Impact of Lanthanoid Substitution on the Structural and Physical

Article pubs.acs.org/IC

Impact of Lanthanoid Substitution on the Structural and Physical Properties of an Infinite-Layer Iron Oxide Takafumi Yamamoto,† Hiroshi Ohkubo,† Cédric Tassel,†,‡ Naoaki Hayashi,§ Shota Kawasaki,† Taku Okada,∥ Takehiko Yagi,∥ James Hester,⊥ Maxim Avdeev,⊥ Yoji Kobayashi,† and Hiroshi Kageyama*,†,# †

Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan ‡ The Hakubi Center for Advanced Research, Kyoto University, Yoshida-Ushinomiya-cho, Sakyo-ku, Kyoto 606-8302, Japan § Micro/Nano Fabrication Hub, Center for the Promotion of Interdisciplinary Education and Research, Kyoto University, Yoshida-Honmachi, Sakyo, Kyoto 606-8501, Japan ∥ Research Institute for Solid State Physics, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581, Japan ⊥ Australian Synchrotron Research Program, Australian Nuclear Science and Technology Organisation, PMB 1, Menai, New South Wales 2234, Australia # CREST, Japan Science and Technology Agency (JST), Kawaguchi, Saitama 332-0012, Japan S Supporting Information *

ABSTRACT: The effect of lanthanoid (Ln = Nd, Sm, Ho) substitution on the structural and physical properties of the infinite-layer iron oxide SrFeO2 was investigated by X-ray diffraction (XRD) at ambient and high pressure, neutron diffraction, and 57Fe Mössbauer spectroscopy. Ln for Sr substituted samples up to ∼30% were synthesized by topochemical reduction using CaH2. While the introduction of the smaller Ln3+ ion reduces the a axis as expected, we found an unusual expansion of the c axis as well as the volume. Rietveld refinements along with pair distribution function analysis revealed the incorporation of oxygen atoms between FeO2 layers with a charge-compensated composition of (Sr1−xLnx)FeO2+x/2, which accounts for the failed electron doping to the FeO2 layer. The incorporated partial apical oxygen or the pyramidal coordination induces incoherent buckling of the FeO2 sheet, leading to a significant reduction of the Néel temperature. High-pressure XRD experiments for (Sr0.75Ho0.25)FeO2.125 suggest a possible stabilization of an intermediate spin state in comparison with SrFeO2, revealing a certain contribution of the in-plane Fe−O distance to the pressure-induced transition.

1. INTRODUCTION The infinite-layer (IL) structure SrCuO2 (P4/mmm space group) composed of stacked CuO2 sheets that are separated by Sr atoms (Figure 1) represents the simplest parent structure of cupric high-temperature superconductors.1 Strong electron− electron correlations make this compound a half-filled S = 1/2 Mott insulator with a G-type magnetic order, but moderate carrier injection into the CuO2 layer leads to superconductivity, as reported in n-type superconductors (Sr1−xLnx)CuO2 (Ln = La, Nd, Sm, Gd).2,3 (Sr1−xLnx)CuO2, synthesized under high pressure, has a solubility limit of x ≈ 0.10 for all Ln atoms. The Ln for Sr substitution decreases the c axis more remarkably than the a axis, reflecting the two-dimensional (2D) structural feature. Despite varied lattice parameters with Ln, the critical temperature Tc in (Sr0.92Ln0.08)CuO2 is nearly independent (Tc ≈ 40 K).3 The iron (S = 2) analogue SrFeO2 is obtained by a topotactic reduction of a perovskite SrFeO3 with calcium hydride.4 Despite its 2D structure, SrFeO2 undergoes an antiferromag© XXXX American Chemical Society

netic transition (G type) far above room temperature (TN = 473 K). The isovalent A-site (Sr) substitution in SrFeO2 without any symmetry change is possible up to 80% by calcium5,6 and 30% by barium,7 bearing almost the same solubility as SrCuO2.1 Unlike cuprates, the iron system shows remarkable flexibility in the FeO4 square-planar coordination. Ca substitution beyond 80% induces a coherent buckling of the FeO2 sheets with FeO4 square planes being tetrahedrally distorted,6 while Ba substitution beyond 30% results in a new oxygen vacancy ordered structure (related to the LaNiO2.5 structure) with rhomboidally distorted FeO4 square planes.8 The robustness and flexibility in FeO4 square-planar coordination arises from a multiorbital nature in SrFeO2, which is distinct from SrCuO2 in which only the 3dx2−y2 orbital is unoccupied. In-plane (superexchange) and out-of-plane (direct exchange) interactions are experimentally and theoretReceived: October 16, 2016

