We have investigated the unreconstructed (0001) surface structure of sapphire (α-Al 2 O 3 ) by grazing incidence X-ray scattering. Modulations along the crystal truncation rods were analyzed in order to determine the chemical nature of the terminating plane, and the structural relaxations of the first few atomic planes below the surface. The most likely model yields a single Al layer termination with relaxations of the first four planes of −51%, +16%, −29% and +20% respectively. These results compare well with the most recent theoretical calculations on this surface.The structure, composition and morphology of ceramic surfaces strongly influence their chemical, mechanical and electrical properties and thus have a dominant contribution in many technologically important processes, such as corrosion, catalysis and sintering. They also affect the nature and strength of bonding at metal/ceramic interfaces used in composites or in electronic packaging. The (0001) surface of sapphire (α-alumina, corrundum) is of major importance; it is one of the most widely used substrates for growth of thin metal, semiconductor or high-T c superconductor films, and its initial state is known to play a dominant role in the overlayer properties. Despite this interest, the nature (Al or O) of its terminating plane is still an open topic because of the lack of experimental result. Moreover the numerous theoretical calculations that concentrate on the surface structure and relaxation 1-6 yield different answers. The single Al-terminated surface is favored by electrostatic considerations 1 as well as surface energy calculations. 2 Indeed, for an Al termination no dipole moment is left across the surface and only the longer, and thus the weaker, anion-cation bonds are broken. However, the (1 × 1) structure is also experimentally observed on alumina surfaces heated in an oxygen-rich atmosphere and could thus be suspected to be oxygen-terminated. As regards the Al-terminated surface, large relaxations have been predicted by pair potential calculations. 3 More recently, ab initio calculations, by the density functional theory combined with pseudopotential techniques, 4 predict a very large relaxation of the last atomic plane (−87%), while Hartree-Fock calculations 5 yield a smaller, although still sizable, relaxation (−40%). A tight-binding, total-energy method 6 also predicts large out-of-plane relaxations of the Al planes and in-plane displacements of the oxygen atoms. Theoreticians crucially need experimental results in order to test the different theories and approximations. 4-6 There is currently no experimental determination of the detailed surface structure, mainly because of the limitations of electron-based techniques such as charging and multiple scattering. Grazing incidence X-ray scattering (GIXS) is now a well-established tool for accurately determining the atomic structure of surfaces, 7 which does not suffer these drawbacks. The aim of the present study is to quantitatively analyze the modulations along the crystal truncation rod...

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We have investigated the unreconstructed (0001) surface structure of sapphire (α-Al2O3) by Grazing Incidence X-ray Scattering. Modulations along the crystal truncation rods were analyzed in order to determine the chemical nature of the terminating plane, and the structural relaxations of the first few atomic planes below the surface. The most likely model yields a single Al layer termination with relaxations of the first four planes of -51%, +16%, -29% and +20% respectively. These results compare well with the most recent theoretical calculations on this surface.