Electronic and optical properties of Θ-Al2O3 and comparison to α-Al2O3

theta-Al2O3 is a metastable phase of alumina with local Al coordinations closely related to those in gamma-Al2O3. The electronic structure and the linear optical properties of theta-Al2O3 are studied by means of first-principles local density calculations. The results are compared with the stable phase alpha-Al2O3. An indirect band gap of 4.64 eV is obtained, which is about 1.6 eV smaller than that of alpha-Al2O3. The other band structure parameters are found to be quite close, and the calculated density of states (DOS) differ from that of alpha-Al2O3 in fine details. Site-and orbital-resolved partial DOS show significant dependence on local coordinations for both Al and O atoms. They are in reasonable agreement with the experimental energy loss near edge spectra of gamma-Al2O3. Effective charge and bond order calculations show theta-Al2O3 to be comparable to alpha-Al2O3. For the linear optical properties, a refractive index 1.73 is obtained, which is close to 1.78 for alpha-Al2O3. The bulk plasmon peak in the energy loss function of theta-Al2O3 is located at 22.6 eV, which is lower by about 1 eV than that in alpha-Al2O3. The anisotropy in the optical properties of alpha-Al2O3 occurs mainly at the low photon energy region of less than 20 eV, and that for the theta-Al2O3 is insignificant.