Analysis of Crystal Structure and Phase Transition of Partially Cation Substituted SrFeO3-
δ
Authors: Hashimoto Takuya , Shido Kosuke , Nagai Keina , Yoshino Taizo , Kato Chihiro , Murakami Hikaru , Yamada Ginga , Maehara Ryutaro , Nimura Shumpei , Matsuo Motoyuki , Morikawa Daisuke , Tsuda Kenji 
Published: December 2024

Transformation of ordered arrangement of oxide ion vacancies in solid oxides to random one is one of the probable methods for development of new oxide ion or mixed conductor. The crystal structures of SrFeO2.875 and SrFeO2.75 at room temperature are tetragonal and orthorhombic distorted perovskite, respectively, with ordered oxide ion vacancies. In this work, partial cation substitution to SrFeO3-δ
was examined as a method to transform the crystal structure to cubic perovskite with random distribution of oxide ion vacancies.

Two kinds of cations were examined as substituents. One was trivalent ions such as Y and lanthanoid (Ln). The other was Ca2+ or Ba2+ for controlling size of A-site ion resulting in tolerance factor. The samples were prepared by Pechini method. SrCO3 and CaCO3 were dissolved in dilute HNO3. BaCO3 was dissolved in citric acid. Y2O3 and Ln2O3 were dissolved in HNO3 or mixture of HNO3 and H2O2 with heating. As an Fe source, Fe(NO3)3•9H2O, whose purity was verified with mass of Fe2O3 after heating at 800 ºC, was employed and 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. The precursor was calcined at 750 ºC for more than 17 h in air. The obtained powder was pressed into pellet after pulverization and sintered at 1200 ºC for more than 17 h in air. The phase of the specimens was identified by X-ray diffraction measurements. Some specimens were subjected to Mössbauer spectroscopy and electron diffraction measurement for minute analysis of crystal structure. Thermal analyses such as TG-DTA and DSC were performed to clarify phase transition behavior.

X-ray diffraction measurements of the samples with nominal composition of Sr1-x
Ln
x
FeO3-δ
and SrFe1-x
Ln
x
O3-δ
revealed that La, Nd, Ho, Y, Tm, Yb, and Lu could be substituted for Sr site with x=0.1-0.2 resulting in cubic perovskite structure, whereas only Tm, Yb, and Lu could be substituted for Fe site. This tendency was successfully explained assuming that Ln3+ substituted site with smaller difference of ionic radius. Although spots assigned as long-range order was observed in electron diffraction pattern of Sr0.9Y0.1FeO2.80, they were diffuse ones with small intensity, indicating large randomness of oxide ion distribution. Broad Mössbauer spectrum of Sr0.9Y0.1FeO2.80 and Sr0.9Yb0.1FeO2.80 identified as summation of Fe3.5+ with main intensity and smaller Fe3+ and Fe4+ also indicated random distribution of oxide ion vacancy. No DTA signal was observed between 25-1000 ºC, indicating arrangement of oxide ion vacancies was maintained.

X-ray diffraction patterns of Sr1-x
Ca
x
FeO3-δ
were not assigned as cubic; whereas those of Sr1-x
Ba
x
FeO3-δ
with x=0.1, 0.15, and 0.2 were apparently identified as cubic perovskite. Iodometric titration revealed that Fe valence in Sr1-x
Ba
x
FeO3-δ
with x=0.1 and 0.2 was 3.5+. Because tolerance factor approaches 1.0 by partial substitution of Ba if Fe valence is 3.5+, the observed variation of crystal structure to apparently cubic could be explained by using tolerance factor. However, clear spots assigned as long-range order observed in electron diffraction patterns and Mössbauer spectrum showing both Fe3+ in octahedron and Fe4+ in pyramid indicated that crystal structure of Sr0.8Ba0.2FeO2.75 was not cubic but orthorhombic with ordered oxide ion vacancies. The X-ray diffraction pattern identified as apparent cubic could be attributed to small distortion of octahedron, pyramid, and their linkage from cubic perovskite. The endothermic peaks without mass variation were observed in TG-DTA and DSC curves in Sr1-x
Ba
x
FeO2.75, indicating the structural phase transition. The phase transition temperature observed at 420 ºC for SrFeO2.75 decreased to 200 ºC for Sr0.8Ba0.2FeO2.75. The crystal structure at higher temperature than the phase transition temperature was identified as cubic with random oxide ion arrangement because the clear spots assigned as long-range order in electron diffraction patterns at room temperature disappeared or became diffused ones at 250 ºC.

The investigation of electrical conductivity of cation substituted SrFeO3-δ
is in progress at present.