System Gd–Rh–O: Thermodynamics and phase relations

Thermodynamic properties of GdRhO3 are investigated in the temperature range from 900 to 1300 K by employing a solid-state electrochemical cell, incorporating calcia-stabilized zirconia as the electrolyte. The standard Gibbs free energy of formation of GdRhO3 from component binary oxide Gd2O3 with C-rare earth structure and Rh2O3 with orthorhombic structure can be expressed as;ΔGf(ox)o(±60)/Jmol-1=-56603+3.78(T/K)Based on the thermodynamic information on GdRhO3 from experiment and auxiliary data for binary oxides from the literature and estimated properties of Gd–Rh alloys, phase relations are computed for the system Gd–Rh–O at 1273 K. Gibbs free energies for intermetallic phases in the binary Gd–Rh are evaluated using calorimetric data available in the literature for two compositions and Miedema’s model, consistent with the binary phase diagram. Isothermal section of the ternary phase diagram, oxygen potential-composition diagram and a 3-D chemical potential diagram for the system Gd–Rh–O at 1273 K are developed. Phase relations in the ternary Gd–Rh–O are also computed as a function of temperature at constant oxygen partial pressures. The ternary oxide, GdRhO3 decomposes to Gd2O3 with B-rare earth structure, metallic Rh and O2 at 1759(±2)1759(±2) K in pure O2 and 1649(±2)1649(±2) K in air at a total pressure P0=0.1MPa.

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► Paper reports the first measurement thermodynamic properties of GdRhO3.
► Isothermal section of the T–X phase diagram for the system Gd–Rh–O at 1273 K.
► 2-D oxygen potential-composition diagram for the ternary system at 1273 K.
► 3-D chemical potential diagram at 1273 K.
► 2-D temperature-composition diagram at constant oxygen partial pressures.