Analysis of structural phase transition from monoclinic Ba2Fe2O5 to cubic Ba2Fe2O5+δ

The phase transition behavior of single-phase monoclinic Ba2Fe2O5 was investigated under various oxygen partial pressure P(O2) values. The first-order phase transition involving mass increase and heat absorption was observed at about 900 °C by thermogravimetry–differential thermal analysis measurements under N2 flow with P(O2) of 10−4 atm. The crystal structure of the high-temperature phase was revealed to be cubic perovskite with disordered oxide ion vacancies. By TG–DTA and DSC measurements under various P(O2) values, it was clarified that the transition temperature and the variation in enthalpy, ΔH, at the phase transition decreased with an increase in P(O2), indicating that the phase transition involved not only the variation in crystal structure but also the uptake of oxygen from a gas phase. The decrease in ΔH at the phase transition with increasing P(O2) can be ascribed to the increase in the amount of oxygen absorbed at the phase transition. The phase boundary between the monoclinic Ba2Fe2O5 and cubic Ba2Fe2O5+δ phases obtained by DSC measurements at various P(O2) values can be explained using the variation in ΔH induced by P(O2).