Abstract
The variation of the oxygen stoichiometry of CaMnO 3− δ with the partial pressure of oxygen has been measured by coulometric titration at 1223, 1273 and 1373 K, and the technique used is described in some detail. CaMnO 3− δ is not stable under reducing conditions; at 1273 K, CaMnO 3− δ decomposes at log( p O 2 / p O 2 o)=−1.8 with (3− δ)≅2.84, under the formation of Ca 2MnO 4− δ and CaMn 2O 4. The partial molar enthalpy and entropy of oxidation of CaMnO 3− δ are derived for 2.92>(3− δ)>2.86. The obtained partial molar enthalpy of oxidation is constant ( H O− H O o=−163.5±7.5 kJ mol −1) within the estimated uncertainty (twice the standard deviation of the mean), whereas the partial molar entropy of oxidation appears to vary with the oxygen stoichiometry.
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