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 (AlO). 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 (AlO, AlO) and two superoxides (AlO, AlO) with uncommon stoichiometries at high pressures using first-principle calculations and crystal structure prediction methods. We find that the AlO becomes stable above ~765 GPa and may survive in the deep mantles or cores of giant planets such as Neptune. Interestingly, the AlO and AlO are metallic and have electride features, in which some electrons are localized in the interstitials between atoms. We find that AlO 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.