Phase Relations in the System La‐Rh‐O and Thermodynamic Properties of LaRhO3

Abstract

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 image 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.