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

Abstract

Based on the most recently determined noncubic structure for $\ensuremath{\gamma}{\text{-Al}}_{2}{\text{O}}_{3}$ 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 $\ensuremath{\alpha}{\text{-Al}}_{2}{\text{O}}_{3}$, we find a smaller lowest zone-center vibrational mode at $97.6\text{ }{\text{cm}}^{\ensuremath{-}1}$, 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 $751\text{ }{\text{cm}}^{\ensuremath{-}1}$. The calculated thermal Gr\"uneisen parameter indicates a strong anharmonicity in $\ensuremath{\gamma}{\text{-Al}}_{2}{\text{O}}_{3}$. 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 $\ensuremath{\gamma}{\text{-Al}}_{2}{\text{O}}_{3}$ has a smaller band gap but a refractive index similar to $\ensuremath{\alpha}{\text{-Al}}_{2}{\text{O}}_{3}$. Highly localized states at the top of the valence band originating from threefold bonded O in the more covalently bonded ${\text{AlO}}_{4}$ tetrahedra are identified. The calculated Mulliken effective charges and bond order values indicate that the structural model for $\ensuremath{\gamma}{\text{-Al}}_{2}{\text{O}}_{3}$ has a high degree of disorder. The octahedral unit $({\text{AlO}}_{6})$ is a stronger polyhedron than the tetrahedral unit $({\text{AlO}}_{4})$ although the latter has stronger Al--O bonds. The calculated $\text{Al-}K$, $\text{Al-}{L}_{3}$, and $\text{O-}K$ edges for Al and O in $\ensuremath{\gamma}{\text{-Al}}_{2}{\text{O}}_{3}$ show strong dependence on their local coordination and environments. These results are in good agreement with available experimental data but the effect of the $\ensuremath{\gamma}{\text{-Al}}_{2}{\text{O}}_{3}$ samples' porosity should be properly assessed. It is argued that the traditional view that stoichiometric $\ensuremath{\gamma}{\text{-Al}}_{2}{\text{O}}_{3}$ is a defective spinel with cation vacancies (or its variations) should be modified. $\ensuremath{\gamma}{\text{-Al}}_{2}{\text{O}}_{3}$ 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.