transition temperature to 870◦C in a N2atmosphere, which is con-tradictory to thermodynamics. In our preceding study, some of theauthors carried out TG-DTA, DSC (differential scanning calorimetry)and high-temperature XRD analysis under various gas atmospheressuch as N2, Ar and O2/N2mixtures [10]. It was clarified that thestructural phase transition from monoclinic Ba2Fe2O5to cubicBa2Fe2O5involves a mass increase and heat absorption. The tem-perature, mass variation and enthalpy variation of the structuralphase transition depended on the oxygen partial pressure in thegas phase, P(O2), suggesting that the following chemical reactionformula could be applied to the structural phase transition: