Thermodynamic Properties and Phase Equilibria for Pt-Rh Alloys

Alloys of platinum and rhodium are extensively used in thermocouples for high-temperature measurement and as clean and inert heating elements in experimental high-temperature furnaces. The phase diagram for the Pt-Rh system is inadequately characterized.[1] The liquidus has been measured by Müller[2] and Acken.[3] The results have been normalized using more recent values for the melting points of the two pure components.[1,4] The liquid-solid two-phase region in this isomorphous system is displaced upward with respect to the straight line joining the melting points of the pure metals Pt and Rh,[1,5] implying that the solid solution is relatively more stable than the liquid alloy. Since Pt and Rh belong to the same group of the periodic table, the behavior of the liquid phase is expected to be close to ideal. The nature of the two-phase lens would then imply negative deviation from ideality for the solid solution. However, Raub[6] has suggested the onset of solid-state immiscibility below ;1033 K, based on information for other binary systems involving platinum-group metals such as Pt-Ir, Pd-Ir, and Pd-Rh. Magnetic susceptibility data of Darling,[7] which show a sharp inflection in the susceptibility as a function of composition at XRh ≅ 0.23, have been cited in support of this hypothesis.[8] The miscibility gap has been retained in recent phase diagram compilations,[1,5] although Raub and Falkenburgh[9] were unable to confirm the existence of a miscibility gap even after annealing for 150 days at 873