Nonstoichiometry and reductive decomposition of CaMnO3-delta

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