Mi. Pownceby et Hsc. O'Neill, Thermodynamic data from redox reactions at high temperatures. VI. Thermodynamic properties of CoO-MnO solid solutions from emf measurements, CONTR MIN P, 140(1), 2000, pp. 28-39
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).