A general theory of surfaces, the MTCIP (modern thermodynamic calculation of interface properties) was developed earlier by one of the authors. Solving a part of its basic equations in a recent and even more applicable second approximation (MTCIP-2A), global surface sublayer compositions for several Pt alloys were recently published. The PtRh alloy is of great practical importance due to its catalytical properties. Nevertheless, experimental data on its mixing behaviour is not readily available, so that at first corresponding bulk and surface thermodynamic quantities had to be estimated. A recent chemically resolved STM study for PtRh(100) shows surface clustering, in contrast to bulk ordering as predicted from ab-initio and other calculations. Extending the MTCIP-2A approach to include local chemical compositions by means of the quasi-chemical theory explains this behaviour. The calculated global surface concentration profiles (for T=1120K), as well as local surface compositions (indicating local clustering) are in agreement with recent experimental results.
Corresponding author: W. Hofer (hofer).
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