Identification of post-pyrite transition in SiO2 by a genetic algorithm*

Here we propose a new phase of SiO$_{2}$ beyond the pyrite-type phase. SiO$_{2}$ is one of the most important minerals in Earth and planetary sciences. So far, the pyrite-type phase has been identified experimentally as the highest-pressure form of SiO$_{2}$. In solar giants and extrasolar planets whose interior pressures are considerably higher than that on Earth, a post-pyrite transition in SiO$_{2}$ may occur at $\sim$ 1 TPa as a result of the dissociation of MgSiO$_{3}$ post-perovskite into MgO and SiO$_{2}$ [Umemtoto et al., Science 311, 983 (2006)]. Several dioxides considered to be low-pressure analogs of SiO$_{2}$ have a phase with cotunnite-type (PbCl$_{2}$-type) structure as the post-pyrite phase. However, a first-principles structural search using a genetic algorithm shows that SiO$_{2}$ should undergo a post-pyrite transition to a hexagonal phase, not to the cotunnite phase. The hexagonal phase is energetically very competitive with the cotunnite-type one.