first-principles calculations to analyze the structural stability and lattice dynamics of SiO2 cristobalite phases, revealing low-energy pathways between different structures and insights into phase transition mechanisms.

Contribution

The paper provides a detailed computational investigation of SiO2 cristobalite phases, proposing a unified energy landscape and low-energy transition pathways between different structural variants.

Findings

Identified low-energy barriers (~5 meV) between P4_12_12 and I-42d structures.

Grouped enantiomorphs into three structural manifolds with high energy barriers between them.

Computed phonon frequencies consistent with experimental observations.

Abstract

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.