Analysis of chemical state of Fe in cubic Sr1-xLnxFeO3-δ and SrFe1‑xLnxO3-δ (Ln: Tm, Yb, Lu) by Mössbauer spectroscopy and its effect on electrical conduction property

Analysis of chemical state of Fe in cubic Sr1-xLnxFeO3-δ and SrFe1‑xLnxO3-δ (Ln: Tm, Yb, Lu) by Mössbauer spectroscopy and its effect on electrical conduction property | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Analysis of chemical state of Fe in cubic Sr 1-x Ln x FeO 3- δ and SrFe 1‑ x Ln x O 3- δ ( Ln : Tm, Yb, Lu) by Mössbauer spectroscopy and its effect on electrical conduction property Keina Nagai, Ryutaro Maehara, Takayuki Sugimoto, Kosuke Shido, and 2 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7605029/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 11 Nov, 2025 Read the published version in Journal of Radioanalytical and Nuclear Chemistry → Version 1 posted You are reading this latest preprint version Abstract Oxide materials with cubic perovskite structure and randomly distributed oxide-ion vacancies were synthesized by partially substituting Ln ( Ln : Tm, Yb, Lu) for Sr or Fe in SrFeO 3‑ δ . Mössbauer spectroscopy showed that the Fe chemical state depended on the substitution site rather than the Ln type. Although Sr 0.9 Ln 0.1 FeO 3- δ and SrFe 0.9 Ln 0.1 O 3- δ exhibited similar mean Fe valence, the former demonstrated greater charge carrier delocalization, higher conductivity and lower activation energy. Notably, Sr 0.9 Tm 0.1 FeO 3‑ δ exhibited the highest conductivity without structural phase transition, indicating strong potential as an electrode material for solid oxide fuel cells. Sr1‑xLnxFeO3−δ SrFe1‑xLnxO3−δ X-ray diffraction Mössbauer spectroscopy electrical conductivity Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Article highlights Tm, Yb, & Lu with small ionic radius can be substituted for both Sr and Fe sites, resulting in cubic perovskite structure Chemical state of Fe and charge delocalization was found to not depend on the type of Ln but on the substitution site With high electrical conductivity comparable to cubic SrFeO 2.875 , Sr 0.9 Tm 0.1 FeO 3‑ δ is a promising material for fuel cells 1. Introduction SrFeO 3− δ -based perovskite oxides have attracted much interest as electrode materials for a number of applications, including in solid oxide fuel cells [ 1 – 4 ], O 2 or N 2 gas separation [ 5 , 6 ], O 2 gas storage [ 7 – 10 ], and catalysts for methane oxidation [ 11 , 12 ]. Under high temperature conditions, SrFeO 3− δ adopts a cubic crystal structure with a random distribution of oxide-ion vacancies. At room temperature, the crystal structure of SrFeO 3− δ is a tetragonal or orthorhombic perovskite with an ordered arrangement of oxide-ion vacancy [ 13 , 14 ]. The structural phase transition from tetragonal or orthorhombic to the cubic phase occurs at approximately 300°C or 420°C [ 15 – 21 ], respectively, which involves an abrupt variation in volume and electrical conduction property [ 19 – 21 ]. Therefore, the stabilization of the cubic phase from room temperature to higher temperatures is necessary. One of the methods for the stabilization of the cubic phase is the partial substitution of trivalent ions such as La, Y, Nd, Sm, Eu, and Gd for the Sr site [ 22 – 28 ]. In our previous work, we clarified that Yb, which has a smaller ionic radius among trivalent ions, could partially substitute for the Sr and Fe sites, resulting in a cubic perovskite structure [ 29 ]. The variation in the chemical state of Fe by substitutional sites or species may affect certain properties of perovskites, such as their electrical conductivity gas separation properties. Therefore, the analysis of the chemical state of Fe in the cubic phase prepared by the partial substitution of trivalent ions is necessary for the design of new functional oxides. X-ray photoelectron spectroscopy (XPS) [ 2 – 4 ] and X-ray absorption spectroscopy [ 8 , 9 ] (XAS) have been proposed as methods for analyzing the chemical state of Fe. However, the signals obtained by XPS reflect only the surface of the specimen, and variations in the surface due to exposure to high vacuum and X-ray irradiation are inevitable. The chemical state of the bulk specimen in air was observed using XAS. However, the chemical shift due to the variation in Fe valence is only 1–3 eV at most, which is an insufficient resolution to clarify the relationship between the function and chemical state of Fe. Mössbauer spectroscopy, which can be performed in air and provide information on the chemical state of Fe from bulk with high resolution, is a powerful tool for clarifying the chemical state of Fe and its correlation to the properties of perovskites. In an earlier study, we reported a clear difference in Mössbauer spectroscopy between Sr 1 − x Yb x FeO 3− δ and SrFe 1- x Yb x O 3- δ owing to the difference in the chemical state of Fe, showing correspondence with the variation in electrical properties by substitutional site [ 29 ]. It is probable that Ln 3+ other than Yb can be partially substituted for both Sr and Fe sites, resulting in a cubic perovskite structure. However, the preparation of cubic SrFe 1 − x Ln x O 3− δ with Ln other than Yb has not yet been reported. In this study, the probability of partial substitution of Tm and Lu, which have almost the same ionic radius as Yb [ 30 ], for the Sr or Fe sites in SrFeO 3− δ was verified. For the specimens with a cubic perovskite structure, the chemical state of Fe was examined using Mössbauer spectroscopy to clarify whether it was affected by substitutional species or sites. The electrical conduction behavior was also measured, and the effect of the chemical state of Fe was examined. It is expected that new partially Ln -substituted SrFeO 3− δ with higher electrical properties could be developed as materials for solid oxide fuel cells and so on. 2. Experimental Specimens with nominal compositions of Sr 1 − x Ln x FeO 3− δ and Sr 1 − x Ln x FeO 3− δ ( Ln = Tm, Yb, Lu) were prepared from SrCO 3 , Ln 2 O 3 , and Fe(NO 3 ) 3 •9H 2 O using the Pechini method. Before weighing, SrCO 3 (99.9%, Kanto Chem. Corp.) was dried at 150°C, and the purity of Fe(NO 3 ) 3 •9H 2 O (99.9%, FUJIFILM Wako) was verified from the mass of residual Fe 2 O 3 after heating at 800°C for > 12 h. Ln 2 O 3 powder (99.9%, Kojundo Chemical Laboratory Co., Ltd.) was heated at 1000°C for > 17 h in air to remove the carbonate or hydroxide at the surface. Nominal amounts of SrCO 3 , Fe(NO 3 ) 3 •9H 2 O, and Ln 2 O 3 were dissolved in dilute HNO 3 , distilled H 2 O, and a mixture of hot H 2 O 2 and concentrated HNO 3 , respectively, and the resulting solutions were mixed. After the addition of citric acid and ethylene glycol, both with more than double the molar content of the total cation content, the solution was heated in a mantle heater, producing a solid precursor. After the calcination of the precursor at 750°C for 17 h in air, the powder was pressed into 10-mmφ pellets via uniaxial pressing and sintered at 1200°C for 17 h in air. The pellets were then reground and pressed into cuboids with dimensions of approximately 3 × 4 × 14 mm 3 via cold isostatic pressing and sintered at 1200°C for 17 h, followed by cooling at 200°C·h − 1 in air. The crystal structures of the specimens at room temperature were analyzed via powder X-ray diffraction (XRD) measurements using a RINT-2500 (Rigaku Corp., CuKα, 50 kV, 250 mA). To analyze the chemical state of Fe in the specimens, 57 Fe Mössbauer spectroscopy was performed. The Mössbauer spectra were measured using a Topologic Systems MFD-110D spectrometer with a 57 Co/Rh source at room temperature. The obtained spectra were curve-fitted using the least-squares method, assuming a Lorentzian line shape in each spectrum. The intensity and half-width of the peaks within each quadrupole doublet were constrained to be equal. The constituent iron species were identified by comparing the Mössbauer parameters with reference data. The relative content of individual iron species in an analyzed sample was calculated from the peak area in the Mössbauer spectrum, assuming identical recoil-free fractions for all species. The isomer shifts were expressed with respect to the centroid of the spectrum of metallic iron foil. TG-DTA (TG8120: Rigaku Corp.) was measured to determine whether the oxygen content was maintained at a temperature as high as 450°C and whether the phase transition observed in SrFeO 3− δ disappeared by Ln substitution. The specimen (~ 30 mg) and Al 2 O 3 (~ 20 mg) powders as the sample and reference, respectively, were placed in a Pt pan, and TG-DTA curves were obtained under air with a heating rate of 10°C/min. The total electrical conductivity of the DC and its temperature dependence were measured using the four-probe method, with a Pt mesh and wire as the current and voltage electrodes, respectively. The applied current range was ± 10 mA, and a linear relationship between the current and voltage was confirmed for each measurement. The measurements were performed during heating at a rate of 1°C·min − 1 in an air atmosphere. 