LEED STUDY OF PT25CO75(111)

A quantitative LEED study of the Pt25Co75(111) surface (in the substit utionally disordered state) shows weak relaxations of interlayer spaci ngs (DELTAd12/d(bulk), DELTAd23/d(bulk), DELTAd34/d(bulk))= (-1.9, app roximately 1.4, 3.6)%, no stacking fault with respect to the ABC stack ing but a significant oscillatory composition profile (C1, C2, C3) = ( 46, 3, 37) at% Pt. This behaviour is very close to the general trend o bserved for PtNi(111) surfaces as a function of the bulk concentration Cb and in good agreement with the predictions of the tight binding Is ing model of Treglia, Legrand and Ducastelle and of the embedded atom method results of Lundberg.