Prediction of new thermodynamically stable aluminum oxides

Recently, it has been shown that under pressure, unexpected and counterintuitive chemical compounds become stable. Laser shock experiments (A. Rode, unpublished) on alumina (Al₂O₃) have shown non-equilibrium decomposition of alumina with the formation of free Al and a mysterious transparent phase. Inspired by these observations, we have explored the possibility of the formation of new chemical compounds in the system Al-O. Using the variable-composition structure prediction algorithm USPEX, in addition to the well-known Al₂O₃, we have found two extraordinary compounds Al₄O₇ and AlO₂ to be thermodynamically stable in the pressure ranges 330-443 GPa and above 332 GPa, respectively. Both of these compounds at the same time contain oxide O²⁻ and peroxide O₂²⁻ ions and both are insulating. Peroxo-groups are responsible for gap states, which significantly reduce the electronic band gap of both Al₄O₇ and AlO₂.