Structural phase transition behavior of tetragonal and orthorhombic SrFeO3-δ and its effects on thermal expansion and electrical conduction properties

Oxides with a high concentration of oxide-ion vacancies and high degree of crystal symmetry have attracted interest as high oxide-ion or hole and oxide-ion mixed conductors. For development of new oxide or mixed conductors, the structural phase transition of SrFeO_3-δ from tetragonal or orthorhombic perovskite with an ordered arrangement of oxide-ion vacancies to cubic perovskite with a random arrangement of oxide-ion vacancies was investigated via thermogravimetric-differential thermal analysis and X-ray diffraction at various temperatures. SrFeO_2.87 with tetragonal perovskite underwent the first-order structural phase transition to cubic perovskite without variation of δ at approximately 300 °C as has been frequently reported; however, the first-order structural phase transition of SrFeO_2.75 from orthorhombic to cubic without variation of δ occurred at approximately 420 °C, which was evidenced for the first time in this study. An abrupt increase was noted in both the thermal expansion and electrical conductivity with each phase transition. Our findings related to the effect of the structural phase transition on the thermal expansion and electrical conductivity of two different SrFeO_3-δ systems may guide their application as electrodes in solid oxide fuel cells and gas sensors.