Entropy of oxidation and redox energetics of CaMnO3−δ

The heat capacities of CaMnO3 and CaMnO2.562 from 10 to 600 K are determined by adiabatic calorimetry and used to derive an average entropy for the following oxidation reaction4CaMnO2.50 + O2(g) = 4CaMnO3The redox energetics of CaMnO3−δ is analysed using four independent sets of experimental data: the entropy of oxidation reported here, the enthalpy of oxidation previously determined by reaction calorimetry and by coloumetric titration as well as data for the oxygen-stoichiometry of CaMnO3−δ as function of partial pressure of oxygen and temperature. We show very clearly the importance of experimental data for the non-configurational entropy. A solution model, in which a random distribution of oxygen atoms and oxygen vacancies on the O-sub-lattice is the main contribution to the structural disorder, largely reproduces the experimental data. The analysis furthermore suggests an energetic preference for square pyramidal coordination for manganese in CaMnO3−δ.