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

Scanning tunnelling microscopy images with chemical contrast allowed the di rect determination of the composition and short-range order behaviour of th e clean Pt25Rh75(111), (110), and (100) alloy surfaces. All measurements we re performed at room temperature and showed a strong platinum enrichment de pending 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 neig hbours. Pt25Rh75(100) exhibits after a preparation temperature of 900 degre es C a preference for clustering whereas after a preparation temperature of 600 degrees C, comparable with Pt25Rh75(111) and (110), ordering tendencie s appear. Additional investigations of the chemical identity of atoms surro unding hollow positions show mostly small deviations from a random distribu tion. However, the number of hollow sites surrounded by Rh atoms only can b e 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 approxima tely 2 nm and a wavelength up to 200 nm depending on the preparation temper ature. (C) 1999 Elsevier Science B.V. All rights reserved.