Electronic Material Properties - und Geowissenschaften ...

Phase diagram of the Pt-Rh alloy studied with a refined
BOS mixing model
Johan Pohl and Karsten Albe
The Pt-Rh alloy is used in automotive exhaust gas converters as a three-way
catalyst. It has attracted much attention from the scientific community due to its
catalytic applications. But although many studies have been performed on the bulk,
surface and nanoparticulate properties of this alloy, some features still remain
puzzling. An answer to the question whether Pt-Rh phase separates or not could only
recently be given. The widely accepted phase diagram, which shows a miscibility gap
and predicts a critical temperature of 1033K, has been constructed by E. Raub [1] in
1959. He inferred from the experimentally confirmed phase separation in Ir-Pt, Ir-Pd
and Pd-Rh, that Pt-Rh would behave similarly. The critical temperature was
estimated from the difference in the melting points of the two constituents. Although
the phase diagram was reprinted many times since, a miscibility gap has never been
observed in experiment. Recent ab-inito density functional theory (DFT) studies [2], in
contrast, provide strong hints that Pt-Rh does not phase separate, but that various
ordered compounds exist at temperatures below 300K. These studies have been
corroborated by experimental results using diffuse X-ray scattering in order to
measure the amount of short-range order present in the alloy at a temperature as
high as 923K [3]. A corrected phase diagram for Pt-Rh, however, that incorporates
the findings of the last 10 years has not been available so far.
Fig. 1: The phase diagram of Pt-Rh based on our calculations, the 40 and D022 structures are the
stable compounds that are found at low temperatures, above 240K there are no stable ordered
structures.
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