Miscibility gap by oxygen content in SrFeO_3-δ: examination using X-ray diffraction and Mössbauer spectroscopy

SrFeO_3-δ is a promising candidate for applications in mixed conductors. However, the dependence of the phase as a function of δ and temperature of SrFeO_3-δ remains unclear. Herein, SrFeO_3-δ specimens with controlled and homogeneous oxygen content were prepared via quenching in liquid N₂ after annealing in air at various temperatures. The relationship between the oxygen content and crystal structure of SrFeO_3-δ was examined via X-ray diffraction (XRD) and Mössbauer spectroscopy at room temperature. XRD results indicated that SrFeO_2.87 and SrFeO_2.76 were apparently single tetragonal and orthorhombic phases, respectively; however, trace amounts of the orthorhombic phase were detected in the Mössbauer spectrum of SrFeO_2.87. SrFeO_3-δ with 3-δ ≥ 2.90 prepared via annealing under P(O₂) ≥ 0.35 bar contained a small amount of cubic SrFeO₃ apart from the tetragonal phase. Hence, it was concluded that tetragonal SrFeO_3-δ was a line phase with oxygen content between 2.87 and 2.90 and that miscibility gap composed of tetragonal and cubic phase presented at 2.90 ≤ 3-δ ≤ 3.00. The specimens with 3-δ ≤ 2.73 contained a small amount of brownmillerite SrFeO_2.5 apart from the orthorhombic, indicating the presence of a miscibility gap of brownmillerite and orthorhombic at 3-δ ≤ 2.73. Both XRD and Mössbauer spectroscopy indicated that SrFeO_3-δ with 2.80 ≤ 3-δ ≤ 2.86 were a mixture of the tetragonal and orthorhombic phases. The ratio of the orthorhombic phase increased with decreasing oxygen content, indicating the presence of a miscibility gap of the tetragonal and orthorhombic phases for 2.76 ≤ 3-δ ≤ 2.87.