Pt25Rh75(111), (110), and (100) studied by scanning tunnelling microscopy with chemical contrast

Scanning tunnelling microscopy images with chemical contrast allowed the direct determination of the composition and short-range order behaviour of the clean Pt25Rh75(111), (110), and (100) alloy surfaces. All measurements were performed at room temperature and showed a strong platinum enrichment depending on the preparation temperature. In the top layers of both Pt25Rh75(111) and Pt25Rh75(110)-(1 x 2) we find a preference for unlike nearest neighbours. Pt25Rh75(100) exhibits after a preparation temperature of 900 degrees C a preference for clustering whereas after a preparation temperature of 600 degrees C, comparable with Pt25Rh75(111) and (110), ordering tendencies appear. Additional investigations of the chemical identity of atoms surrounding hollow positions show mostly small deviations from a random distribution. However, the number of hollow sites surrounded by Rh atoms only can be significantly affected by the short-range order. Pt25Rh75(110) exhibits a (1 x 2) missing-row reconstruction after annealing above 700 degrees C. After the first annealing of the sputtered surface it is accompanied by mesoscopic long-range 'waves' with a height of approximately 2 nm and a wavelength up to 200 nm depending on the preparation temperature. (C) 1999 Elsevier Science B.V. All rights reserved.