Elucidation of Structural Phase Diagram of SrFeO3-δ
-Evidence of Miscibility Gap and Effect of Structural Phase Transition on Electrical Conduction Behavior
Authors: Yoshino Taizo , Shido Kosuke , Hatano Shiho , Sugimoto Takayuki , Hashimoto Takuya , Matsuo Motoyuki 
Published: December 2024

The crystal structures of SrFeO2.875 and SrFeO2.75 at room temperature are tetragonal and orthorhombic distorted perovskite, respectively, with ordered oxide ion vacancies. It was reported that SrFeO3-δ
underwent the structural phase transition to cubic with random oxide ion distribution around 300 ºC. However, there are contradictions in literature on the phase transition temperature. It was probably because the preparation method for specimens with homogeneous oxygen content was not established. In this work, SrFeO3-δ
specimens with homogeneous oxygen content was prepared and their crystal structures and phase transition behavior were investigated by X-ray diffraction, Mössbauer spectroscopy, TG-DTA, and DSC. The structural phase diagram of SrFeO3-δ
on oxygen content and temperature was proposed and existence of miscibility gap and its disappearance with increasing temperature were clarified. Impact of the phase transition on electrical conductivity was also investigated.

The SrFeO3-δ
samples were prepared by Pechini method. SrCO3 was dissolved in dilute HNO3. Fe(NO3)3•9H2O, whose purity was verified with mass of Fe2O3 after heating at 800 ºC, was dissolved in distilled H2O. The dissolved raw materials were mixed with nominal composition. After addition of citric acid and ethylene glycol, the solution was heated, resulting in precursor, which was calcined at 750 ºC for 17 h in air. The obtained powder was pressed into pellet and sintered at 1200 ºC for 17 h in air. The phase of the pellet was confirmed to be single perovskite structure by X-ray diffraction measurements. Oxygen content in the pellet was determined to be 2.872 by iodometric titration. For preparation of the specimens with controlled oxygen content, 3-δ, some pellets were annealed in air at 400-800 ºC followed by quenching into liquid N2. The oxygen content, determined by iodometric titration, agreed with ones prospected by TG curves measured in air, indicating specimens with homogeneous oxygen content were obtained. For preparation of the specimens with larger oxygen content, some pellets were annealed at 400 ºC in O2/N2 mixed gas with O2 partial pressure of 0.35 atm or more. The crystal structures of the specimens with controlled oxygen content were evaluated by X-ray diffraction and Mössbauer spectroscopy. TG-DTA, DSC and X-ray diffraction at high temperature were performed to clarify the phase transition behavior. The electrical conductivity was measured at high temperature for investigation of impact of the phase transition.

The oxygen content of the specimens quenched from 400 ºC and 600 ºC were 2.87 and 2.76, respectively. X-ray diffraction measurements revealed that the former and the latter specimens were single phase of tetragonal and orthorhombic perovskite. The Mössbauer spectrum of the former specimen was composed of Fe4+ in pyramid, Fe4+ and Fe3.5+ in octahedron, with mean Fe valence of 3.75+. That of the latter one was composed of Fe4+ in pyramid and Fe3+ in octahedron with mean Fe valence of 3.50+. The former and the latter Mössbauer spectra showed agreement with structural parameters of tetragonal and orthorhombic SrFeO3-δ
, respectively. The specimens with oxygen content between 2.76 and 2.87 were successfully prepared by controlling quenching temperature between 400 and 600 ºC. The X-ray diffraction patterns and Mössbauer spectra indicated that the specimens with oxygen content between 2.76 and 2.87 was mixture of tetragonal and orthorhombic perovskite. Content of orthorhombic phase systematically decreased with increasing oxygen content, indicating miscibility gap existed between tetragonal and orthorhombic phase. By annealing under oxygen partial pressure of 0.35 atm or more, the specimens with mixture of tetragonal and cubic were obtained, showing miscibility gap also existed between tetragonal and cubic phase.

TG-DTA, DSC, and high temperature X-ray diffraction revealed that tetragonal single phase and orthorhombic one underwent the reversible phase transition to cubic perovskite at 300 ºC and 420 ºC, respectively. For the specimens identified as mixture of the both phases, phase transition from tetragonal to cubic was observed at 300 ºC; whereas the DTA and DSC peaks identified as the phase transition from orthorhombic to cubic was broad and its temperature decreased with increasing content of tetragonal phase. Fig. 1 shows the proposed phase diagram of SrFeO3-δ
. The stable phase below 300 ºC was only cubic SrFeO3, tetragonal SrFeO2.875, and orthorhombic SrFeO2.75. The miscibility gaps existed between cubic and tetragonal and between tetragonal and orthorhombic phases. The miscibility gap between tetragonal and orthorhombic phase disappeared and cubic perovskite phase was obtained above the temperature between 300 ºC and 420 ºC, which increased with increasing orthorhombic content. The slight but sudden increase of electrical conductivity was observed at the phase transition from tetragonal to cubic, which will be minutely reported at the presentation.

Figure 1