Nonstoichiometry and reductive decomposition of CaMnO3−δ

The variation of the oxygen stoichiometry of CaMnO3−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. CaMnO3−δ is not stable under reducing conditions; at 1273 K, CaMnO3−δ decomposes at log(pO2/pO2o)=−1.8 with (3−δ)≅2.84, under the formation of Ca2MnO4−δ and CaMn2O4. The partial molar enthalpy and entropy of oxidation of CaMnO3−δ are derived for 2.92>(3−δ)>2.86. The obtained partial molar enthalpy of oxidation is constant (HO−HOo=−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.