A

DOI: 10.1021/acs.inorgchem.6b02513 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Table 1. Synthesis Conditions for Hydride Reduction of (Sr1−xLnx)FeO3−δ (δ < 0.5) Ln

x

reacn temp (K)

reacn time (days)

Nd

0.1, 0.2 0.3 0.35a 0.1 0.2 0.3 0.35 0.05, 0.1 0.15, 0.2, 0.25 0.3

568 673 723 553 568 623 673 623 623 723

6 8

Sm

Ho

Figure 1. Infinite-layer structure ABO2. Orange, gray, and blue spheres represent oxygen and A- and B-site atoms. Dotted circles represent vacant apical oxygen sites.

a

3 5 4 7 1 4 2

Reduction was unsuccessful for x = 0.35 (Nd).

diffractometer at the OPAL reactor, ANSTO.15 A polycrystalline sample (3 g) was placed into a vanadium cylinder. ND data were collected with a step scan procedure using 128 neutron detectors in a 2θ range from 2.75 to 163.95° with a step width of 0.05°. The neutron beam was monochromated to a wavelength of 1.6220(5) Å by using a Ge335 monochromator crystal. Primary and secondary collimators were left open. The SXRD and ND data were analyzed by the Rietveld method using the RIETAN-FP program.16 For hydride-reduced Ho samples with x = 0, 0.2, we conducted a pair distribution function (PDF) analysis for SXRD data at room temperature collected at the SPring-8 beamline BL04B2 (λ = 0.20154 Å) using the PDFgui program.17 Raw data over a large Q range (Qmax = 25 Å−1) were corrected for scattering of the quartz capillary, incident X-ray spectrum, absorption, and multiple scattering and finally normalized, followed by a Fourier transform to obtain the atomic pair distribution G(r). Hydrogen release for the hydride-reduced Ho sample (x = 0.25) was monitored by a Bruker MS9610 quadrupole mass spectrometer connected to the TG-DTA instrument. The experiment was conducted with an Ar flow of 300 mL/min. 57Fe Mössbauer spectra on the hydride-reduced Ho samples (x = 0.05, 0.1, 0.2, 0.25, 0.3) were collected at room temperature in transmission geometry using a 57Co/ Rh γ-ray source. The source velocity was calibrated by α-Fe as a standard. High-pressure SXRD experiments were performed for a reduced x = 0.25 (Ho) sample at room temperature up to 45 GPa using the NE1A synchrotron beamline of the Photon Factory-Advanced Ring for Pulse X-rays (PF-AR) at the High Energy Accelerator Research Organization (KEK) of Japan. The incident X-ray beam was monochromated to λ = 0.4130 Å, and the beam was collimated to a diameter of about 50 μm. Powder samples were loaded into a 120 μm hole of preindented rhenium gaskets of the diamond anvil cell. Glycerine was used as a pressure-transmitting medium. The fluorescence shift of ruby was used to calibrate the pressure, revealing that the pressure gradient was less than 5 GPa at the maximum pressure.

ically shown to be fairly strong (6.58 and 1.75 meV, respectively),9−11 which explains the high TN value of this material. Furthermore, application of external pressure enhances the out-of-plane interaction and eventually leads to a spin-state transition at Pc = 33 GPa from S = 2 to 1, accompanied by an antiferromagnetic to ferromagnetic and an insulator to metal transition.12 Up to now, there have been several studies that attempted to introduce carriers in square-planar iron oxides under ambient pressure. However, B-site substitution by Mn and Co with different d electron counts did not induce metallicity.13,14 In this work, we study the topochemical reaction of the Lnsubstituted perovskite phases (Sr1−xLnx)FeO3−δ (Ln = Nd, Sm, Ho). We found that the Ln3+/Sr2+ substitution enables the incorporation of oxygen atoms between FeO2 layers, leading to a charge-compensated composition of (Sr1−xLnx)FeO2+x/2 with unvaried Fe2+ state. The incorporated oxygen (providing a pyramidal iron) induces incoherent buckling of the FeO2 plane, leading to a significant reduction in TN. We will also discuss the effect of Ho substitution on the spin state transition under high pressure.