3. Results and discussion 3.1. Analysis of crystal structure by X-ray diffraction Figure 1 shows the XRD patterns of the representative peaks of the specimens with nominal compositions of (a) Sr 1 − x Tm x FeO 3− δ and SrFe 1 − x Tm x O 3− δ and (b) Sr 1 − x Lu x FeO 3− δ and SrFe 1 − x Lu x O 3‑ δ . For SrFeO 3− δ , peak splitting owing to distortion of the tetragonal and orthorhombic structures was observed in the 2 θ range of 68.0–69.5° and 86.5–88.0°. Single peaks indexed as 220 and 222 of the cubic structure were observed in the 2 θ range of 67.0–69.5° and 85.0–88.0°, respectively, for Sr 1- x Tm x FeO 3− δ with 0.10 ≤ x ≤ 0.20 and SrFe 1 − x Tm x O 3− δ with x = 0.10. No peak identified as the second phase was observed in the XRD patterns in the 2 θ range of 10.0–110.0° of these specimens, indicating the successful single-phase preparation of Sr 1- x Tm x FeO 3− δ with 0.10 ≤ x ≤ 0.20 and SrFe 0.9 Tm 0.1 O 3− δ with a cubic perovskite structure. Peaks that could not be indexed as cubic perovskite were observed in the XRD patterns of Sr 1- x Tm x FeO 3− δ with x ≥ 0.25 and SrFe 0.8 Tm 0.2 O 3− δ , indicating the solubility limit of Sr 1- x Tm x FeO 3- δ and SrFe 1 − x Tm x O 3− δ . Similarly, it was concluded that a single phase with a cubic perovskite structure was obtained for Sr 1 − x Lu x FeO 3− δ with 0.10 ≤ x ≤ 0.20 and SrFe 1 − x Lu x O 3− δ with 0.10 ≤ x ≤ 0.20, showing the same tendency as the Yb-substituted specimens [ 29 ]. Figure 2 shows the lattice constants of the Tm- and Lu-substituted specimens with a cubic structure calculated from the Bragg angles of the 222 peaks. The lattice constants of the Yb-substituted specimens [ 28 ] are presented for comparison. A slight decrease and a large increase in the lattice constants were observed with an increase in the amount of Ln 3+ substituted for the Sr and Fe sites, respectively, indicating successful substitution for every Ln 3+ examined in this study. 3.2. Evaluation of chemical state of Fe in cubic Sr 0.9 Ln 0.1 FeO 3− δ and SrFe 0.9 Ln 0.1 O 3− δ by Mössbauer spectroscopy Figure 3 shows the Mössbauer spectra of Sr 0.9 Ln 0.1 FeO 3− δ . The Mössbauer spectrum of Sr 0.9 Y 0.1 FeO 2.80 [ 29 ], with which mean valence of Fe and oxygen content were determined to be 3.50 + and 2.80, respectively, by iodometric titration, is shown for comparison. A similar peak shape was observed for each Sr 0.9 Ln 0.1 FeO 3− δ sample as for Sr 0.9 Y 0.1 FeO 2.80 , suggesting an identical chemical state of Fe regardless of the type of Ln 3+ . Mössbauer spectra of cubic SrFe 0.9 Ln 0.1 O 3− δ are shown in Fig. 4 . Although almost identical peak shapes were observed for SrFe 0.9 Ln 0.1 O 3− δ regardless of the type of Ln , different peak shapes were observed for Sr 0.9 Ln 0.1 FeO 3−δ . To elucidate the difference in the chemical state of Fe between Sr 0.9 Ln 0.1 FeO 3− δ and SrFe 0.9 Ln 0.1 O 3− δ , curve fitting was performed according to the following guidelines. 1) The chemical state of Fe can be separated into three types: 3+, 3.5+, and 4+, as observed in the Mössbauer spectra of SrFeO 3− δ [ 31 – 35 ]. 2) The mean Fe valence in Sr 0.9 Y 0.1 FeO 3−δ calculated from the ratio of the peak area should be 3.50+, as confirmed by iodometric titration. The results of the curve fitting are shown in Figs. 3 and 4 , and the optimized parameters are summarized in Table 1. The mean Fe valences of Sr 0.9 Tm 0.1 FeO 3− δ , Sr 0.9 Yb 0.1 FeO 3− δ , and Sr 0.9 Lu 0.1 FeO 3− δ were 3.51(7)+, 3.44(7)+, and 3.52(7), respectively. Those of SrFe 0.9 Tm 0.1 O 3− δ , SrFe 0.9 Yb 0.1 O 3− δ , and SrFe 0.9 Lu 0.1 O 3− δ were 3.51(6), 3.50(7), and 3.52(7), respectively. The mean Fe valence of all the specimens examined in this study was concluded to be 3.50+. In addition, the equilibrium formula described as follows is satisfied in all the specimens because of the coexistence of Fe 3+ , Fe 4+ , and Fe 3.5+ : Fe 3+ + Fe 4+ ↔2Fe 3.5+ (1) This chemical equilibrium formula is also applicable to orthorhombic SrFeO 2.75 [ 34 ]. For Sr 0.9 Ln 0.1 FeO 3− δ , the intensity of the peak assigned to Fe 3.5+ was the largest regardless of the type of Ln , indicating chemical equilibrium, as represented by Eq. (1) proceeded to the right side, resulting in the delocalization of the charge carrier. In SrFe 0.9 Ln 0.1 O 3− δ , the chemical equilibrium was deduced as Eq. (1) proceeded relatively to the left side, resulting in a lower degree of charge delocalization because the intensities of the peaks assigned to Fe 3+ , Fe 4+ , and Fe 3.5+ were almost similar. However, the intensity of the peak assigned to Fe 3.5+ in SrFe 0.9 Ln 0.1 O 3− δ was larger than that in orthorhombic SrFeO 2.75 [ 34 ], indicating that the charge carrier was delocalized by the structural phase transition to cubic with Ln substitution for both Sr and Fe sites. The concentration of oxide-ion vacancies should be larger in SrFe 0.9 Ln 0.1 O 3− δ than in Sr 0.9 Ln 0.1 FeO 3− δ because the mean Fe valence in all the specimens was constant at 3.5+. Therefore, the repulsion between the Fe cations is expected to be larger in SrFe 0.9 Ln 0.1 O 3− δ , resulting in longer lattice constants, as shown in Fig. 2 . A longer distance between Fe ions can be the origin of charge localization, that is, proceeding to the left side of Eq. (1). The effect of charge delocalization on electrical conductivity is discussed in Section 3 . 3. 3.3. Electrical conductivity of cubic Sr 0.9 Ln 0.1 FeO 3− δ and SrFe 0.9 Ln 0.1 O 3− δ Figure 5 shows the temperature dependence of the electrical conductivity of Sr 0.9 Ln 0.1 FeO 3− δ and SrFe 0.9 Ln 0.1 O 3− δ in air. The data for SrFeO 2.86 and SrFeO 2.74 [ 21 ] are also shown for comparison. The semiconducting temperature dependence with activation energies of 0.09(2) and 0.16(2) eV without abrupt variation was observed for Sr 0.9 Ln 0.1 FeO 3− δ and SrFe 0.9 Ln 0.1 O 3− δ , respectively, while an abrupt variation originating from the structural phase transition to cubic was detected for SrFeO 2.86 and SrFeO 2.74 . The absence of abrupt variations in the electrical conductivity of Sr 0.9 Ln 0.1 FeO 3− δ and SrFe 0.9 Ln 0.1 O 3− δ is attributed to the absence of structural phase transitions and variations in the oxygen content, as observed in the TG-DTA curves shown in Fig. 6 . Although the mean Fe valence was almost the same at 3.50 + for all the Ln -substituted specimens and SrFeO 2.75 , the electrical conductivities of Sr 0.9 Ln 0.1 FeO 3− δ were higher than that of SrFeO 2.75 below 400°C and those of SrFe 0.9 Ln 0.1 O 3− δ . The higher electrical conductivity of the Ln -substituted specimens than that of SrFeO 2.75 below 400°C was attributed to the cubic structure with a larger degree of carrier delocalization observed by Mössbauer spectroscopy. Sr 0.9 Ln 0.1 FeO 3− δ exhibits higher electrical conductivity and activation energy than SrFe 0.9 Ln 0.1 O 3- δ , which is in agreement with the higher degree of carrier delocalization clarified by Mössbauer spectroscopy. The higher carrier delocalization can be attributed to the larger overlap of the Fe3d and O2p orbitals due to the shorter bond length observed in the XRD measurements. Among the examined species, Sr 0.9 Tm 0.1 FeO 3− δ exhibited the highest electrical conductivity, exceeding that of cubic SrFeO 2.87 with an Fe valence of 3.75+. These findings establish that Sr 0.9 Tm 0.1 FeO 3− δ has high potential as an electrode material for use in solid oxide fuel cells. 4. Conclusions Sr 1 − x Ln x FeO 3− δ and SrFe 1 − x Ln x O 3− δ ( Ln : Tm, Yb, and Lu) were prepared with a cubic perovskite structure and random distribution of oxide-ion vacancies. Mössbauer spectroscopy revealed the mean Fe valence of cubic Sr 0.9 Ln 0.1 FeO 3− δ and SrFe 0.9 Ln 0.1 O 3− δ to be 3.50+. Further, the chemical state of Fe was not found to be dependent on the type of Ln but on the substitution site. Mössbauer spectroscopy showed that the charge carrier delocalization was larger in Sr 0.9 Ln 0.1 FeO 3− δ than in SrFe 0.9 Ln 0.1 O 3− δ , indicating that Sr 0.9 Ln 0.1 FeO 3− δ possesses higher electrical conductivity than SrFe 0.9 Ln 0.1 O 3− δ . Among the prepared cubic specimens, Sr 0.9 Tm 0.1 FeO 3− δ was found to be the most promising electrode material for solid oxide fuel cells because it was free from abrupt variations in electrical conductivity caused by the structural phase transition, and exhibited the highest conductivity. By examining other Ln 3+ as substituents for both Sr and Fe sites, new cubic perovskite oxides with higher electrical properties could be developed as materials for solid oxide fuel cells. Trials for the preparation of other Ln 3+ substituted specimens and investigation of the chemical state of Fe and its electrical property are currently in progress and will be reported in due course. Declarations Acknowledgements Mössbauer measurements were carried out with the support of the Radioisotope Laboratory in the Graduate School of Science and Isotope Science Center, University of Tokyo. We thank Editage (www.editage.jp) for the English language editing. Author contributions KN: Investigation, Data curation, Formal analysis, Writing – original draft; RM: Investigation, Formal analysis, Writing – review and editing; TS: Methodology, Formal analysis, Visualization, Writing – review and editing; KS: Data curation, Investigation, Writing – review and editing; TH: Conceptualization, Supervision, Validation, Writing – review and editing; MM: Methodology, Data curation, Writing – review and editing. Data availability Data will be made available on request. Conflict of interests The authors have no competing interests to declare. References Fernández-Ropero AJ, Porras-Vázquez JM, Cabeza A, Slater PR, Marrero-López D, Losilla ER (2014) High valence transition metal doped strontium ferrites for electrode materials in symmetrical SOFCs. 