Formation of solid SiO2He compound at high pressure and high temperature

Abstract

Silica, a main mineral inside the Earth and super-Earth, is generally assumed to be resistant to react with noble gas elements at ambient conditions. Here, combining crystal structural prediction and first-principles calculation, we predict a $\mathrm{Si}{\mathrm{O}}_{2}\mathrm{He}$ compound that becomes stable at high pressure. Each Si (He) atom in the $\mathrm{Si}{\mathrm{O}}_{2}\mathrm{He}$ is bonded with seven O atoms and two He (Si) atoms, forming a Si-centered $\mathrm{Si}{\mathrm{O}}_{7}{\mathrm{He}}_{2}$ (He-centered $\mathrm{He}{\mathrm{O}}_{7}{\mathrm{Si}}_{2}$) polyhedron. Further calculations indicate that the $\mathrm{Si}{\mathrm{O}}_{2}\mathrm{He}$ compound remains solid over a wide range of pressures exceeding 607 GPa and temperatures of 0--9000 K, covering the extreme conditions of the core-mantle boundary in super-Earth exoplanets or even in the ice giants of our solar system. Our results may provide theoretical guidance for the discovery of other silicides at high pressures, which promote the exploration of materials at planetary core-mantle boundaries, and enable planetary models to be refined as well.