2. EXPERIMENTAL PROCEDURES Precursor perovskite iron oxides (Sr1−xLnx)FeO3−δ (Ln = Nd, Sm, Ho; x ≤ 0.35; δ < 0.5) were prepared by thoroughly mixing stoichiometric amounts of SrCO3 (99.99%, Rare Metallic Co., Ltd.), predried Ln2O3 (99.99%, Rare Metallic Co., Ltd.), and Fe2O3 (99.99%, Rare Metallic Co., Ltd.), pelletizing, and heating at 1473 K in air for 48 h with one intermediate grinding. The obtained precursors were ground with a 2 molar excess of CaH2 (99.9%, Sigma-Aldrich Co.) in an Ar-filled glovebox, sealed in an evacuated Pyrex tube (volume 15 cm3) with a residual pressure less than 2.0 × 10−2 Pa, and reacted at various temperatures and durations as given in Table 1. The residual CaH2 and CaO byproduct were removed by washing with a 0.1 M NH4Cl/ methanol solution. The purity of the precursors and the reduced products was checked by X-ray diffraction (XRD) using a D8 ADVANCE diffractometer (Burker AXS) with Cu Kα radiation. High-resolution synchrotron XRD (SXRD) experiments for the hydride-reduced Ho compound (Sr0.75Ho0.25)FeO3−δ were performed at room temperature using the large Debye−Scherrer camera installed at the beamline BL02B2 of the Japan Synchrotron Radiation Research Institute (SPring-8). An imaging plate was used as a detector. Incident beams from a bending magnet were monochromated to 0.399633 Å. A finely ground powder sample was sieved through a 32 μm mesh sieve and put into a Pyrex capillary (0.1 mm inner diameter). The sealed capillary was rotated during measurements to reduce the effect of preferential orientation. A powder neutron diffraction (ND) experiment on the same specimen was carried out at room temperature at the Echidna neutron

3. RESULTS AND DISCUSSIONS The pure phase of the (Sr1−xHox)FeO3−δ precursor (δ < 0.5) was obtained up to x = 0.3 (Figure S1 in the Supporting Information) with a cubic unit cell, consistent with a previous report.18 A deviation of the lattice parameter evolution from Vegard’s law should correlate with the x dependence of the oxygen vacancy content δ.19 Upon hydride reduction, tetragonal phases with a ≈ 3.9 Å and c ≈ 3.5 Å were obtained as a single phase (Figure S2 in the Supporting Information). Both the a and c parameters, as calculated by Le Bail analysis (Table S1 in the Supporting Information), exhibit nonlinear x dependence (Figure 2), implying compositional and/or structural modification. In particular, a significant increase in the c axis and even the B


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Inorganic Chemistry

Figure 3. (a) Structural characterization of (Sr0.75Ho0.25)FeO2.125 by Rietveld refinement of SXRD data at room temperature and (b) ND data at room temperature. The lower ticks in (b) correspond to the position of the magnetic Bragg peaks.

