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 ½La₂O₃+Rh₂O₃→LaRhO₃, ΔG°=-70 780+4.89T(±90)J·mol^-1 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.