Thermodynamic data from redox reactions at high temperatures. VI. Thermodynamic properties of CoO-MnO solid solutions from emf measurements

Activities of CoO in (Co,Mn)O solid solutions in contact with metallic Co h ave been determined on ten compositions ranging from 0.12 to 0.84 X-CoO in order to calibrate the divariant equilibrium between (Co,Mn)O oxide solutio ns and Co metal as an oxygen fugacity sensor for application in experimenta l petrology. Experiments were conducted over the temperature range 900-1300 K at 1 bar, using an electrochemical technique with oxygen-specific calcia -stabilized zirconia (CSZ) electrolytes. Co + CoO or Fe + FeO was used as t he reference electrode. Compositions of the (Co,Mn)O solid solutions were m easured after each run by electron microprobe, and these were checked for i nternal consistency by measuring the lattice parameter by X-ray diffraction . Activity-composition relations were fitted to the Redlich-Kister formalis m. (Co,Mn)O solid solutions exhibit slight positive deviations from idealit y, which are symmetrical (corresponding to a regular solution mixing model) across the entire composition range with A(0)(G) = 3690(+/-47) Jmol(-1). E xcess entropies and enthalpies were also derived from the emf data and gave S-ex = 0.77(+/-0.08) JK(-1) mol(-1) and H-ex = 4558(+/-90) Jmol(-1) respec tively. The experimental data from this study have been used to formulate t he (Co,Mn)O/Co oxygen fugacity sensor to give an expression: muO(2) = muO(2)(CoCoO) + 2RTlnX(CoO) + 2(1 - X-CoO)(2)[4558 - 0.773T] (900 < T (K) < 1300) where muO(2)(CoCoO) = -492,186 + 509.322 T - 53.284 T lnT + 0.02518 T-2, ta ken from O'Neill and Pownceby (1993).