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Interactions 245:299. https://doi.org/10.1007/s10751-024-02129-z Table Table 1 Parameters of Mössbauer spectra for Sr 0.9 Y 0.1 FeO 2.80 , Sr 0.9 Ln 0.1 FeO 3- δ , and SrFe 0.9 Ln 0.1 O 3‑ δ . I. S.: isomer shift, Q. S.: quadrupole splitting, H. W.: full width at half maximum Sample Species Area / % I.S. / mm·s -1 Q.S. / mm·s -1 H.W. / mm·s -1 Sr 0.9 Y 0.1 FeO 2.80 Fe 4+ 19.9±0.4 -0.016±0.006 0.420±0.009 0.310±0.013 Fe 3+ 20.2±0.6 0.187±0.003 1.257±0.019 0.365±0.027 Fe 3.5+ 59.9±0.7 0.243±0.003 0.574±0.015 0.492±0.023 Sr 0.9 Tm 0.1 FeO 3- δ Fe 4+ 20.0±0.3 -0.014±0.005 0.435±0.008 0.315±0.011 Fe 3+ 18.9±0.5 0.180±0.003 1.259±0.016 0.361±0.022 Fe 3.5+ 61.2±0.5 0.240±0.003 0.586±0.012 0.490±0.017 Sr 0.9 Yb 0.1 FeO 3- δ Fe 4+ 17.2±0.3 -0.004±0.006 0.412±0.008 0.295±0.012 Fe 3+ 28.6±0.5 0.198±0.002 1.191±0.016 0.394±0.019 Fe 3.5+ 54.2±0.6 0.240±0.003 0.554±0.015 0.458±0.022 Sr 0.9 Lu 0.1 FeO 3- δ Fe 4+ 18.0±0.3 -0.016±0.005 0.436±0.008 0.299±0.011 Fe 3+ 21.7±0.5 0.178±0.003 1.255±0.015 0.367±0.020 Fe 3.5+ 60.3±0.5 0.239±0.003 0.585±0.012 0.488±0.017 SrFe 0.9 Tm 0.1 O 3- δ Fe 4+ 33.6±0.4 -0.047±0.005 0.430±0.005 0.362±0.006 Fe 3+ 30.7±0.6 0.209±0.002 1.307±0.014 0.419±0.016 Fe 3.5+ 35.7±0.7 0.266±0.004 0.615±0.024 0.514±0.034 SrFe 0.9 Yb 0.1 O 3- δ Fe 4+ 36.0±0.5 -0.053±0.005 0.415±0.005 0.352±0.007 Fe 3+ 35.2±0.5 0.224±0.002 1.320±0.015 0.437±0.016 Fe 3.5+ 28.7±0.8 0.278±0.006 0.618±0.033 0.511±0.047 SrFe 0.9 Lu 0.1 O 3- δ Fe 4+ 37.1±0.5 -0.052±0.005 0.417±0.004 0.364±0.007 Fe 3+ 32.1±0.6 0.216±0.002 1.295±0.014 0.435±0.015 Fe 3.5+ 30.8±0.7 0.267±0.005 0.594±0.025 0.506±0.040 Additional Declarations No competing interests reported. Cite Share Download PDF Status: Published Journal Publication published 11 Nov, 2025 Read the published version in Journal of Radioanalytical and Nuclear Chemistry → Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. 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SrFe\u003csub\u003e1-\u003c/sub\u003e\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eTm\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eO\u003csub\u003e3-\u003c/sub\u003e\u003csub\u003e\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e and (b) Sr\u003csub\u003e1-\u003c/sub\u003e\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eLu\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eFeO\u003csub\u003e3-\u003c/sub\u003e\u003csub\u003e\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e and SrFe\u003csub\u003e1-\u003c/sub\u003e\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eLu\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eO\u003csub\u003e3-\u003c/sub\u003e\u003csub\u003e\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e. A and B represent the total cation contents of the A- and B-site ions, respectively, in the perovskite structure. Miller indices representing the cubic perovskite structure are attached. The peaks that cannot be assigned to the cubic perovskite are represented by ▲.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-7605029/v1/6b39088bbdd75c90ea990b90.png"},{"id":92408297,"identity":"8ab25991-ca1b-404b-a3d3-00998ad1c028","added_by":"auto","created_at":"2025-09-29 11:43:58","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":43743,"visible":true,"origin":"","legend":"\u003cp\u003eLattice constants of cubic Sr\u003csub\u003e1-\u003c/sub\u003e\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eTm\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eFeO\u003csub\u003e3-\u003c/sub\u003e\u003csub\u003e\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e, SrFe\u003csub\u003e1-\u003c/sub\u003e\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eTm\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eO\u003csub\u003e3-\u003c/sub\u003e\u003csub\u003e\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e, Sr\u003csub\u003e1-\u003c/sub\u003e\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eLu\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eFeO\u003csub\u003e3-\u003c/sub\u003e\u003csub\u003e\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e, and SrFe\u003csub\u003e1‑\u003c/sub\u003e\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eLu\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eO\u003csub\u003e3-\u003c/sub\u003e\u003csub\u003e\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e. The results for Sr\u003csub\u003e1-\u003c/sub\u003e\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eYb\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eFeO\u003csub\u003e3-\u003c/sub\u003e\u003csub\u003e\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e and SrFe\u003csub\u003e1-\u003c/sub\u003e\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eYb\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eO\u003csub\u003e3-\u003c/sub\u003e\u003csub\u003e\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e\u003csub\u003e \u003c/sub\u003e[29] are also depicted for comparison.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-7605029/v1/7260658d569fa3552a728db0.png"},{"id":92407658,"identity":"9dd5ca5d-41c3-4bde-9646-cecd10edb5c1","added_by":"auto","created_at":"2025-09-29 11:35:58","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":56161,"visible":true,"origin":"","legend":"\u003cp\u003eMössbauer spectra of Sr\u003csub\u003e0.9\u003c/sub\u003e\u003cem\u003eLn\u003c/em\u003e\u003csub\u003e0.1\u003c/sub\u003eFeO\u003csub\u003e3-\u003c/sub\u003e\u003csub\u003e\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e (\u003cem\u003eLn\u003c/em\u003e: Tm, Yb, Lu) with cubic perovskite structure. Mössbauer spectrum of Sr\u003csub\u003e0.9\u003c/sub\u003eY\u003csub\u003e0.1\u003c/sub\u003eFeO\u003csub\u003e3-\u003c/sub\u003e\u003csub\u003e\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e with a mean Fe valence of 3.50+ determined by iodometric titration [29] is shown for comparison. The observed data are represented by ●. The red, blue, and green curves represent the results of the curve fitting for Fe\u003csup\u003e4+\u003c/sup\u003e, Fe\u003csup\u003e3+\u003c/sup\u003e, and Fe\u003csup\u003e3.5+\u003c/sup\u003e, respectively. The black curves are the summation of the curves of Fe\u003csup\u003e4+\u003c/sup\u003e, Fe\u003csup\u003e3+\u003c/sup\u003e, and Fe\u003csup\u003e3.5+\u003c/sup\u003e, showing agreement with the observed data. The parameters for the curve fittings are listed in Table 1.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-7605029/v1/a6b9b7540ca30c31c6c5b774.png"},{"id":92407425,"identity":"21764837-2a38-4a4c-895d-e2a6ef699b9d","added_by":"auto","created_at":"2025-09-29 11:27:58","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":44753,"visible":true,"origin":"","legend":"\u003cp\u003eMössbauer spectra of cubic SrFe\u003csub\u003e0.9\u003c/sub\u003e\u003cem\u003eLn\u003c/em\u003e\u003csub\u003e0.1\u003c/sub\u003eO\u003csub\u003e3-\u003c/sub\u003e\u003csub\u003e\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e (\u003cem\u003eLn\u003c/em\u003e: Tm, Yb, Lu). The observed data are represented by ●. Red, blue, and green curves represent results of curve fitting as Fe\u003csup\u003e4+\u003c/sup\u003e, Fe\u003csup\u003e3+\u003c/sup\u003e, and Fe\u003csup\u003e3.5+\u003c/sup\u003e, respectively. Black curves are summation of the curves of Fe\u003csup\u003e4+\u003c/sup\u003e, Fe\u003csup\u003e3+\u003c/sup\u003e, and Fe\u003csup\u003e3.5+\u003c/sup\u003e, showing agreement with the observed data. The parameters of the curve fittings are listed in Table 1.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-7605029/v1/caaac86a60821095c355107d.png"},{"id":92407428,"identity":"0485a7f7-be89-41f1-8159-c0ca9fa73084","added_by":"auto","created_at":"2025-09-29 11:27:58","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":56869,"visible":true,"origin":"","legend":"\u003cp\u003eArrhenius plots of electrical conductivity of cubic Sr\u003csub\u003e0.9\u003c/sub\u003e\u003cem\u003eLn\u003c/em\u003e\u003csub\u003e0.1\u003c/sub\u003eFeO\u003csub\u003e3-\u003c/sub\u003e\u003csub\u003e\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e and SrFe\u003csub\u003e0.9\u003c/sub\u003e\u003cem\u003eLn\u003c/em\u003e\u003csub\u003e0.1\u003c/sub\u003eO\u003csub\u003e3-\u003c/sub\u003e\u003csub\u003e\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e (\u003cem\u003eLn\u003c/em\u003e: Tm, Yb, Lu) in air. The data for tetragonal SrFeO\u003csub\u003e2.87\u003c/sub\u003e and orthorhombic SrFeO\u003csub\u003e2.76\u003c/sub\u003e [21] are shown for comparison.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-7605029/v1/a77090ad9e7392b5ee7897aa.png"},{"id":92407660,"identity":"cbe3a613-3844-4f93-b0e7-14f66aba360b","added_by":"auto","created_at":"2025-09-29 11:35:58","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":45934,"visible":true,"origin":"","legend":"\u003cp\u003eTG-DTA curves of cubic Sr\u003csub\u003e0.9\u003c/sub\u003e\u003cem\u003eLn\u003c/em\u003e\u003csub\u003e0.1\u003c/sub\u003eFeO\u003csub\u003e3-\u003c/sub\u003e\u003csub\u003e\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e and SrFe\u003csub\u003e0.9\u003c/sub\u003e\u003cem\u003eLn\u003c/em\u003e\u003csub\u003e0.1\u003c/sub\u003eO\u003csub\u003e3-\u003c/sub\u003e\u003csub\u003e\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e (\u003cem\u003eLn\u003c/em\u003e: (a) Tm, (b) Yb, (c) Lu) in air. Mass variation was converted to oxygen content in the specimens, assuming a mean Fe valence of 3.50 + at room temperature.