The structural phase transition of Ba2Fe2O5+ı from monoclinic to cubic in a N2 atmosphere has been
analyzed using simultaneous thermogravimetry-differential thermal analysis and mass spectroscopy
(TG-DTA-MS). It has been confirmed that the structural phase transition is reversible, of the first order,
endothermic, and involves a mass increase with increasing temperature. From the mass spectrometry,
direct evidence that the origin of the mass variation in the phase transition is the variation of oxygen
nonstoichiometry has been obtained. From the simultaneously observed TG curves, the variation of ı for
the transition from the monoclinic phase to the cubic phase has been evaluated to be about 0.008.