Computational Investigation of Al2O3 Structures at Super-Earth Interior Conditions

Al2O3 is an important component of rocky planetary interiors, yet its behavior at the multimegabar pressures relevant to super-Earth mantles is not completely understood. Here, we combine crystal structure prediction with state-of-the-art density functional theory (DFT) calculations to investigate the structural, thermodynamic, and electronic properties of alumina from ambient pressure to 1 TPa and up to 8000 K. Evolutionary searches uncovered a previously unreported monoclinic P21/m polymorph that becomes nearly degenerate in enthalpy with the U2S3-type and α-Gd2S3-type structures above 500 GPa. Cold enthalpy calculations using the r2SCAN meta-GGA (meta-generalized-gradient approximation) functional predict the transition sequence α-Al2O3 → Rh2O3(II)-type → CaIrO3-type → U2S3-type, with the CaIrO3- to U2S3-type transition occurring at ∼450 GPa, significantly higher than earlier local-density approximation (LDA) estimates, suggesting that the U2S3-type phase may be accessible primarily under dynamic compression conditions. Quasi-harmonic free energy calculations further indicate that this transition pressure is temperature dependent up to 8000 K, with a positive slope of 94.1 K/GPa (or 10.6 MPa/K) for the phase boundary of the CaIrO3-type/U2S3-type structures. Across the stable phases, we analyze the trends of the atomic volumes, Bader charges, and electronic band gaps, revealing limited compressibility of Al3+ above 200 GPa. The predicted small enthalpy differences among high-pressure polymorphs imply the possible coexistence of numerous metastable structures under super-Earth interior conditions. These results refine the phase diagram of Al2O3 at extreme conditions and provide quantitative constraints for modeling the mineralogy, density profiles, and transport properties of rocky exoplanet mantles. We further predict that alumina remains a wide-band gap insulator throughout the investigated pressure range, with phase-dependent discontinuities of the band gap at structural transitions, but no indication of metallization up to 1 TPa and 0 K.