\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-7605029/v1/81292ae714e35bf1c5e87bb3.png"},{"id":96105068,"identity":"fecd4317-4de9-439c-b5ba-10719223a3e7","added_by":"auto","created_at":"2025-11-17 16:08:09","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1162311,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7605029/v1/bda94d93-3811-47cd-9259-51f9d9f1cbb7.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"\u003cp\u003eAnalysis of chemical state of Fe in cubic Sr\u003csub\u003e1-x\u003c/sub\u003e\u003cem\u003eLn\u003c/em\u003e\u003csub\u003ex\u003c/sub\u003eFeO\u003csub\u003e3-\u003c/sub\u003e\u003csub\u003e\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e and SrFe\u003csub\u003e1‑\u003c/sub\u003e\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eLn\u003c/em\u003e\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eO\u003csub\u003e3-\u003c/sub\u003e\u003csub\u003e\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e (\u003cem\u003eLn\u003c/em\u003e: Tm, Yb, Lu) by Mössbauer spectroscopy and its effect on electrical conduction property\u003c/p\u003e","fulltext":[{"header":"Article highlights","content":"\u003cul class=\"decimal_type\"\u003e\n \u003cli\u003eTm, Yb, \u0026amp; Lu with small ionic radius can be substituted for both Sr and Fe sites, resulting in cubic perovskite structure\u003c/li\u003e\n \u003cli\u003eChemical state of Fe and charge delocalization was found to not depend on the type of \u003cem\u003eLn\u003c/em\u003e but on the substitution site\u003c/li\u003e\n \u003cli\u003eWith high electrical conductivity comparable to cubic SrFeO\u003csub\u003e2.875\u003c/sub\u003e, Sr\u003csub\u003e0.9\u003c/sub\u003eTm\u003csub\u003e0.1\u003c/sub\u003eFeO\u003csub\u003e3‑\u003cem\u003e\u0026delta;\u003c/em\u003e\u003c/sub\u003e is a promising material for fuel cells\u0026nbsp;\u003c/li\u003e\n\u003c/ul\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e"},{"header":"1. Introduction","content":"\u003cp\u003eSrFeO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e-based perovskite oxides have attracted much interest as electrode materials for a number of applications, including in solid oxide fuel cells [\u003cspan additionalcitationids=\"CR2 CR3\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e], O\u003csub\u003e2\u003c/sub\u003e or N\u003csub\u003e2\u003c/sub\u003e gas separation [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e], O\u003csub\u003e2\u003c/sub\u003e gas storage [\u003cspan additionalcitationids=\"CR8 CR9\" citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e], and catalysts for methane oxidation [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. Under high temperature conditions, SrFeO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e adopts a cubic crystal structure with a random distribution of oxide-ion vacancies. At room temperature, the crystal structure of SrFeO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e is a tetragonal or orthorhombic perovskite with an ordered arrangement of oxide-ion vacancy [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e, \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. The structural phase transition from tetragonal or orthorhombic to the cubic phase occurs at approximately 300\u0026deg;C or 420\u0026deg;C [\u003cspan additionalcitationids=\"CR16 CR17 CR18 CR19 CR20\" citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e], respectively, which involves an abrupt variation in volume and electrical conduction property [\u003cspan additionalcitationids=\"CR20\" citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. Therefore, the stabilization of the cubic phase from room temperature to higher temperatures is necessary. One of the methods for the stabilization of the cubic phase is the partial substitution of trivalent ions such as La, Y, Nd, Sm, Eu, and Gd for the Sr site [\u003cspan additionalcitationids=\"CR23 CR24 CR25 CR26 CR27\" citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]. In our previous work, we clarified that Yb, which has a smaller ionic radius among trivalent ions, could partially substitute for the Sr and Fe sites, resulting in a cubic perovskite structure [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. The variation in the chemical state of Fe by substitutional sites or species may affect certain properties of perovskites, such as their electrical conductivity gas separation properties. Therefore, the analysis of the chemical state of Fe in the cubic phase prepared by the partial substitution of trivalent ions is necessary for the design of new functional oxides.\u003c/p\u003e\u003cp\u003eX-ray photoelectron spectroscopy (XPS) [\u003cspan additionalcitationids=\"CR3\" citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e] and X-ray absorption spectroscopy [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e] (XAS) have been proposed as methods for analyzing the chemical state of Fe. However, the signals obtained by XPS reflect only the surface of the specimen, and variations in the surface due to exposure to high vacuum and X-ray irradiation are inevitable. The chemical state of the bulk specimen in air was observed using XAS. However, the chemical shift due to the variation in Fe valence is only 1\u0026ndash;3 eV at most, which is an insufficient resolution to clarify the relationship between the function and chemical state of Fe.\u003c/p\u003e\u003cp\u003eM\u0026ouml;ssbauer spectroscopy, which can be performed in air and provide information on the chemical state of Fe from bulk with high resolution, is a powerful tool for clarifying the chemical state of Fe and its correlation to the properties of perovskites. In an earlier study, we reported a clear difference in M\u0026ouml;ssbauer spectroscopy between Sr\u003csub\u003e1\u0026thinsp;\u0026minus;\u0026thinsp;\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eYb\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eFeO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e and SrFe\u003csub\u003e1-\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eYb\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eO\u003csub\u003e3-\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e owing to the difference in the chemical state of Fe, showing correspondence with the variation in electrical properties by substitutional site [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. It is probable that \u003cem\u003eLn\u003c/em\u003e\u003csup\u003e3+\u003c/sup\u003e other than Yb can be partially substituted for both Sr and Fe sites, resulting in a cubic perovskite structure. However, the preparation of cubic SrFe\u003csub\u003e1\u0026thinsp;\u0026minus;\u0026thinsp;\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eLn\u003c/em\u003e\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e with \u003cem\u003eLn\u003c/em\u003e other than Yb has not yet been reported. In this study, the probability of partial substitution of Tm and Lu, which have almost the same ionic radius as Yb [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e], for the Sr or Fe sites in SrFeO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e was verified. For the specimens with a cubic perovskite structure, the chemical state of Fe was examined using M\u0026ouml;ssbauer spectroscopy to clarify whether it was affected by substitutional species or sites. The electrical conduction behavior was also measured, and the effect of the chemical state of Fe was examined. It is expected that new partially \u003cem\u003eLn\u003c/em\u003e-substituted SrFeO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e with higher electrical properties could be developed as materials for solid oxide fuel cells and so on.