as shown in Figure 3b. Anisotropic displacement parameters Uij were used for O(1) and O(2). The magnetic structure of Gtype was included in the refinement, as will be discussed later. We found that the values of U33 for O(1) and O(2) are fairly large, an implication of a large displacement of both atoms along the c axis. However, attempts to off-center the oxygen atoms from the ideal positions did not improve the fitting. We returned to the SXRD analysis to refine Uij for Sr, Ho, and Fe atoms while retaining adjustable parameters of O atoms. Although Uij for Sr/Ho and U33 for Fe are relatively high, they are much smaller than that of U33 for oxygen. Subsequently, the ND data were refined to optimize the oxygen parameters while fixing the cationic parameters. After this process was repeated several times, the g value for O(2) was converged to 0.11(3), which is close to the proposed composition of (Sr0.75Ho0.25)FeO2.125. The final agreement parameters are Rp = 3.32%, Rwp = 2.40%, RI = 6.16%, and RF = 6.17% for SXRD and Rp = 5.59%, Rwp = 4.88%, RI = 6.61%, and RF = 3.71% for ND. Other space groups with lower symmetry did not lead to better results. The final results are summarized in Table 2, with the structure shown in Figure 4a. In (Sr1−xHox)FeO2+x/2, square-planar and pyramidal coordination geometries coexist with a ratio of 1 − x and x (Figure 4b). The 57Fe Mössbauer spectrum of the x = 0.3 sample was successfully deconvoluted into two sets of doublets assigned as square-planar (blue) and pyramidal (purple) sites, in addition to a sextet coming from an over-reduced iron metal (green) (Figure 5a and Figure S8 and Table S3 in the Supporting Information). The isomer shift (IS) of pyramidal iron (∼0.8 mm/s) is typical for divalent iron in a high-spin state. As found in SrFeO2,4 the IS for the square-planar site is fairly small (0.5 mm/s) owing to extremely strong covalency. In order to gain further insights into the local geometry, we performed a PDF analysis of SXRD data for the x = 0.2 sample (i.e., (Sr0.8Ho0.2)FeO2.1), together with SrFeO2 for comparison. As shown in Figure 6a,b, the result for SrFeO2 on the basis of the ideal IL structure is quite satisfactory, suggesting the absence of any local distortion and disorder. On the other hand,

Figure 2. Lattice parameters (a, b) and the volume (c) for (Sr1−xLnx)FeO2+x/2 (Ln = Nd, Sm, Ho) as a function of lanthanide concentration x. The solid lines are guides to the eye. The data for x = 0.0 are taken from the previous literature.4

volume are unexpectedly recognized, despite the Ho 3+ substitution with a smaller ionic radius of 1.26 Å in comparison to 1.015 Å for Sr2+ (8-fold coordination).20 This behavior contrasts with that of (Sr1−xLnx)CuO2, where both axes are reduced by Ln substitution.2 Furthermore, the room-temperature resistivity of a hand-pressed pellet of each reduced compound was very high with magnitude greater than >1 MΩ cm, implying a failure in electron doping to the FeO2 sheet. The mass spectroscopy for the reduced sample (x = 0.25) up to 700 °C (Figure S7 in the Supporting Information) probed no hydrogen from the sample. These observations suggest incorporation of excess oxygen atoms between the FeO2 sheets so as to compensate the aliovalent Ln3+/Sr2+ substitution and preserve the iron’s valence of +2, giving a chemical composition of (Sr1−xHox)Fe2+O2+x/2. In order to validate the suggested composition and structure, we carried out a Rietveld analysis of SXRD data for x = 0.25 (Figure 3a). We started the Rietveld structural refinement using the ideal IL structure with the P4/mmm space group, where Fe, O(1), and Sr/Ho atoms were placed at 1a (0, 0, 0), 2f (1/2, 0, 0), and 1d (1/2, 1/2, 1/2), and Sr and Ho atoms were distributed randomly. However, this model gave rather poor reliability factors of RI = 9.51% and RF = 6.98%. Hence, we included the additional oxygen O(2) at the apical site, 1b (0, 0, 0.5). The occupancy factors of O(1) and O(2) sites were refined respectively to g = 1.0 and 0.15, implying a full and partial occupation of equatorial and apical sites. Relatively large isotropic displacement parameters, Uiso, for both O(1) and O(2) (>0.04 Å2) indicate a local displacement from the ideal site. Since the scattering length of oxygen is large for neutrons, we performed a Rietveld refinement of ND using the same sample, C


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Inorganic Chemistry Table 2. Crystallographic Data for (Sr0.75Ho0.25)FeO2.11(3)a atom

site

g

x

y

z

U11 (Å2)

U22 (Å2)

U33 (Å2)

Sr Ho Fe O(1) O(2)

1d 1d 1a 2f 1b

3/4 1/4 1 0.980(8) 0.15(1)

1/2 1/2 0 1/2 0

1/2 1/2 0 0 0

1/2 1/2 0 0 1/2

0.0259(5) 0.0259(5) 0.0067(6) 0.021(1) 0.009(7)

0.0259(5) 0.0259(5) 0.0067(6) 0.032(2) 0.009(7)

0.0246(6) 0.0246(6) 0.030(1) 0.097(2) 0.11(3)

a Crystal data: space group, P4/mmm, a = 3.95349(9) Å, c = 3.57172(9) Å, Rp = 3.32%, Rwp = 2.40%, RI = 6.16%, RF = 6.17% for SXRD data; a = 3.9566(2) Å, c = 3.5392(2) Å, Rp = 5.59%, Rwp = 4.88%, RI = 6.61%, RF = 3.71% for ND data. U12 = U13= U23 = 0. The magnetic structure is of Gtype with 1.2 μB per iron.

