Entropy of oxidation and redox energetics of CaMnO3−δ

Abstract The heat capacities of CaMnO 3 and CaMnO 2.562 from 10 to 600 K are determined by adiabatic calorimetry and used to derive an average entropy for the following oxidation reaction 4CaMnO 2.50  + O 2 (g) = 4CaMnO 3 The redox energetics of CaMnO 3− 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 CaMnO 3− 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 CaMnO 3− δ .