  Grazing-incidence X-ray diffraction data are combined with
energy-minimization calculations to analyse the atomic structure of the Al-rich
(3sqrt{3} x 3sqrt{3})R 30 deg reconstructed surface of sapphire
alpha-Al_2O_3(0001). The experiments on the BM32 beamline of the ESRF provide
the non-integer-order diffraction intensities and, after Fourier transform, an
incomplete Patterson map. The computer simulations are implemented to obtain
structural information from this map. In the simulations, the interactions
between the Al overlayer atoms were described with the Sutton-Chen potential
and the interactions between the overlayer and the sapphire substrate with a
laterally modulated Lennard-Jones potential. We have shown that the hexagonal
reconstructed unit cell is composed of triangles where the two layers of Al
adatoms are FCC(111) ordered whereas between the triangles the stacking is
FCC(001).
