Origin of the change of the electrical and optical properties in shocked Al₂O₃ and prediction of an increase in electrical conductivity in MgSiO₃ at pressure-temperature conditions of the Earth's D'' layer

Shock-wave experiments of Al₂O₃ indicated the onset of an increase in electrical conductivity and observed the optical transparency loss at ∼130 GPa. Here, based on first-principles calculations, we determine the pressure dependence of the band gap of perfect Al₂O₃ to 220 GPa, and investigate the optical absorption of Al₂O₃ without and with oxygen and aluminum vacancies within 220 GPa. Our results indicate that: 1) the onset of the conductivity increase is attributed to a band-gap decrease due to the Rh₂O₃(II)-CaIrO₃ transition at ∼130 GPa and ∼1500 K; 2) this transition is not responsible for the transparency loss, but heterogeneous absorption in the visible-light region, induced by the +2 charge oxygen vacancy, should be a source of this phenomenon. The calculations of perfect MgSiO₃, a material analogous to Al₂O₃, suggest that a perovskite to post-povskite transition in MgSiO₃ at ∼125 GPa and ∼2500 K also yields a band-gap reduction. This causes an increase in electrical conductivity in MgSiO₃ at pressure-temperature conditions of the Earth's D'' layer.