Figure 4. (a) Structure of (Sr0.75Ho0.25)FeO2.125 depicted by thermal ellipsoids. Orange, gray, and blue spheres represent O, A-site, and Fe atoms. (b) Proposed local structure by the insertion of the apical oxygen.

PDF patterns in (Sr0.8Ho0.2)FeO2.1 are obviously different, in both the short and long ranges (Figure 6 and Figure S9 in the Supporting Information). For example, peaks centered at 2.0 and 2.6 Å representing Fe−O and A−O bonding, respectively, are asymmetric (see the inset of Figure 6c), indicating unequal bond distances. We analyzed the PDF data assuming the structure obtained above (Figure 6c,d). Similar to the Rietveld refinement, the U33 values for O(1) and O(2), related to the asymmetric peaks around 2.0 and 2.6 Å, became remarkably high (Table S4 in the Supporting Information), thus supporting the structural model obtained by Rietveld analysis. As shown above, the presence of partial apical oxygen (10− 12.5%) induces a significant structural disorder at the oxygen sites. We consider that the incorporated O(2) pushes the neighboring eight equatorial O(1) atoms away along the c axis, as illustrated in Figure 4b. Then, the displaced equatorial oxygen atoms belonging to FeO5 pyramidal coordination induce a distortion of FeO4 square-planar coordination by displacing further neighboring equatorial oxygen atoms along the c axis, resulting in a corrugation spreading over the FeO2 plane. As found in CaFeO2 and BaFeO2,6,8 the flexible nature of the square-planar iron possibly explains the formation of the highly buckled layers. The incoherent buckling should be due to the random occupation of the apical oxygen atoms. Nonlinear evolution of lattice parameters as shown in Figure 2 may be related to the enhanced buckling with x. Such an incoherent buckling in turn exerts an influence on the apical O(2) site, leading to a large U33 value. Unlike the present system, CaFeO2 has a coherent buckling with a cell expansion

Figure 5. 57Fe Mössbauer spectra for (Sr1−xHox)FeO2+x/2 (x = 0.05, 0.1, 0.2, 0.25, 0.3) at room temperature. The red lines represent total fitting curves. The blue, purple, and green lines in x = 0.3 represent subspectra corresponding respectively to square-planar Fe2+, pyramidal Fe2+, and amorphous Fe metal impurity. (b) Distribution of the hyperfine field HF. The spectrum was analyzed as a superposition of sextets having different HF values with a full width at half-maximum of 0.27 mm/s.

to √2ap × √2ap × cp (P4̅21m space group), as a result of a distortion of the square planes toward tetrahedral and their tilting. This coherent buckling reduces the coordination number of Ca to 6. We also examined the reduction behavior of (Sr1−xLnx)FeO3−δ (Ln = Nd, Sm; x ≤ 0.35), and the XRD patterns before and after the hydride reduction are shown in Figures S3−S6 in the Supporting Information. Similarly to the Ho substitution, a tetragonal phase with a ≈ 3.9 Å and c ≈ 3.5 Å was obtained as a single phase, except for the x = 0.35 (Nd) sample, which did not react at all. The x dependence of lattice parameters (Figure 2 and Table S1 in the Supporting Information) revealed a significant increase of the c axis as well as the volume, as found in the Ho case. The lattice expansion and the high electric D


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Inorganic Chemistry

occurs as a consequence of a competition between antiferromagnetic (Fe−Fe) and ferromagnetic (Fe−Mn) interactions along the c axis.13,25 In the present case, however, such frustration is absent, since the out-of-plane Fe−O(2)−Fe interaction must be antiferromagnetic. Ln substitution along with partial occupation of the apical oxygen site exerts a considerable influence on the spin state transition under high pressure. The lattice parameters of (Sr0.75Ho0.25)FeO2.125 as a function of pressure are given in Figure 7, with the XRD profiles appearing in Figure S10 in the

