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 ($R\overline{3}$, 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,$\begin{array}{cc}\hfill & 1/2{\text{Cr}}_2{\text{O}}_3(\text{solid})+1/2{\text{Rh}}_2{\text{O}}_3(\text{solid})\to {\text{CrRhO}}_3(\text{solid})\text{,}\hfill \\ \hfill & \mathrm{\Delta }{G}^{°}\pm 140/(\text{J}·{\text{mol}}^{-1})=-31967+5.418(T/\text{K})\text{,}\hfill \end{array}$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.