Phase relations in the system (chromium + rhodium + oxygen) and thermodynamic properties of CrRhO3

Phase relations in the system (chromium + rhodium + oxygen) at T = 1273 K have been determined by examination of equilibrated samples by optical and scanning electron microscopy, powder X-ray diffraction (XRD), and energy dispersive spectroscopy (EDS). Only one ternary oxide, CrRhO3 with rhombohedral structure (R3¯, a = 0.5031, and c = 1.3767 nm) has been identified. Alloys and the intermetallics along the (chromium + rhodium) binary were in equilibrium with Cr2O3. The thermodynamic properties of the CrRhO3 have been determined in the temperature range (900 to 1300) K by using a solid-state electrochemical cell incorporating calcia-stabilized zirconia as the electrolyte. For the reaction,1/2Cr2O3(solid)+1/2Rh2O3(solid)→CrRhO3(solid),ΔG∘±140/(J·mol-1)=-31967+5.418(T/K),where Cr2O3 has the corundum structure and Rh2O3 has the orthorhombic structure. Thermodynamic properties of CrRhO3 at T = 298.15 K have been evaluated. The compound decomposes on heating to a mixture of Cr2O3-rich sesquioxide solid solution, Rh, and O2. The calculated decomposition temperatures are T = 1567 ± 5 K in pure O2 and T = 1470 ± 5 K in air at a total pressure p∘ = 0.1 MPa. The temperature-composition phase diagrams for the system (chromium + rhodium + oxygen) at different partial pressures of oxygen and an oxygen potential diagram at T = 1273 K are calculated from the thermodynamic information.