HYBRIDIZATION OF AL AND O STATES IN ALPHA-ALUMINA AND GAMMA-ALUMINA

The partial Al and O valence spectral densities of alpha- and gamma-alumina in bulk and in the superficial zone of the samples are investigated using x-ray emission spectroscopy induced by electrons. These valence states are mixed over the whole width of the band. We show that changes in the atomic environment affect the hybridization of states in a narrow energy range. For the gamma-phase and superficial zones of the two phases, increased hybridization is observed. This is correlated with an increase in the covalent character of Al-O bonds at the surface and the gamma-phase. Defect states are observed both in the gap between O 2s and O 2p states and in the optical gap; for the alpha-phase, a structure is seen at about 1 eV above the top of the valence band which we interpret as due to an oxygen-vacancy state.