Melting and transport properties ofAl2⁢O3at extreme conditions

The high-pressure temperature phase diagram and transport properties of materials are of broad interest to planetary sciences and high-energy-density sciences and applications. Alumina (Al2⁢O3) or its various forms (e.g., solutions with other oxides or silicates) are important constituents in Earth's and super-Earths' mantles, common window materials in dynamic compression experiments, and standard pressure calibrators in diamond-anvil-cell experiments. Its structures and transport properties are of particular importance but have not been well studied at above 100 GPa pressures. Based on extensive first-principles molecular dynamics calculations, we obtain atomic level insights on structural differences and accurate results on electrical conductivity (𝜎dc), thermal conductivity (𝜅), and reflectivity of Al2⁢O3 in several phases at pressures of ∼130−1300GPa and temperatures of 4000–20 000 K. We find the solid-to-liquid state changes of Al2⁢O3 are accompanied by an insulator-to-semimetal transition with enhanced 𝜎dc and 𝜅, which is similar to other silicates and oxides and can facilitate magnetic field generation in deep interiors of rocky planets. We also find the solid-to-solid transition [from Rh2⁢O3⁡(II) to CaIrO3] is accompanied by negligible changes in 𝜎dc and 𝜅 at 160 GPa, contrary to previous expectations, whereas another higher pressure phase transition (CaIrO3 to U2⁢S3 at 450 GPa) is associated with increases in 𝜎dc and 𝜅. Furthermore, we show the transport properties can be enhanced by oxygen vacancy in Al2⁢O3, but are not significantly affected by forming solutions with MgSiO3, particularly when in the solid states.