Thermodynamics of NdRhO3 and phase relations in the system Nd–Rh–O

Phase equilibrium experiments indicate that NdRhO3 is the only ternary oxide in the system Nd–Rh–O at 1273 K; it has orthorhombically-distorted perovskite structure. By employing a solid-state electrochemical cell incorporating calcia-stabilized zirconia as the electrolyte, thermodynamic properties of NdRhO3 are determined. The standard Gibbs energy of formation of NdRhO3 from its component binary oxides in the temperature ranges from 900 to 1300 K can be expressed as: 1/2Rh2O3(ortho)+1/2Nd2O3(hex)=NdRhO3(ortho), Δ f ( o x ) G o / J mol − 1 ( ± 197 ) = − 66256 + 5.64 ( T / K ) . composition temperature of NdRhO3 computed from extrapolated thermodynamic data is 1803 (±4) K in pure oxygen and 1692 (±4) K in air at standard pressure. Oxygen partial pressure–composition diagram and three-dimensional chemical potential diagram at 1273 K are developed from thermodynamic data obtained in this study and auxiliary information from the literature. Equilibrium temperature–composition phase diagrams at constant oxygen partial pressures are also constructed.