An atom-probe field ion microscope(AP-FIM) has been employed to determine the surface composition and its depth profile of the (100) plane of Pt-20, 40wt.%(32.1, 55.8at.%)Rh alloys. The enrichment of Pt at the top surface layer on annealing at 700 C( 1000K) was found, agreeing with previous studies by AES, ISS, and AP-FIM. On annealing below 600 C, however, a reversed surface segregation, Rh enrichment at the first layer, has been observed without any impurity atoms such as S and P. An

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... ry behavior was observed in composition of both Pt and Rh over ten atomic layers. Annealing up to 700 C in the presence of oxygen, we found no appreciable oxidation of the alloy samples, instead,there appeared chemisorbed oxygen forming an overlayer.