Formation of an anataselike phase in silica under anisotropic stress: Anab initioconstant-pressure study

Abstract

We study the behavior of $\ensuremath{\alpha}$ cristobalite under anisotropic stresses using constant-pressure ab initio simulations and observe the formation of the anataselike, stishovite, and ${\text{CaCl}}_{2}$-type structures depending on the degree of hydrostatic compression. These phase transformations proceed via a tetragonal intermediate phase (phase X-I) within $P{4}_{1}{2}_{1}2$ symmetry or an orthorhombic intermediate state, having space group of $P{2}_{1}{2}_{1}{2}_{1}$. The phase transitions into stishovite and the anataselike phase are based on the same intermediate state. The compression of $\ensuremath{\alpha}$ cristobalite along the $c$ direction produces stishovite while the expansion of $\ensuremath{\alpha}$ cristobalite along the $c$ direction yields an anataselike phase. The energy-volume calculations suggest that $\ensuremath{\alpha}$ cristobalite can transform into the anataselike structure if and only if the phase transition into stishovite is suppressed. A phase transition between stishovite and anatase, however, is unlikely to occur.