Metallic Aluminum Suboxides with Ultrahigh Electrical Conductivity at High Pressure.

Aluminum, as the most abundant metallic elemental content in the Earth's crust, usually exists in the form of alumina (Al 2 O 3 ). However, the oxidation state of aluminum and the crystal structures of aluminum oxides in the pressure range of planetary interiors are not well established. Here, we predicted two aluminum suboxides (Al 2 O, AlO) and two superoxides (Al 4 O 7 , AlO 3 ) with uncommon stoichiometries at high pressures using first-principle calculations and crystal structure prediction methods. We find that the P4/nmm Al 2 O becomes stable above ~765 GPa and may survive in the deep mantles or cores of giant planets such as Neptune. Interestingly, the Al 2 O and AlO are metallic and have electride features, in which some electrons are localized in the interstitials between atoms. We find that Al 2 O has an electrical conductivity one order of magnitude higher than that of iron under the same pressure-temperature conditions, which may influence the total conductivity of giant planets. Our findings enrich the high-pressure phase diagram of aluminum oxides and improve our understanding of the interior structure of giant planets.