ADSORBATE-INDUCED VARIATION OF THE STRUCTURE AND COMPOSITION OF THE Mo0.75Re0.25(100) SURFACE

Abstract

Clean random alloy surfaces usually exhibit non-bulklike concentration profiles. Adsorption at low temperatures does not modify these profiles as there is thermal activation for interdiffusion of surface atoms required. For strongly chemisorbed species like oxygen or carbon, however, the activation barrier can be overcome prior to desorption leading to a new equilibrium distribution of the surface constituents. This paper summarizes the results of different LEED-I(V) studies on the adsorption of hydrogen, oxygen and carbon on the (100) face of the random alloy Mo 0.75 Re 0.25. Hydrogen, which desorbs near room temperature, is found to affect the interlayer spacings only, whilst the concentration profile remains completely unchanged. In contrast, enhanced molybdenum segregation at temperatures above 600° C is shown to happen with oxygen and carbon adsorption leading to exclusive Mo-C and Mo-O bonds, respectively. The surface structures formed for high coverages differ substantially from each other. Oxygen induces a missing row structure and is bound in a “quasi” three-fold coordinated site. In contrast, carbon is adsorbed in five-fold hollow sites at the surface and in six-fold coordinated octahedral subsurface sites.