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

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 ox ide, AgRhO2, was found to be stable and a three phase region involving Ag, AgRhO2 and Rh2O3 was identified. The thermodynamic properties of AgRhO2 wer e 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 oxyge n gas at a pressure of 0.1 MPa was used as the reference electrode. The Gib bs free energy of formation of the ternary oxide from the elements, Delta ( f)G(o) (AgRhO2), can be represented by two linear equations that join at th e melting temperature of silver. In the temperature range 980 K to 1235 K, Delta (f)G(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, Delta (f)G(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 + rho dium + oxygen) at T = 1200 K was also computed on the basis of the results of this study. (C) 2000 Academic Press.