Pt-Rh (Platinum-Rhodium)

Abstract

The Pt-Rh phase diagram in [Massalski2] was redrawn from [Moffatt]. The liquidus and solidus of this phase diagram are shown in Fig. 1. In addition, [Moffatt] showed a miscibility gap in the (Pt,Rh) phase with a critical point at 760 C, 50 at.% Rh, as predicted by [1959Rau] based on the difference in the melting points of Pt and Rh. [1994Oka] questioned the existence of the miscibility gap because of the following contradictory reports: The location of the critical point of the miscibility gap demands a large positive excess Gibbs free energy of about 17,180X(1 X) J/mol, i.e., a large positive Dmix H(L). On the other hand, [1983Nie] estimated that Dmix H(L) has a large negative value of about 6000X(1 X) J/mol based on a semi-empirical model. The estimation of [1983Nie] is more likely because this miscibility gap has never been confirmed experimentally. [2012Mai] confirmed the absence of the miscibility gap. Also, the absence of the miscibility gap can be foreseen just by observing the curvature of the solidus, as pointed out by [1994Oka]. Moreover, the existence of ordered phases at low temperatures, rather than a miscibility gap, was predicted by [1991Lu] and [1995Lu] based on firstprinciple quantum-mechanical calculations. [2009Poh] calculated the Pt-Rh equilibrium phase diagram by means of semi-grand canonical lattice Monte Carlo simulation. The result is shown in Fig. 1 (below 0 C). Pt-Rh crystal structure data shown in Table 1 have been adopted from [2009Poh].