Phase relations in the system Ta–Rh–O and thermodynamic properties of TaRhO₄

Abstract

Phase relations in the system Ta–Rh–O were determined by analysis of quenched samples corresponding to thirteen compositions inside the ternary triangle after equilibration at 1273 K. All the Ta–Rh alloys were found to be in equilibrium with Ta₂O₅. Only one ternary oxide TaRhO4 was detected. Based on phase relations in the ternary system, a solid-state electrochemical cell, incorporating calcia-stabilized zirconia as the electrolyte, was designed to measure the standard Gibbs energy of formation (ΔG°, J mol−1) of TaRhO₄ in the temperature range from 900 to 1300 K. For the reaction, 1/2β-Ta₂O₅+1/2Rh₂O₃(ortho)→TaRhO₄ ΔG°=−42993+5.676T   (±85) The calculated decomposition temperatures of TaRhO₄ are 1644 ± 5 K in pure O₂ and 1543 ± 5 K in air at a total pressure po = 0.1 MPa. Thermodynamic properties of TaRhO₄ at 298.15 K have been evaluated from the results. The limited experimental thermodynamic data for Rh-rich alloys available in the literature are in fair accord with Miedema's model. The Gibbs energies of formation of the different phases in the binary system Ta–Rh were estimated based on these inputs, consistent with the binary phase diagram. Based on the thermodynamic information on the stability of various phases, an oxygen potential diagram for the system Ta–Rh–O at 1273 K was constructed. Also presented are temperature–composition diagrams for the ternary system at constant oxygen partial pressures (pO₂/po=0.212 and 10⁻⁶) calculated form the basic data.