\u003c/p\u003e"},{"header":"2. Experimental","content":"\u003cp\u003eSpecimens with nominal compositions of Sr\u003csub\u003e1\u0026thinsp;\u0026minus;\u0026thinsp;\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eLn\u003c/em\u003e\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eFeO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e and Sr\u003csub\u003e1\u0026thinsp;\u0026minus;\u0026thinsp;\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eLn\u003c/em\u003e\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eFeO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e (\u003cem\u003eLn\u003c/em\u003e\u0026thinsp;=\u0026thinsp;Tm, Yb, Lu) were prepared from SrCO\u003csub\u003e3\u003c/sub\u003e, \u003cem\u003eLn\u003c/em\u003e\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, and Fe(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e3\u003c/sub\u003e\u0026bull;9H\u003csub\u003e2\u003c/sub\u003eO using the Pechini method. Before weighing, SrCO\u003csub\u003e3\u003c/sub\u003e (99.9%, Kanto Chem. Corp.) was dried at 150\u0026deg;C, and the purity of Fe(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e3\u003c/sub\u003e\u0026bull;9H\u003csub\u003e2\u003c/sub\u003eO (99.9%, FUJIFILM Wako) was verified from the mass of residual Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e after heating at 800\u0026deg;C for \u0026gt;\u0026thinsp;12 h. \u003cem\u003eLn\u003c/em\u003e\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e powder (99.9%, Kojundo Chemical Laboratory Co., Ltd.) was heated at 1000\u0026deg;C for \u0026gt;\u0026thinsp;17 h in air to remove the carbonate or hydroxide at the surface. Nominal amounts of SrCO\u003csub\u003e3\u003c/sub\u003e, Fe(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e3\u003c/sub\u003e\u0026bull;9H\u003csub\u003e2\u003c/sub\u003eO, and \u003cem\u003eLn\u003c/em\u003e\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e were dissolved in dilute HNO\u003csub\u003e3\u003c/sub\u003e, distilled H\u003csub\u003e2\u003c/sub\u003eO, and a mixture of hot H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e and concentrated HNO\u003csub\u003e3\u003c/sub\u003e, respectively, and the resulting solutions were mixed. After the addition of citric acid and ethylene glycol, both with more than double the molar content of the total cation content, the solution was heated in a mantle heater, producing a solid precursor. After the calcination of the precursor at 750\u0026deg;C for 17 h in air, the powder was pressed into 10-mmφ pellets via uniaxial pressing and sintered at 1200\u0026deg;C for 17 h in air. The pellets were then reground and pressed into cuboids with dimensions of approximately 3 \u0026times; 4 \u0026times; 14 mm\u003csup\u003e3\u003c/sup\u003e via cold isostatic pressing and sintered at 1200\u0026deg;C for 17 h, followed by cooling at 200\u0026deg;C\u0026middot;h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in air.\u003c/p\u003e\u003cp\u003eThe crystal structures of the specimens at room temperature were analyzed via powder X-ray diffraction (XRD) measurements using a RINT-2500 (Rigaku Corp., CuKα, 50 kV, 250 mA). To analyze the chemical state of Fe in the specimens, \u003csup\u003e57\u003c/sup\u003eFe M\u0026ouml;ssbauer spectroscopy was performed. The M\u0026ouml;ssbauer spectra were measured using a Topologic Systems MFD-110D spectrometer with a \u003csup\u003e57\u003c/sup\u003eCo/Rh source at room temperature. The obtained spectra were curve-fitted using the least-squares method, assuming a Lorentzian line shape in each spectrum. The intensity and half-width of the peaks within each quadrupole doublet were constrained to be equal. The constituent iron species were identified by comparing the M\u0026ouml;ssbauer parameters with reference data. The relative content of individual iron species in an analyzed sample was calculated from the peak area in the M\u0026ouml;ssbauer spectrum, assuming identical recoil-free fractions for all species. The isomer shifts were expressed with respect to the centroid of the spectrum of metallic iron foil. TG-DTA (TG8120: Rigaku Corp.) was measured to determine whether the oxygen content was maintained at a temperature as high as 450\u0026deg;C and whether the phase transition observed in SrFeO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e disappeared by \u003cem\u003eLn\u003c/em\u003e substitution. The specimen (~\u0026thinsp;30 mg) and Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e (~\u0026thinsp;20 mg) powders as the sample and reference, respectively, were placed in a Pt pan, and TG-DTA curves were obtained under air with a heating rate of 10\u0026deg;C/min. The total electrical conductivity of the DC and its temperature dependence were measured using the four-probe method, with a Pt mesh and wire as the current and voltage electrodes, respectively. The applied current range was \u0026plusmn;\u0026thinsp;10 mA, and a linear relationship between the current and voltage was confirmed for each measurement. The measurements were performed during heating at a rate of 1\u0026deg;C\u0026middot;min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in an air atmosphere.\u003c/p\u003e"},{"header":"3. Results and discussion","content":"\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\u003ch2\u003e3.1. Analysis of crystal structure by X-ray diffraction\u003c/h2\u003e\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e shows the XRD patterns of the representative peaks of the specimens with nominal compositions of (a) Sr\u003csub\u003e1\u0026thinsp;\u0026minus;\u0026thinsp;\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eTm\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eFeO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e and SrFe\u003csub\u003e1\u0026thinsp;\u0026minus;\u0026thinsp;\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eTm\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e and (b) Sr\u003csub\u003e1\u0026thinsp;\u0026minus;\u0026thinsp;\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eLu\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eFeO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e and SrFe\u003csub\u003e1\u0026thinsp;\u0026minus;\u0026thinsp;\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eLu\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eO\u003csub\u003e3‑\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e. For SrFeO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e, peak splitting owing to distortion of the tetragonal and orthorhombic structures was observed in the 2\u003cem\u003eθ\u003c/em\u003e range of 68.0\u0026ndash;69.5\u0026deg; and 86.5\u0026ndash;88.0\u0026deg;. Single peaks indexed as 220 and 222 of the cubic structure were observed in the 2\u003cem\u003eθ\u003c/em\u003e range of 67.0\u0026ndash;69.5\u0026deg; and 85.0\u0026ndash;88.0\u0026deg;, respectively, for Sr\u003csub\u003e1-\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eTm\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eFeO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e with 0.10\u0026thinsp;\u0026le;\u0026thinsp;\u003cem\u003ex\u003c/em\u003e\u0026thinsp;\u0026le;\u0026thinsp;0.20 and SrFe\u003csub\u003e1\u0026thinsp;\u0026minus;\u0026thinsp;\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eTm\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e with \u003cem\u003ex\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.10. No peak identified as the second phase was observed in the XRD patterns in the 2\u003cem\u003eθ\u003c/em\u003e range of 10.0\u0026ndash;110.0\u0026deg; of these specimens, indicating the successful single-phase preparation of Sr\u003csub\u003e1-\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eTm\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eFeO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e with 0.10\u0026thinsp;\u0026le;\u0026thinsp;\u003cem\u003ex\u003c/em\u003e\u0026thinsp;\u0026le;\u0026thinsp;0.20 and SrFe\u003csub\u003e0.9\u003c/sub\u003eTm\u003csub\u003e0.1\u003c/sub\u003eO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e with a cubic perovskite structure. Peaks that could not be indexed as cubic perovskite were observed in the XRD patterns of Sr\u003csub\u003e1-\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eTm\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eFeO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e with \u003cem\u003ex\u003c/em\u003e\u0026thinsp;\u0026ge;\u0026thinsp;0.25 and SrFe\u003csub\u003e0.8\u003c/sub\u003eTm\u003csub\u003e0.2\u003c/sub\u003eO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e, indicating the solubility limit of Sr\u003csub\u003e1-\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eTm\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eFeO\u003csub\u003e3-\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e and SrFe\u003csub\u003e1\u0026thinsp;\u0026minus;\u0026thinsp;\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eTm\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e. Similarly, it was concluded that a single phase with a cubic perovskite structure was obtained for Sr\u003csub\u003e1\u0026thinsp;\u0026minus;\u0026thinsp;\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eLu\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eFeO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e with 0.10\u0026thinsp;\u0026le;\u0026thinsp;\u003cem\u003ex\u003c/em\u003e\u0026thinsp;\u0026le;\u0026thinsp;0.20 and SrFe\u003csub\u003e1\u0026thinsp;\u0026minus;\u0026thinsp;\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eLu\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e with 0.10\u0026thinsp;\u0026le;\u0026thinsp;\u003cem\u003ex\u003c/em\u003e\u0026thinsp;\u0026le;\u0026thinsp;0.20, showing the same tendency as the Yb-substituted specimens [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e shows the lattice constants of the Tm- and Lu-substituted specimens with a cubic structure calculated from the Bragg angles of the 222 peaks. The lattice constants of the Yb-substituted specimens [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e] are presented for comparison. A slight decrease and a large increase in the lattice constants were observed with an increase in the amount of \u003cem\u003eLn\u003c/em\u003e\u003csup\u003e3+\u003c/sup\u003e substituted for the Sr and Fe sites, respectively, indicating successful substitution for every \u003cem\u003eLn\u003c/em\u003e\u003csup\u003e3+\u003c/sup\u003e examined in this study.