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Laboratoire pour I'Utilisation du Rayonnement Electromagnetique, 91045New results on the (@ X @ ) R -+ 9" reconstructed a-Al,O, (0001) surface, which can be obtained after heating at high temperature (1400°C) under vacuum, are presented. The atomic structure has been studied by combining lowenergy electron diffractometry and grazing incidence X-ray scattering. The surface structure is found to be perfectly commensurable with the underlying bulk lattice. The surface consists of hexagonal zones of two, nearly perfect, close-packed Al(111) planes separated by a Cefect of hexagonal periodicity with a parameter of 26.44 A. This model is consistent with previous surface studies of this reconstruction. The electronic structure has been investigated using valence band photoemission spectroscopy, X-ray absorption spectroscopy at the 0 Kedge, electron energy loss spectroscopy, and X-ray-induced Auger electron spectroscopy. Interpretation of these experimental data in the frame of a self-consistent, tight-binding calculation leads to the conclusion that the (@ X @)Rf:9" reconstructed surface is more covalent than the (1 X 1) surface. Significant changes in the AI-0 hybridizations are observed; these are likely due to a difference in the interatomic distances along the [OOOlI axis (relaxations). The increase of covalent character is mainly due to a strong decrease of the Madelung field on the reconstructed surface.