Ordering tendencies in Pd-Pt, Rh-Pt, and Ag-Au alloys

First-principles quantum-mechanical calculations indicate that the mixing enthalpies for Pd-Pt and Rh-Pt solid solutions are negative, in agreement with experiment, Calculations of the diffuse-scattering intensity due to short-range order also exhibits ordering tendencies, Further, the directly calculated enthalpies of formation of ordered intermetallic compounds are negative, These ordering tendencies are in direct conflict with a 1959 prediction of Raub that Pd-Pt and Rh-Pt will phase-separate below similar to 760 degrees C (hence their mixing energy will be positive), a position that has been adopted by all binary alloy phase diagram compilations, The present authors predict that Pd1-xPtx will order in the L1(2), L1(0), and L1(2) structures ([001] superstructures) at compositions x = 1/4, 1/2, 3/4 and respectively, while the ordered structures of Rh1-xPtx are predicted to be superlattices stacked along the [012] directions, While the calculated ordering temperatures for these intermetallic compounds are too low to enable direct growth into the ordered phase, diffuse-scattering experiments at higher temperatures should reveal ordering rather than phase-separation characteristics (i.e., off-Gamma peaks), The situation is very similar to the case of Ag-Au, where an ordering tendency is manifested both by a diffuse scattering intensity and by a negative enthalpy of mixing, An experimental reexamination of Pd-Pt and Rh-Pt is needed.