Computational Investigation of Al2O3 Structures at Super-Earth Interior Conditions

Abstract

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.