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\u003ch2\u003e3.2. Evaluation of chemical state of Fe in cubic Sr\u003csub\u003e0.9\u003c/sub\u003e\u003cem\u003eLn\u003c/em\u003e\u003csub\u003e0.1\u003c/sub\u003eFeO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e and SrFe\u003csub\u003e0.9\u003c/sub\u003e\u003cem\u003eLn\u003c/em\u003e\u003csub\u003e0.1\u003c/sub\u003eO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e by M\u0026ouml;ssbauer spectroscopy\u003c/h2\u003e\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e shows the M\u0026ouml;ssbauer spectra of Sr\u003csub\u003e0.9\u003c/sub\u003e\u003cem\u003eLn\u003c/em\u003e\u003csub\u003e0.1\u003c/sub\u003eFeO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e. The M\u0026ouml;ssbauer spectrum of Sr\u003csub\u003e0.9\u003c/sub\u003eY\u003csub\u003e0.1\u003c/sub\u003eFeO\u003csub\u003e2.80\u003c/sub\u003e [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e], with which mean valence of Fe and oxygen content were determined to be 3.50\u0026thinsp;+\u0026thinsp;and 2.80, respectively, by iodometric titration, is shown for comparison. A similar peak shape was observed for each Sr\u003csub\u003e0.9\u003c/sub\u003e\u003cem\u003eLn\u003c/em\u003e\u003csub\u003e0.1\u003c/sub\u003eFeO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e sample as for Sr\u003csub\u003e0.9\u003c/sub\u003eY\u003csub\u003e0.1\u003c/sub\u003eFeO\u003csub\u003e2.80\u003c/sub\u003e, suggesting an identical chemical state of Fe regardless of the type of \u003cem\u003eLn\u003c/em\u003e\u003csup\u003e3+\u003c/sup\u003e. M\u0026ouml;ssbauer spectra of cubic SrFe\u003csub\u003e0.9\u003c/sub\u003e\u003cem\u003eLn\u003c/em\u003e\u003csub\u003e0.1\u003c/sub\u003eO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e. Although almost identical peak shapes were observed for SrFe\u003csub\u003e0.9\u003c/sub\u003e\u003cem\u003eLn\u003c/em\u003e\u003csub\u003e0.1\u003c/sub\u003eO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e regardless of the type of \u003cem\u003eLn\u003c/em\u003e, different peak shapes were observed for Sr\u003csub\u003e0.9\u003c/sub\u003e\u003cem\u003eLn\u003c/em\u003e\u003csub\u003e0.1\u003c/sub\u003eFeO\u003csub\u003e3\u0026minus;δ\u003c/sub\u003e.\u003c/p\u003e\u003cp\u003eTo elucidate the difference in the chemical state of Fe between Sr\u003csub\u003e0.9\u003c/sub\u003e\u003cem\u003eLn\u003c/em\u003e\u003csub\u003e0.1\u003c/sub\u003eFeO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e and SrFe\u003csub\u003e0.9\u003c/sub\u003e\u003cem\u003eLn\u003c/em\u003e\u003csub\u003e0.1\u003c/sub\u003eO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e, curve fitting was performed according to the following guidelines.\u003c/p\u003e\u003cp\u003e1) The chemical state of Fe can be separated into three types: 3+, 3.5+, and 4+, as observed in the M\u0026ouml;ssbauer spectra of SrFeO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e [\u003cspan additionalcitationids=\"CR32 CR33 CR34\" citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e].\u003c/p\u003e\u003cp\u003e2) The mean Fe valence in Sr\u003csub\u003e0.9\u003c/sub\u003eY\u003csub\u003e0.1\u003c/sub\u003eFeO\u003csub\u003e3\u0026minus;δ\u003c/sub\u003e calculated from the ratio of the peak area should be 3.50+, as confirmed by iodometric titration.\u003c/p\u003e\u003cp\u003eThe results of the curve fitting are shown in Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e and \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e, and the optimized parameters are summarized in Table\u0026nbsp;1. The mean Fe valences of Sr\u003csub\u003e0.9\u003c/sub\u003eTm\u003csub\u003e0.1\u003c/sub\u003eFeO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e, Sr\u003csub\u003e0.9\u003c/sub\u003eYb\u003csub\u003e0.1\u003c/sub\u003eFeO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e, and Sr\u003csub\u003e0.9\u003c/sub\u003eLu\u003csub\u003e0.1\u003c/sub\u003eFeO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e were 3.51(7)+, 3.44(7)+, and 3.52(7), respectively. Those of SrFe\u003csub\u003e0.9\u003c/sub\u003eTm\u003csub\u003e0.1\u003c/sub\u003eO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e, SrFe\u003csub\u003e0.9\u003c/sub\u003eYb\u003csub\u003e0.1\u003c/sub\u003eO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e, and SrFe\u003csub\u003e0.9\u003c/sub\u003eLu\u003csub\u003e0.1\u003c/sub\u003eO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e were 3.51(6), 3.50(7), and 3.52(7), respectively. The mean Fe valence of all the specimens examined in this study was concluded to be 3.50+. In addition, the equilibrium formula described as follows is satisfied in all the specimens because of the coexistence of Fe\u003csup\u003e3+\u003c/sup\u003e, Fe\u003csup\u003e4+\u003c/sup\u003e, and Fe\u003csup\u003e3.5+\u003c/sup\u003e:\u003c/p\u003e\u003cp\u003eFe\u003csup\u003e3+\u003c/sup\u003e + Fe\u003csup\u003e4+\u003c/sup\u003e \u0026harr;2Fe\u003csup\u003e3.5+\u003c/sup\u003e (1)\u003c/p\u003e\u003cp\u003eThis chemical equilibrium formula is also applicable to orthorhombic SrFeO\u003csub\u003e2.75\u003c/sub\u003e [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e]. For Sr\u003csub\u003e0.9\u003c/sub\u003e\u003cem\u003eLn\u003c/em\u003e\u003csub\u003e0.1\u003c/sub\u003eFeO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e, the intensity of the peak assigned to Fe\u003csup\u003e3.5+\u003c/sup\u003e was the largest regardless of the type of \u003cem\u003eLn\u003c/em\u003e, indicating chemical equilibrium, as represented by Eq.\u0026nbsp;(1) proceeded to the right side, resulting in the delocalization of the charge carrier. In SrFe\u003csub\u003e0.9\u003c/sub\u003e\u003cem\u003eLn\u003c/em\u003e\u003csub\u003e0.1\u003c/sub\u003eO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e, the chemical equilibrium was deduced as Eq.\u0026nbsp;(1) proceeded relatively to the left side, resulting in a lower degree of charge delocalization because the intensities of the peaks assigned to Fe\u003csup\u003e3+\u003c/sup\u003e, Fe\u003csup\u003e4+\u003c/sup\u003e, and Fe\u003csup\u003e3.5+\u003c/sup\u003e were almost similar. However, the intensity of the peak assigned to Fe\u003csup\u003e3.5+\u003c/sup\u003e in SrFe\u003csub\u003e0.9\u003c/sub\u003e\u003cem\u003eLn\u003c/em\u003e\u003csub\u003e0.1\u003c/sub\u003eO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e was larger than that in orthorhombic SrFeO\u003csub\u003e2.75\u003c/sub\u003e [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e], indicating that the charge carrier was delocalized by the structural phase transition to cubic with \u003cem\u003eLn\u003c/em\u003e substitution for both Sr and Fe sites.\u003c/p\u003e\u003cp\u003eThe concentration of oxide-ion vacancies should be larger in SrFe\u003csub\u003e0.9\u003c/sub\u003e\u003cem\u003eLn\u003c/em\u003e\u003csub\u003e0.1\u003c/sub\u003eO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e than in Sr\u003csub\u003e0.9\u003c/sub\u003e\u003cem\u003eLn\u003c/em\u003e\u003csub\u003e0.1\u003c/sub\u003eFeO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e because the mean Fe valence in all the specimens was constant at 3.5+. Therefore, the repulsion between the Fe cations is expected to be larger in SrFe\u003csub\u003e0.9\u003c/sub\u003e\u003cem\u003eLn\u003c/em\u003e\u003csub\u003e0.1\u003c/sub\u003eO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e, resulting in longer lattice constants, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. A longer distance between Fe ions can be the origin of charge localization, that is, proceeding to the left side of Eq.\u0026nbsp;(1). The effect of charge delocalization on electrical conductivity is discussed in Section \u003cspan refid=\"Sec3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. 3.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\u003ch2\u003e3.3. Electrical conductivity of cubic Sr\u003csub\u003e0.9\u003c/sub\u003e\u003cem\u003eLn\u003c/em\u003e\u003csub\u003e0.1\u003c/sub\u003eFeO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e and SrFe\u003csub\u003e0.9\u003c/sub\u003e\u003cem\u003eLn\u003c/em\u003e\u003csub\u003e0.1\u003c/sub\u003eO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e\u003c/h2\u003e\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e shows the temperature dependence of the electrical conductivity of Sr\u003csub\u003e0.9\u003c/sub\u003e\u003cem\u003eLn\u003c/em\u003e\u003csub\u003e0.1\u003c/sub\u003eFeO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e and SrFe\u003csub\u003e0.9\u003c/sub\u003e\u003cem\u003eLn\u003c/em\u003e\u003csub\u003e0.1\u003c/sub\u003eO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e in air. The data for SrFeO\u003csub\u003e2.86\u003c/sub\u003e and SrFeO\u003csub\u003e2.74\u003c/sub\u003e [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e] are also shown for comparison. The semiconducting temperature dependence with activation energies of 0.09(2) and 0.16(2) eV without abrupt variation was observed for Sr\u003csub\u003e0.9\u003c/sub\u003e\u003cem\u003eLn\u003c/em\u003e\u003csub\u003e0.1\u003c/sub\u003eFeO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e and SrFe\u003csub\u003e0.9\u003c/sub\u003e\u003cem\u003eLn\u003c/em\u003e\u003csub\u003e0.1\u003c/sub\u003eO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e, respectively, while an abrupt variation originating from the structural phase transition to cubic was detected for SrFeO\u003csub\u003e2.86\u003c/sub\u003e and SrFeO\u003csub\u003e2.74\u003c/sub\u003e. The absence of abrupt variations in the electrical conductivity of Sr\u003csub\u003e0.9\u003c/sub\u003e\u003cem\u003eLn\u003c/em\u003e\u003csub\u003e0.1\u003c/sub\u003eFeO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e and SrFe\u003csub\u003e0.9\u003c/sub\u003e\u003cem\u003eLn\u003c/em\u003e\u003csub\u003e0.1\u003c/sub\u003eO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e is attributed to the absence of structural phase transitions and variations in the oxygen content, as observed in the TG-DTA curves shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e.