Surface structure and segregation of bimetallic bcc-type alloys

Abstract

We report on the surface structure and segregation of bimetallic bcc-type alloys, on which so far there has been comparably little emphasis in the literature - unlike the case for fcc-type material. The focus is both on ordered and on substitutionally disordered alloys: it is shown that for a given slab of the surface, ideal chemical order or disorder, respectively, are rather limiting cases. For chemically ordered samples, surface segregation can lead to both some disorder and new ordered phases. In the case of substitutionally disordered alloys, segregation leads to a layer-dependent compositional profile. Depending on the openness of the surface, as is evident from the investigation of different low-index surfaces, this profile may extend deep into the surface. Correspondingly, the geometrical multilayer relaxation is unusually deeply extended, too, as comparison to the related pure-metal surfaces shows. Clearly, there is a correlation between geometrical relaxation and chemical segregation: though the relaxation of the first few layer spacings can be lifted by, for example, hydrogen adsorption, it is stabilized in deeper layers by the frozen-in segregation profile. On the other hand, surface segregation is modified by adsorbates strongly bonding to one of the constituents.