Ab initio study of the physical properties of γ-Al2O3 : Lattice dynamics, bulk properties, electronic structure, bonding, optical properties, and ELNES/XANES spectra

Based on the most recently determined noncubic structure for -Al2O3 by Menendez-Proupin and Gutierrez, a comprehensive list of physical properties is investigated theoretically. These include lattice dynamics and phonon spectra, elastic constants and bulk structural parameters, electronic structure and interatomic bonding, optical properties, and x-ray absorption near-edge structure XANES spectra. Compared to similar calculations of -Al2O3, we find a smaller lowest zone-center vibrational mode at 97.6 cm −1, a lower heat capacity, a smaller bulk modulus, and a much larger thermal-expansion coefficient. The threefold bonded O ions introduce highly localized vibrational modes near 751 cm−1. The calculated thermal Gruneisen parameter indicates a strong anharmonicity in -Al2O3. The elastic tensor and the elastic wave velocities are also evaluated showing the longitudinal wave to be nearly isotropic. For the electronic structure, we find that -Al2O3 has a smaller band gap but a refractive index similar to -Al2O3. Highly localized states at the top of the valence band originating from threefold bonded O in the more covalently bonded AlO4 tetrahedra are identified. The calculated Mulliken effective charges and bond order values indicate that the structural model for -Al2O3 has a high degree of disorder. The octahedral unit AlO6 is a stronger polyhedron than the tetrahedral unit AlO4 although the latter has stronger Al–O bonds. The calculated Al-K, Al-L3, and O-K edges for Al and O in -Al2O3 show strong dependence on their local coordination and environments. These results are in good agreement with available experimental data but the effect of the -Al2O3 samples’ porosity should be properly assessed. It is argued that the traditional view that stoichiometric -Al2O3 is a defective spinel with cation vacancies or its variations should be modified. -Al2O3 is better described as an amorphous networklike structure such that the ratio of tetrahedrally coordinated Al to octahedrally coordinated Al is close to 0.6; and the O ions are bonded to Al in either a threefold or fourfold configurations in about equal proportion.