Phase relations in the system La-Rh-O at 1223 K have been determined by examination of equilibrated samples by optical and scanning electron microscopy, powder X-ray diffraction (XRD), and energy-dispersive analysis of X-rays (EDAX). Only one ternary oxide, LaRhO3, with distorted orthorhombic perovskite structure (Pbnm, a = 0.5525, b= 0.5680, and c= 0.7901 nm) was identified. The alloys and intermetallics along the La-Rh binary are in equilibrium with La2O3. The thermodynamic properties of LaRhO3 were determined in the temperature range 890 to 1310 K, using a solid-state cell incorporating yttria-stabilized zirconia as the electrolyte. A new four-compartment design of the emf cell was used to enhance the accuracy of measurement. For the reaction

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The compound decomposes on heating to a mixture of La2O3, Rh and O2. The calculated decomposition temperatures are 1843 (± 5) K in pure O2 and 1728 (± 5) K in air at a pressure of 1.01 × 105 Pa. The phase diagrams for the system La-Rh-O at different partial pressures of oxygen are calculated from the thermodynamic information.