Segregation and reconstructions of PtxNi1-x(100)

It is known that on (100) surfaces of PtxNi1-x single crystals Pt segregates. With increasing Pt concentration in the surface the transition from unreconstructed Ni(100) to the pseudo hexagonal Pt(100) reconstruction occurs via a shifted row reconstruction and several pseudo hexagonal (n x 1) superstructures (n = 7, 12 and 19) consisting of similar (7 x 1) and (5 x 1) subcells. This was revealed by atomically resolved scanning tunnelling microscopy (STM). From low energy ion scattering measurements it becomes clear that the formation of the pseudo hexagonal structure leads to strong amplification of Pt segregation. Chemically resolved STM on the atomic scale shows that Pt prefers the highly coordinated four-fold hollow sites in the pseudo hexagonal structures and Ni is pushed into nearly on-top or bridge sites. Therefore the strong tendency of Pt to increase its coordination is proposed as the driving force of the reconstructions. Corrugations and chemical ordering measured by STM within the pseudo hexagonal reconstructions are confirmed by simulations based on embedded atom method potentials. (C) 1997 Elsevier Science B.V.