Prediction of stability of tenfold-coordinated silica up to 200 TPa pressure based on ab initio calculations with all-electron pseudopotentials

The ultrahigh-pressure structural evolution and phase diagram of silica (${\mathrm{SiO}}_2$) from 10 TPa (${10}^{12}\mathrm{Pa}$) to 200 TPa are studied. Using a combination of ab initio simulations and a structure search algorithm, we reveal the phase diagram of ${\mathrm{SiO}}_2$ above 10 TPa and confirm that $I4/mmm{\mathrm{SiO}}_2$ with the coordination number of 10 is the most stable phase of ${\mathrm{SiO}}_2$ at ultrahigh pressures. The phase transition pressures from ${\mathrm{Fe}}_{2}P$-type to $I4/mmm$ phase are obtained for different temperatures. To correctly model the ultrahigh-pressure structures, the inner-shell electron interactions are necessary to be considered. The pseudopotentials with different valence electron configurations are tested and the results show that neglecting the inner-shell electrons can predict incorrect phase transition sequence and thermodynamic stability. Based on simulations with all-electron pseudopotentials, it is found that the $I4/mmm{\mathrm{SiO}}_2$ is the most stable ten-coordinated structure up to 200 TPa. The thermodynamic stability of $I4/mmm{\mathrm{SiO}}_2$ up to a temperature of 8 kK is inferred from the Gibbs free energy calculations and the dynamic stability of $I4/mmm{\mathrm{SiO}}_2$ at 180 TPa is demonstrated by phonon calculations.