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A solid‐state electrochemical cell with yttria‐stabilized zirconia (YSZ) as the electrolyte is used to measure accurately thermodynamic properties of PrRhO3 in the range of temperature from 875 to 1325 K. The standard Gibbs energy of formation of PrRhO3 with orthorhombically distorted perovskite structure from its binary oxides β‐Rh2O3 and hexagonal Pr2O3 is given by, ΔGf)(oxo±50/Jmol-1=-67813+5.299T/K. Invoking the Newmann‐Kopp rule, the standard enthalpy of formation from elements and standard entropy of PrRhO3 at 298.15 K are evaluated: ΔHfo=-1175.53±3.26kJmol-1 and Snormalo=108.89±0.1.3J mol-1normalK-1. Phase relations in the system Pr–Rh–O at 1200 K are computed from thermodynamic data. Calorimetric data on enthalpy of formation of two intermetallic compounds are combined with the semi‐empirical model of Miedema and phase diagram to estimate Gibbs energy of formation for the intermetallics and liquid alloys. An isothermal section of ternary phase diagram, an oxygen potential‐composition diagram in 2‐D and a 3‐D chemical potential diagram for the system Pr–Rh–O at 1200 K are presented. In addition, temperature‐composition diagrams at different oxygen pressures are developed. The diagrams provide a road map for design and optimization of processing routes for catalysts based on Rh.

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