| Abstract
| - The enthalpies of oxidation of CaMnO3-δ, Ca2MnO4-δ, and SrMnO3-δ have been determinedby in situ oxidation in a high-temperature adiabatic calorimeter. CaMnO3, Ca2MnO4, andSrMnO3 were synthesized by the EDTA precursor or the ceramic method, and partly reducedmaterials were prepared by topotactic reduction in diluted H2 (5−10%) at 290−355 °C. Unitcell dimensions and oxygen stoichiometry of partly reduced and reoxidized materials weredetermined by powder X-ray diffraction, thermogravimetry, and iodometric titration. Theenthalpy of oxidation of SrMnO3-δ was considerably less exothermic than the correspondingvalue for CaMnO3-δ, and the enthalpy of oxidation of Ca2MnO4-δ was more exothermic thanfor CaMnO3-δ. The enthalpy of oxidation is discussed in terms of the basicity of oxides, theGoldschmidt tolerance factor, and the crystal structures of the oxidized and reducedmaterials. Finally, based on the enthalpy of oxidation, the oxygen defect chemistry of thesematerials is estimated using simple thermodynamic models. Using the measured enthalpyof oxidation and estimated entropy of oxidation, the measured oxygen nonstoichiometry isreproduced rather well. A free fit of the same models to the experimental data givesenthalpies of oxidation that are far more exothermic than the measured values. The presentfindings demonstrate that simple models based on point defect equilibrium may givesignificant errors in the enthalpy and entropy of formation of point defects, despite a goodfit to the free energy.
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