α–βtransformation and disorder inβ-cristobalite silica

Abstract

Disorder in $\ensuremath{\beta}$-cristobalite is shown to arise from the conformational changes between $\ensuremath{\alpha}$- and $\ensuremath{\beta}$-rings (energy barriers of 10--30 meV/SiO${}_{2}$). $\ensuremath{\alpha}$-rings, remnants from the $\ensuremath{\alpha}--\ensuremath{\beta}$ transformation, constitute $\ensuremath{\alpha}$-cristobalite. $\ensuremath{\beta}$-rings are local structures of a new ${\ensuremath{\beta}}^{\ensuremath{'}}$-cristobalite of symmetry $I\overline{4}2d,$confirmed herein by both molecular dynamics simulations and first-principles calculations. The ring conformation in $\ensuremath{\beta}$-cristobalite features a short correlation time ($<50$ fs) and short correlation length ($<1$ nm), giving the underlying topology framework the ability to support the dynamic disorder.