Enthalpies of oxidation of CaMnO3-delta, Ca2MnO4-delta and SrMnO3-delta - Deduced redox properties

The enthalpies of oxidation of CaMnO3-delta, Ca2MnO4-delta, and SrMnO3-delt a have been determined by in situ oxidation in a high-temperature adiabatic calorimeter. CaMnO3, Ca2MnO4, and SrMnO3 were synthesized by the EDTA prec ursor or the ceramic method, and partly reduced materials were prepared by topotactic reduction in diluted H-2 (5-10%) at 290-355 degreesC. Unit cell dimensions and oxygen stoichiometry of partly reduced and reoxidized materi als were determined by powder X-ray diffraction, thermogravimetry, and iodo metric titration. The enthalpy of oxidation of SrMnO3-delta was considerabl y less exothermic than the corresponding value for CaMnO3-delta, and the en thalpy of oxidation of Ca2MnO4-delta was more exothermic than for CaMnO3-de lta. The enthalpy of oxidation is discussed in terms of the basicity of oxi des, the Goldschmidt tolerance factor, and the crystal structures of the ox idized and reduced materials. Finally, based on the enthalpy of oxidation, the oxygen defect chemistry of these materials is estimated using simple th ermodynamic models. Using the measured enthalpy of oxidation and estimated entropy of oxidation, the measured oxygen nonstoichiometry is reproduced ra ther well. A free fit of the same models to the experimental data gives ent halpies of oxidation that are far more exothermic than the measured values. The present findings demonstrate that simple models based on point defect equilibrium may give significant errors in the enthalpy and entropy of form ation of point defects, despite a good fit to the free energy.