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 reaction 4CaMnO(2.50) + O-2(g) = 4CaMnO(3) The redox energetics of CaMnO3-delta 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-delta 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-delta. (c) 2005 Elsevier B.V. All rights reserved.