\u003c/p\u003e\u003cp\u003eAlthough the mean Fe valence was almost the same at 3.50\u0026thinsp;+\u0026thinsp;for all the \u003cem\u003eLn\u003c/em\u003e-substituted specimens and SrFeO\u003csub\u003e2.75\u003c/sub\u003e, the electrical conductivities of Sr\u003csub\u003e0.9\u003c/sub\u003e\u003cem\u003eLn\u003c/em\u003e\u003csub\u003e0.1\u003c/sub\u003eFeO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e were higher than that of SrFeO\u003csub\u003e2.75\u003c/sub\u003e below 400\u0026deg;C and those of SrFe\u003csub\u003e0.9\u003c/sub\u003e\u003cem\u003eLn\u003c/em\u003e\u003csub\u003e0.1\u003c/sub\u003eO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e. The higher electrical conductivity of the \u003cem\u003eLn\u003c/em\u003e-substituted specimens than that of SrFeO\u003csub\u003e2.75\u003c/sub\u003e below 400\u0026deg;C was attributed to the cubic structure with a larger degree of carrier delocalization observed by M\u0026ouml;ssbauer spectroscopy. Sr\u003csub\u003e0.9\u003c/sub\u003e\u003cem\u003eLn\u003c/em\u003e\u003csub\u003e0.1\u003c/sub\u003eFeO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e exhibits higher electrical conductivity and activation energy than SrFe\u003csub\u003e0.9\u003c/sub\u003e\u003cem\u003eLn\u003c/em\u003e\u003csub\u003e0.1\u003c/sub\u003eO\u003csub\u003e3-\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e, which is in agreement with the higher degree of carrier delocalization clarified by M\u0026ouml;ssbauer spectroscopy. The higher carrier delocalization can be attributed to the larger overlap of the Fe3d and O2p orbitals due to the shorter bond length observed in the XRD measurements.\u003c/p\u003e\u003cp\u003eAmong the examined species, Sr\u003csub\u003e0.9\u003c/sub\u003eTm\u003csub\u003e0.1\u003c/sub\u003eFeO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e exhibited the highest electrical conductivity, exceeding that of cubic SrFeO\u003csub\u003e2.87\u003c/sub\u003e with an Fe valence of 3.75+. These findings establish that Sr\u003csub\u003e0.9\u003c/sub\u003eTm\u003csub\u003e0.1\u003c/sub\u003eFeO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e has high potential as an electrode material for use in solid oxide fuel cells.\u003c/p\u003e\u003c/div\u003e"},{"header":"4. Conclusions","content":"\u003cp\u003eSr\u003csub\u003e1\u0026thinsp;\u0026minus;\u0026thinsp;\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eLn\u003c/em\u003e\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eFeO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e and SrFe\u003csub\u003e1\u0026thinsp;\u0026minus;\u0026thinsp;\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eLn\u003c/em\u003e\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e (\u003cem\u003eLn\u003c/em\u003e: Tm, Yb, and Lu) were prepared with a cubic perovskite structure and random distribution of oxide-ion vacancies. M\u0026ouml;ssbauer spectroscopy revealed the mean Fe valence of cubic Sr\u003csub\u003e0.9\u003c/sub\u003e\u003cem\u003eLn\u003c/em\u003e\u003csub\u003e0.1\u003c/sub\u003eFeO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e and SrFe\u003csub\u003e0.9\u003c/sub\u003e\u003cem\u003eLn\u003c/em\u003e\u003csub\u003e0.1\u003c/sub\u003eO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e to be 3.50+. Further, the chemical state of Fe was not found to be dependent on the type of \u003cem\u003eLn\u003c/em\u003e but on the substitution site. M\u0026ouml;ssbauer spectroscopy showed that the charge carrier delocalization was larger in Sr\u003csub\u003e0.9\u003c/sub\u003e\u003cem\u003eLn\u003c/em\u003e\u003csub\u003e0.1\u003c/sub\u003eFeO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e than in SrFe\u003csub\u003e0.9\u003c/sub\u003e\u003cem\u003eLn\u003c/em\u003e\u003csub\u003e0.1\u003c/sub\u003eO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e, indicating that Sr\u003csub\u003e0.9\u003c/sub\u003e\u003cem\u003eLn\u003c/em\u003e\u003csub\u003e0.1\u003c/sub\u003eFeO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e possesses higher electrical conductivity than SrFe\u003csub\u003e0.9\u003c/sub\u003e\u003cem\u003eLn\u003c/em\u003e\u003csub\u003e0.1\u003c/sub\u003eO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e. Among the prepared cubic specimens, Sr\u003csub\u003e0.9\u003c/sub\u003eTm\u003csub\u003e0.1\u003c/sub\u003eFeO\u003csub\u003e3\u0026minus;\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e was found to be the most promising electrode material for solid oxide fuel cells because it was free from abrupt variations in electrical conductivity caused by the structural phase transition, and exhibited the highest conductivity. By examining other \u003cem\u003eLn\u003c/em\u003e\u003csup\u003e3+\u003c/sup\u003e as substituents for both Sr and Fe sites, new cubic perovskite oxides with higher electrical properties could be developed as materials for solid oxide fuel cells. Trials for the preparation of other \u003cem\u003eLn\u003c/em\u003e\u003csup\u003e3+\u003c/sup\u003e substituted specimens and investigation of the chemical state of Fe and its electrical property are currently in progress and will be reported in due course.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eM\u0026ouml;ssbauer measurements were carried out with the support of the Radioisotope Laboratory in the Graduate School of Science and Isotope Science Center, University of Tokyo. We thank Editage (www.editage.jp) for the English language editing.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eKN: Investigation, Data curation, Formal analysis, Writing \u0026ndash; original draft; RM: Investigation, Formal analysis, Writing \u0026ndash; review and editing; TS: Methodology, Formal analysis, Visualization, Writing \u0026ndash; review and editing; KS: Data curation, Investigation, Writing \u0026ndash; review and editing; TH: Conceptualization, Supervision, Validation, Writing \u0026ndash; review and editing; MM: Methodology, Data curation, Writing \u0026ndash; review and editing.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData availability\u003c/strong\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eData will be made available on request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConflict of interests\u003c/strong\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe authors have no competing interests to declare.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eFern\u0026aacute;ndez-Ropero AJ, Porras-V\u0026aacute;zquez JM, Cabeza A, Slater PR, Marrero-L\u0026oacute;pez D, Losilla ER (2014) High valence transition metal doped strontium ferrites for electrode materials in symmetrical SOFCs. 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J Solid State Chem 267:113\u0026ndash;118. https://doi.org/10.1016/j.jssc.2018.08.007\u003c/li\u003e\n\u003cli\u003eAksenova TV, Volkova NE, Gavrilova LY, Cherepanov VA (2025) Phase equilibria, crystal structure and oxygen nonstoichiometry of complex oxides in the \u0026frac12;Eu\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u0026mdash;SrO\u0026mdash;\u0026frac12;Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e system. J Alloys Compd 1036:182000. https://doi.org/10.1016/j.jallcom.2025.182000\u003c/li\u003e\n\u003cli\u003eТugova ЕА, Кrasilin АА, Panchuk VV, Semenov VG, Gusarov VV (2020) Subsolidus phase equilibria in the GdFeO\u003csub\u003e3\u003c/sub\u003e-SrFeO\u003csub\u003e3\u0026minus;\u003cem\u003e\u0026delta;\u003c/em\u003e\u003c/sub\u003e system in air. 