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 (SiO₂) from 10 TPa (10¹²0.16em0exPa) to 200 TPa are studied. Using a combination of ab initio simulations and a structure search algorithm, we reveal the phase diagram of SiO₂ above 10 TPa and confirm that I4/mmm4pt0exSiO₂ with the coordination number of 10 is the most stable phase of SiO₂ at ultrahigh pressures. The phase transition pressures from Fe₂P-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/mmm4pt0exSiO₂ is the most stable ten-coordinated structure up to 200 TPa. The thermodynamic stability of I4/mmm4pt0exSiO₂ up to a temperature of 8 kK is inferred from the Gibbs free energy calculations and the dynamic stability of I4/mmm4pt0exSiO₂ at 180 TPa is demonstrated by phonon calculations.