Phys. Rev. B 78, 054117 (2008) [14 pages]
Structural stability and lattice dynamics of SiO2 cristobalite
Abstract
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Received 23 June 2008; published 21 August 2008
Among the phases of SiO2 are  and  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 phase has space group I![[overline 4]](http://scitation.aip.org/servlet/GetImg?key=PRBMDO000078000005054117000001%3A0%3A0%3A28&t=a&d=a) 2d and, by assuming a group-subgroup relationship between phases, they argued that the lower-temperature phase should have lower symmetry than that of the widely accepted P41212 space group. With this motivation, we use first-principles calculations to investigate the energy, structure, and local stability of P41212 and I2d structures. We also compute the frequencies of the zone-center phonon modes in both structures, as well as certain zone-boundary modes in the I2d structure, and compare with experiment. We then argue that the various P41212 and I2d enantiomorphs can be grouped into three clusters, each of which is identified with a three-dimensional manifold of structures of P212121 symmetry in which the P41212 and I2d 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 P41212 and I2d structures with a surprisingly low barrier of ~5 meV per formula unit. Possible implications for the phase-transition mechanism are discussed.
©2008 The American Physical Society
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