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Phys Rev B 73:094451. https://doi.org/10.1103/PhysRevB.73.094451\u003c/li\u003e\n\u003cli\u003eSedykh VD, Rybchenko OG, Nekrasov AN, Koneva IE, Kulakov VI (2019) Effect of the oxygen content on the local environment of Fe Atoms in anion-deficient SrFeO\u003csub\u003e3-\u003cem\u003e\u0026delta;\u003c/em\u003e\u003c/sub\u003e. Phys Solid State 61:1099\u0026ndash;1106. https://doi.org/10.1134/S1063783419060210\u003c/li\u003e\n\u003cli\u003eSedykh VD, Rybchenko OG, Suvorov EV, Ivanov AI, Kulakov VI (2020) Oxygen vacancies and valence states of iron in SrFeO\u003csub\u003e3-\u003cem\u003e\u0026delta;\u003c/em\u003e\u003c/sub\u003e compounds. Phys Solid State 62:1916\u0026ndash;1923. https://doi.org/10.1134/S1063783420100297\u003c/li\u003e\n\u003cli\u003eBarkalov OI, Zaitsev SV, Sedykh VD (2022) Strontium ferrite SrFeO\u003csub\u003e3-\u003cem\u003e\u0026delta;\u003c/em\u003e\u003c/sub\u003e (2.50 \u0026le; 3-\u0026delta; \u0026le; 2.87) studied by Raman and M\u0026ouml;ssbauer spectroscopy. Solid State Commun 354:114912. https://doi.org/10.1016/j.ssc.2022.114912\u003c/li\u003e\n\u003cli\u003eShido K, Yoshino T, Hatano S, Matsuo M, Hashimoto T (2024) Miscibility gap by oxygen content in SrFeO\u003csub\u003e3-\u003cem\u003e\u0026delta;\u003c/em\u003e\u003c/sub\u003e: Examination using X-ray diffraction and M\u0026ouml;ssbauer spectroscopy. Interactions 245:299. https://doi.org/10.1007/s10751-024-02129-z\u003c/li\u003e\n\u003c/ol\u003e"},{"header":"Table","content":"\u003cp\u003e\u003cstrong\u003eTable 1\u003c/strong\u003e Parameters of M\u0026ouml;ssbauer spectra for Sr\u003csub\u003e0.9\u003c/sub\u003eY\u003csub\u003e0.1\u003c/sub\u003eFeO\u003csub\u003e2.80\u003c/sub\u003e, Sr\u003csub\u003e0.9\u003c/sub\u003e\u003cem\u003eLn\u003c/em\u003e\u003csub\u003e0.1\u003c/sub\u003eFeO\u003csub\u003e3-\u003cem\u003e\u0026delta;\u003c/em\u003e\u003c/sub\u003e, and SrFe\u003csub\u003e0.9\u003c/sub\u003e\u003cem\u003eLn\u003c/em\u003e\u003csub\u003e0.1\u003c/sub\u003eO\u003csub\u003e3‑\u003cem\u003e\u0026delta;\u003c/em\u003e\u003c/sub\u003e. I. S.: isomer shift, Q. S.: quadrupole splitting, H. W.: full width at half maximum\u003c/p\u003e\n\u003ctable border=\"1\" cellspacing=\"0\" cellpadding=\"0\" width=\"612\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 113px;\"\u003e\n \u003cp\u003eSample\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 76px;\"\u003e\n \u003cp\u003eSpecies\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 80px;\"\u003e\n \u003cp\u003eArea / %\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 100px;\"\u003e\n \u003cp\u003eI.S. / mm\u0026middot;s\u003csup\u003e-1\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 100px;\"\u003e\n \u003cp\u003eQ.S. / mm\u0026middot;s\u003csup\u003e-1\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 143px;\"\u003e\n \u003cp\u003eH.W. / mm\u0026middot;s\u003csup\u003e-1\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 113px;\"\u003e\n \u003cp\u003eSr\u003csub\u003e0.9\u003c/sub\u003eY\u003csub\u003e0.1\u003c/sub\u003eFeO\u003csub\u003e2.80\u003c/sub\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 76px;\"\u003e\n \u003cp\u003eFe\u003csup\u003e4+\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 80px;\"\u003e\n \u003cp\u003e\u0026nbsp;19.9\u0026plusmn;0.4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 100px;\"\u003e\n \u003cp\u003e-0.016\u0026plusmn;0.006\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 100px;\"\u003e\n \u003cp\u003e0.420\u0026plusmn;0.009\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 143px;\"\u003e\n \u003cp\u003e0.310\u0026plusmn;0.013\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 113px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 76px;\"\u003e\n \u003cp\u003eFe\u003csup\u003e3+\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 80px;\"\u003e\n \u003cp\u003e\u0026nbsp;20.2\u0026plusmn;0.6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 100px;\"\u003e\n \u003cp\u003e0.187\u0026plusmn;0.003\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 100px;\"\u003e\n \u003cp\u003e\u0026nbsp;1.257\u0026plusmn;0.019\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 143px;\"\u003e\n \u003cp\u003e0.365\u0026plusmn;0.027\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 113px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 76px;\"\u003e\n \u003cp\u003eFe\u003csup\u003e3.5+\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 80px;\"\u003e\n \u003cp\u003e\u0026nbsp;59.9\u0026plusmn;0.7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 100px;\"\u003e\n \u003cp\u003e0.243\u0026plusmn;0.003\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 100px;\"\u003e\n \u003cp\u003e\u0026nbsp;0.574\u0026plusmn;0.015\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 143px;\"\u003e\n \u003cp\u003e0.492\u0026plusmn;0.023\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 113px;\"\u003e\n \u003cp\u003eSr\u003csub\u003e0.9\u003c/sub\u003eTm\u003csub\u003e0.1\u003c/sub\u003eFeO\u003csub\u003e3-\u003cem\u003e\u0026delta;\u003c/em\u003e\u003c/sub\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 76px;\"\u003e\n \u003cp\u003eFe\u003csup\u003e4+\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 80px;\"\u003e\n \u003cp\u003e\u0026nbsp;20.0\u0026plusmn;0.3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 100px;\"\u003e\n \u003cp\u003e-0.014\u0026plusmn;0.005\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 100px;\"\u003e\n \u003cp\u003e0.435\u0026plusmn;0.008\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 143px;\"\u003e\n \u003cp\u003e0.315\u0026plusmn;0.011\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 113px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 76px;\"\u003e\n \u003cp\u003eFe\u003csup\u003e3+\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 80px;\"\u003e\n \u003cp\u003e\u0026nbsp;18.9\u0026plusmn;0.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 100px;\"\u003e\n \u003cp\u003e0.180\u0026plusmn;0.003\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 100px;\"\u003e\n \u003cp\u003e\u0026nbsp;1.259\u0026plusmn;0.016\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 143px;\"\u003e\n \u003cp\u003e0.361\u0026plusmn;0.022\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 113px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 76px;\"\u003e\n \u003cp\u003eFe\u003csup\u003e3.5+\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 80px;\"\u003e\n \u003cp\u003e\u0026nbsp;61.2\u0026plusmn;0.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 100px;\"\u003e\n \u003cp\u003e0.240\u0026plusmn;0.003\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 100px;\"\u003e\n \u003cp\u003e\u0026nbsp;0.586\u0026plusmn;0.012\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 143px;\"\u003e\n \u003cp\u003e0.490\u0026plusmn;0.017\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 113px;\"\u003e\n \u003cp\u003eSr\u003csub\u003e0.9\u003c/sub\u003eYb\u003csub\u003e0.1\u003c/sub\u003eFeO\u003csub\u003e3-\u003cem\u003e\u0026delta;\u003c/em\u003e\u003c/sub\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 76px;\"\u003e\n \u003cp\u003eFe\u003csup\u003e4+\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 80px;\"\u003e\n 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\u003c/tbody\u003e\n\u003c/table\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Sr1‑xLnxFeO3−δ, SrFe1‑xLnxO3−δ, X-ray diffraction, Mössbauer spectroscopy, electrical conductivity","lastPublishedDoi":"10.21203/rs.3.rs-7605029/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7605029/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eOxide materials with cubic perovskite structure and randomly distributed oxide-ion vacancies were synthesized by partially substituting \u003cem\u003eLn\u003c/em\u003e (\u003cem\u003eLn\u003c/em\u003e: Tm, Yb, Lu) for Sr or Fe in SrFeO\u003csub\u003e3‑\u003c/sub\u003e\u003csub\u003e\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e. Mössbauer spectroscopy showed that the Fe chemical state depended on the substitution site rather than the \u003cem\u003eLn\u003c/em\u003e type. Although Sr\u003csub\u003e0.9\u003c/sub\u003e\u003cem\u003eLn\u003c/em\u003e\u003csub\u003e0.1\u003c/sub\u003eFeO\u003csub\u003e3-\u003c/sub\u003e\u003csub\u003e\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e and SrFe\u003csub\u003e0.9\u003c/sub\u003e\u003cem\u003eLn\u003c/em\u003e\u003csub\u003e0.1\u003c/sub\u003eO\u003csub\u003e3-\u003c/sub\u003e\u003csub\u003e\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e exhibited similar mean Fe valence, the former demonstrated greater charge carrier delocalization, higher conductivity and lower activation energy. Notably, Sr\u003csub\u003e0.9\u003c/sub\u003eTm\u003csub\u003e0.1\u003c/sub\u003eFeO\u003csub\u003e3‑\u003c/sub\u003e\u003csub\u003e\u003cem\u003eδ\u003c/em\u003e\u003c/sub\u003e exhibited the highest conductivity without structural phase transition, indicating strong potential as an electrode material for solid oxide fuel cells.\u003c/p\u003e","manuscriptTitle":"Analysis of chemical state of Fe in cubic Sr1-xLnxFeO3-δ and SrFe1‑xLnxO3-δ (Ln: Tm, Yb, Lu) by Mössbauer spectroscopy and its effect on electrical conduction property","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-09-29 11:27:53","doi":"10.21203/rs.3.rs-7605029/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"07f07951-2be2-4f0d-b6e6-9429c5aaf644","owner":[],"postedDate":"September 29th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-11-17T16:02:14+00:00","versionOfRecord":{"articleIdentity":"rs-7605029","link":"https://doi.org/10.1007/s10967-025-10512-5","journal":{"identity":"journal-of-radioanalytical-and-nuclear-chemistry","isVorOnly":false,"title":"Journal of Radioanalytical and Nuclear Chemistry"},"publishedOn":"2025-11-11 15:58:02","publishedOnDateReadable":"November 11th, 2025"},"versionCreatedAt":"2025-09-29 11:27:53","video":"","vorDoi":"10.1007/s10967-025-10512-5","vorDoiUrl":"https://doi.org/10.1007/s10967-025-10512-5","workflowStages":[]},"version":"v1","identity":"rs-7605029","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7605029","identity":"rs-7605029","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}