Ordering tendencies in the binary alloys of Rh, Pd, Ir, and Pt: Density functional calculations

The binary alloys of Rh, Pd, Ir, and Pt are important because of their high catalytic potential. We report in this paper that the Rh1xIrx and Rh1xPtx systems, long thought to phase separate at low temperatures, actually exhibit miscibility over the entire concentration and temperature range. We find low critical ordering tempera- tures, indicating that long-range order is unlikely to be observed experimentally. These results are compared with previous theoretical predictions for the other binary alloys of Rh, Pd, Ir, and Pt and with calculations performed here on the Pt1xIrx and Pd1xIrx systems. We discuss these results and investigate the mechanisms governing ordering vs phase separating behavior in all six binaries of the Rh-Pd-Ir-Pt quartet. In particular, we examine why, given the chemical similarity of all members of the quartet, three of their binaries PdIr, PtIr, and RhPd phase separate while the remaining three RhIr, RhPt, and PdPt order.