OXYGEN-INDUCED MISSING-ROW RECONSTRUCTION OF SUBSTITUTIONALLY DISORDERED MO0.75RE0.25(100)

Adsorption of oxygen on Mo0.75Re0.25(100) with exposures in the range 0.5-100 L and subsequent annealing at or above about 1400 K produces a 2 X 1 superstructure as observed by low energy electron diffraction. The superstructure spots are very bright indicating a strong oxygen in duced substrate reconstruction. By full dynamical LEED intensity analy sis this is proved to be of the missing row type in agreement with ear lier results by ion scattering. As usual, the missing of rows induces row pairing and buckling in the second and third substrate layers, res pectively, by amplitudes of the order of 0.1 Angstrom. Substrate inter layer distances change drastically. Oxygen atoms reside in 3-fold coor dinated sites binding to both first and second layer atoms with bond l engths of 2.02 Angstrom and 2.04 Angstrom (oxygen hard core radii 0.66 Angstrom and 0.68 Angstrom), respectively. The Mo-O bonds being stron ger than Re-O bonds seems to cause and dominate an oxygen induced surf ace segregation process activated by the high annealing temperature wh ich results in a considerable enrichment of Mo in the surface. If one views the oxygen coordinated atoms as belonging to a composite first l ayer, both the stoichiometry and distances of deeper layers are very s imilar to those found for the clean surface.