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)
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).
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