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

E.L.D. Hebenstreit*, W. Hebenstreit*, M. Schmid, and P. Varga

Institut für Allgemeine Physik, Technische Universität Wien, A-1040 Wien, Austria
*Present address: Department of Physics, Tulane University, New Orleans, LA, 70118, USA

Surf. Sci. 441 (1999) 441-453

Scanning tunneling 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)-(1x2) we find a preference for unlike nearest neighbors. Pt25Rh75(100) exhibits after a preparation temperature of 900°C a preference for clustering whereas after a preparation temperature of 600°C, comparable to 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 (1x2) missing-row reconstruction after annealing above 700°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.

Corresponding author: E.L.D. Hebenstreit (currently at Tulane University, U.S.A.). Reprints also available from M. Schmid (schmid< encoded email address >).

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