Thermodynamics of TmRhO3, Phase Equilibria, and Chemical Potentials in the System Tm–Rh–O

Abstract

Phase equilibria in the system Tm–Rh–O at 1200 K is established by isothermal equilibration of selected compositions and phase identification after quenching to room temperature. Six intermetallic phases (Tm3Rh, Tm7Rh3, Tm5Rh3, Tm3Rh2, TmRh, TmRh2±δ) and a ternary oxide TmRhO3 are identified. Based on experimentally determined phase relations, a solid-state electrochemical cell is devised to measure the standard free energy of formation of orthorhombic perovskite TmRhO3 from cubic Tm2O3 and β-Rh2O3 in the temperature range from (900 to 1300) K. The results can be summarized as: ΔGf,ox° ± 104/J·mol–1 = −46474 + 3.925(T/K). Invoking the Neumann-Kopp rule, the standard enthalpy of formation of TmRhO3 from its constituent elements at 298.15 K is estimated as −1193.89 (± 2.86) kJ·mol–1. The standard entropy of TmRhO3 at 298.15 K is evaluated as 103.8 (± 1.6) J·mol–1·K–1. The oxygen potential-composition diagram and three-dimensional chemical potential diagram at 1200 K and temperature–composition diagrams at c...