Experimental investigation of phase equilibria of the Ir-Pt binary system in subsolidus region

Binary phase diagram Ir-Pt system has a wide miscibility gap whose boundaries are difficult to determine due to low diffusion of refractory metals. The system itself has a number of potential applications primarily in catalysis, where addition of iridium to pure platinum promotes higher activity. Therefore, the information about thermodynamic stability of bimetallic particles is necessary for research in this area. The Ir-Pt system was reinvestigated in subsolidus region by means of X-ray diffraction, SEM and TEM. The true equilibrium state of solid solutions in miscibility gap region was achieved via double-sided approach. Pairs of samples, mixture of pure metals and single-phase solid solution, were annealed at different temperatures until the equal phase composition is reached. To shorten annealing time needed for reaching the state of equilibrium, the porous nanosized samples were used, which were prepared from the mixture {(NH4)2[PtCl6] + (NH4)2[IrCl6]} and solid solution of corresponding complex salts(NH4)2[Ir0.5Pt0.5Cl6]. The temperature range was 600–1400 °C. The boundaries of miscibility gap revealed to be significantly lower than stated before, the critical point position was estimated to be ~ 790 °C at 55 at% Pt. The approach can be suitable for investigation in other refractory metals binary systems.