Oxygen induced missing row reconstruction of substitutionally disordered Mo0.75Re0.25(100)

Abstract

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 × 1 superstructure as observed by low energy electron diffraction. The superstructure spots are very bright indicating a strong oxygen induced substrate reconstruction. By full dynamical LEED intensity analysis this is proved to be of the missing row type in agreement with earlier results by ion scattering. As usual, the missing of rows induces row pairing and buckling in the second and third substrate layers, respectively, by amplitudes of the order of 0.1 Å. Substrate interlayer distances change drastically. Oxygen atoms reside in 3-fold coordinated sites binding to both first and second layer atoms with bond lengths of 2.02 Å and 2.04 Å (oxygen hard core radii 0.66 Å and 0.68 Å), respectively. The MoO bonds being stronger than ReO bonds seems to cause and dominate an oxygen induced surface segregation process activated by the high annealing temperature which results in a considerable enrichment of Mo in the surface. If one views the oxygen coordinated atoms as belonging to a composite first layer, both the stoichiometry and distances of deeper layers are very similar to those found for the clean surface.