Interpretation of the Al K- and L-II/III-edges of aluminium oxides: differences between tetrahedral and octahedral Al explained by different local symmetries

The Al K- and L-II/III-edge XANES of aluminium oxide are interpreted using empirical molecular orbital theory (EHMO) and ab initio self-consistent fie ld real space multiple scattering calculations (FEFF8). Most features in th e XANES at the K- and L-II/III-edges are interpreted as shape resonances; a lthough some fine structure, visible at both edges, arises from multiple sc attering over the medium range (similar to 15 Angstrom). The change in loca l symmetry between octahedral and tetrahedral Al explains the observed diff erences in the electronic structure. First, Al p-d hybridization is allowed only in tetrahedral symmetry, resulting in a lower absorption edge in tetr ahedral Al than in the octahedral. Second, only in octahedral Al do the oxy gen orbitals near the valence band maximum (the HOMOs) have the right symme try to mix with the Al p orbitals just above the band gap (the LUMOs). This gives a more screened core hole in the octahedral case. Calculations on di storted octahedral Al sites reveal both p-d and s-d hybridizations; however , the latter is less prominent. The diffuse d orbitals, which hybridize wit h the p or s orbitals in tetrahedral or distorted octahedral symmetry, are primarily responsible for the fine structure in the near-edge region (0-15 eV) that is determined by medium-range scattering (up to similar to 15 Angs trom). The observed difference in the magnitude of this fine structure at t he K- and L-II/III-edges is caused by the different degrees of d orbital hy bridization with the s and p orbitals.