Phase equilibria and thermodynamics of (silver+rhodium+oxygen)

Abstract

An isothermal section of the phase diagram for (silver + rhodium + oxygen) at T = 1173 K has been established by equilibration of samples representing twelve different compositions, and phase identification after quenching by optical and scanning electron microscopy (s.e.m.), X-ray diffraction (x.r.d.), and energy dispersive analysis of X-rays (e.d.x.). Only one ternary oxide, AgRhO₂, was found to be stable and a three phase region involving Ag, AgRhO₂ and Rh₂O₃ was identified. The thermodynamic properties of AgRhO₂ were measured using a galvanic cell in the temperature range 980 K to 1320 K. Yttria-stabilized zirconia was used as the solid electrolyte and pure oxygen gas at a pressure of 0.1 MPa was used as the reference electrode. The Gibbs free energy of formation of the ternary oxide from the elements, Δ_fG^o(AgRhO₂), can be represented by two linear equations that join at the melting temperature of silver. In the temperature range 980 K to 1235 K, Δ_fG^o(AgRHO₂)/(J·mol^-1)=-249080+179.08 T/K (±120). Above the melting temperature of silver, in the temperature range 1235 K to 1320 K, Δ_fG^o(AgRHO₂)/(J·mol^-1)=-260400+188.24 T/K (±95). The thermodynamic properties of AgRhO₂ at T = 298.15 K were evaluated from the high temperature data. The chemical potential diagram for (silver + rhodium + oxygen) at T = 1200 K was also computed on the basis of the results of this study.