Phase relations in the system (chromium + rhodium + oxygen) and thermodynamic properties of CrRhO 3

Abstract

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, CrRhO 3 with rhombohedral structure ( R 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 Cr 2O 3. The thermodynamic properties of the CrRhO 3 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 / 2 Cr 2 O 3 ( solid ) + 1 / 2 Rh 2 O 3 ( solid ) → CrRhO 3 ( solid ) , Δ G ∘ ± 140 / ( J · mol - 1 ) = - 31 967 + 5.418 ( T / K ) , where Cr 2O 3 has the corundum structure and Rh 2O 3 has the orthorhombic structure. Thermodynamic properties of CrRhO 3 at T = 298.15 K have been evaluated. The compound decomposes on heating to a mixture of Cr 2O 3-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 ∘ = 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.