System Ho-Rh-O: Phase Equilibria, Chemical Potentials and Gibbs Energy of Formation of HoRhO3

Abstract

The thermodynamic properties of the HoRhO3 were determined in the temperature range from 900 to 1300 K by using a solid-state electrochemical cell incorporating calcia-stabilized zirconia as the electrolyte. The standard Gibbs free energy of formation of orthorhombic perovskite HoRhO3, from Ho2O3 with C-rare earth structure and Rh2O3 with orthorhombic structure, can be expressed by the equation; $$ \Updelta G_{{{\text{f}}({\text{ox}})}}^{ \circ } \left( { \pm 78} \right)/({\text{J}}/{\text{mol}}) = - 50535 + 3.85\left( {T/{\text{K}}} \right) $$ Using the thermodynamic data of HoRhO3 and auxiliary data for binary oxides from the literature, the phase relations in the Ho-Rh-O system were computed at 1273 K. Thermodynamic data for intermetallic phases in the binary Ho-Rh were estimated from experimental enthalpy of formation for three compositions from the literature and Miedema’s model, consistent with the phase diagram. The oxygen potential-composition diagram and three-dimensional chemical potential diagram at 1273 K, and temperature-composition diagrams at constant oxygen partial pressures were computed for the system Ho-Rh-O. The decomposition temperature of HoRhO3 is 1717(±2) K in pure O2 and 1610(±2) K in air at a total pressure p o = 0.1 MPa.