INFLUENCE OF THE TRANSITION-METAL AND OF ORDER ON THE COMPOSITION PROFILE OF PT80M20(111) (M = NI, CO FE) ALLOY SURFACES - LEED STUDY OF PT80CO20(111)

Low energy electron diffraction (LEED) was used to determine the surface structure and composition profile versus the depth of the disordered alloy Pt80Co20(111). The Pt concentration oscillates around the bulk value and damps out over three layers: the top layer is a pure Pt layer (C1 = 100at% Pt), the second layer is strongly Pt depleted (C2 = 48at%) and the third layer is only slightly enriched in Pt. The structural parameters are typical of a (111) face of fcc metals, i.e., weak relaxations of interlayer spacings (DELTAd12 = -0.3, DELTAd23 = -0.9%) and ABC stacking. The present results are compared to those obtained on the same face of Pt78Ni22 and of Pt80Fe20, in the light of changes concerning nearest neighbours. More particularly, we show that the composition of the second layer is strongly related to the contraction of the interatomic Pt-Pt distance in the surface region and/or, possibly, to the presence of order in the underlying layers of the bulk.