CONCENTRATION PROFILES OF LOW-INDEX SURFACES OF PTCO ALLOYS - APPLICATION OF AN IMPROVED THERMODYNAMIC THEORY

Several years ago a general theory of solid surfaces was developed by one of us (MTCIP: modem thermodynamic calculation of interface propert ies). A part of it describes the surface concentration profile of the thermodynamic equilibrium state by a system of coupled, non-linear equ ations. This system was solved in a first approximation for dilute bin ary mixtures and applied to several bimetallic alloys, with reliable r esults in several publications, even in the case of surface interactio ns with environmental (oxygen) atoms. Recently a much more detailed se cond approximation (MTCIP-2A) has been developed (under publication no w). It has been shown to give good agreement with experimental concent ration profiles for the three low-index surfaces of PtNi for all the b ulk concentrations investigated experimentally (for T approximate to 1 120 K). Here the MTCIP-2A is shortly outlined and applied to obtain co ncentration profiles for Pt25Co75(111), Pt25Co75(111) and Pt80Co20((11 1)) surfaces (for T = 1120 K), These results, too, are in agreement wi th recent LEED experiments for all the surface layers investigated. To our knowledge, these are the first calculated surface concentration p rofiles for PtCo.