Phase relations in the system (chromium plus rhodium plus 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 (3) over bar, 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. 1/2Cr(2)O(3) (solid) + 1/2Rh(2)O(3) (solid) -> CrRhO3 (solid), Delta G(o) +/- 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 O-2. The calculated decomposition temperatures are T = 1567 +/- 5 K in pure O-2 and T = 1470 +/- 5 K in air at a total pressure p(o) = 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. (C) 2008 Elsevier Ltd. All rights reserved.