ALPHA-AL2O3-(0001) SURFACES - ATOMIC AND ELECTRONIC-STRUCTURE

New results on the (root 31 X root 31)R+/-9 degrees reconstructed alph a-Al2O3 (0001) surface, which can be obtained after heating at high te mperature (1400 degrees C) under vacuum, are presented. The atomic str ucture has been studied by combining low-energy electron diffractometr y and grazing incidence X-ray scattering. The surface structure is fou nd to be perfectly commensurable with the underlying bulk lattice. The surface consists of hexagonal zones of two, nearly perfect, close-pac ked Al (111) planes separated by a effect of hexagonal periodicity wit h a parameter of 26.44 Angstrom. This model is consistent with previou s surface studies of this reconstruction. The electronic structure has been investigated using valence band photoemission spectroscopy, X-ra y absorption spectroscopy at the O Kedge, electron energy loss spectro scopy, and X-ray-induced Auger electron spectroscopy. Interpretation o f these experimental data in the frame of a self-consistent, tight-bin ding calculation leads to the conclusion that the (root 31 X root 31)R +/-9 degrees reconstructed surface is more covalent than the (1 X 1) s urface, Significant changes in the Al-O hybridizations are observed; t hese are likely due to a difference in the interatomic distances along the [0001] axis (relaxations). The increase of covalent character is mainly due to a strong decrease of the Madelung field on the reconstru cted surface.