Figure 6. PDF analyses for SrFeO2 (a, b) and for (Sr0.8Ho0.2)FeO2.1 (c, d). The overlying circles and the solid lines represent the observed G(r) intensities and the calculated G(r) intensities. The bottom solid lines represent the residual. The inset of (c) shows the PDF pattern for (Sr0.8Ho0.2)FeO2.1 at 1.5−3 Å.

resistivity of the reduced phases suggest that the Nd- and Smsubstituted samples also adopt the same structure, with the chemical formula of (Sr1−xLnx)FeO2+x/2. The evolution of the a axis with Ln matches with their ionic radii (Nd3+ > Sm3+ > Ho3+) as observed in the cuprates.2 However, there is an opposite trend as to the c axis, implying an enhanced buckling of the FeO2 sheet in the Ho-substituted samples. Despite extensive studies of substitutional chemistry on the IL cuprate,1−3,21−23 the apical oxygen incorporated IL-type structure was not reported. This in turn reflects the robustness of the divalent state and structural flexibility of the squareplanar coordinated iron. A similar charge compensation is suggested in the La-substituted BaFeO2 system (LaNiO2.5-type structure).8 In the IL nickelate, a partially oxidized phase with a stoichiometry of LaNiO2.09 is reported, but it is in fact a lamellar structure consisting of a random intergrowth of the squareplanar and pyramidal layers, due to incomplete reduction from LaNiO2.5.24 Thus, the present study represents a rare example where the oxygen content of the IL-derived structure deviates from 2. Let us move onto physical properties of the Ho-substituted samples. Shown in Figures 5a are Mössbauer spectra for (Sr1−xHox)FeO2+x/2 at room temperature. As x increases, a well-developed sextet becomes smaller and it becomes a doublet at x = 0.3, indicating a significant reduction of TN. This parallels with the neutron refinement of x = 0.25 at room temperature with 1.2 ± 0.1 μB per iron, much smaller than the magnetic moment for a high-spin state of Fe2+. This value is also significantly smaller than 3.1 μB (room temperature) for SrFeO2 with TN of 473 K,4 suggesting a significant reduction in TN. The peak width becomes broader so that each spectrum was fitted allowing a distribution of HF (Figure 5b). It is interesting to address that the TN value of CaFeO2 with a coherent buckling is much higher (420 K)6 in comparison with (Sr0.7Ho0.3)FeO2.15 (TN < room temperature), suggesting that the incoherent buckling in (Sr0.7Ho0.3)FeO2.15 reduces the TN drastically. Here, the elongation of the c axis should not be the main reason for the reduction of TN, since Ba-substituted (Sr0.7Ba0.3)FeO2 with a similar c axis retains a high TN, judging from the HF value of 39 T at room temperature.7 A drastic decrease of TN has been observed in Sr(Fe,Mn)O2, but this

Figure 7. (a) Pressure dependence of the volume for (Sr0.75Ho0.25)FeO2.125. The solid line in (a) represents Birch−Murnaghan fitting. (b−d) Comparisons of (Sr0.75Ho0.25)FeO2.125 and SrFeO2: pressure dependence of the volumes (b) and lattice parameters (c, d). The arrows correspond to Pc.

Supporting Information. The pressure dependence of the volume analyzed by the Birch−Murnaghan equation of state26 shows a deviation above 20 GPa (Figure 7a). The bulk modulus K below 20 GPa is 99 ± 4 GPa, which is slightly smaller than that of SrFeO2 (K = 126 GPa).12 A sudden volume reduction is observed at 27 ± 2 GPa, which suggests the occurrence to a spin state transition, as similar volume reductions at the spin state transition have been reported in SrFeO 2 and Sr3Fe2O5.12,27 The critical pressure Pc is certainly lower than 33−34 GPa for SrFeO2 and Sr3Fe2O5.12,27 We previously reported that the Fe−Fe distance between adjacent face-to-face FeO4 square-planar units (i.e., c axis for SrFeO2 and a axis for Sr3Fe2O5) is an essential parameter for the pressure-induced spin state transition, since squeezing the Fe−Fe distance stabilizes dxz and dyz orbitals.27 However, the Fe−Fe distance of (Sr0.75Ho0.25)FeO2.125 at Pc is 3.20 Å, which is obviously longer than 3.09 Å in SrFeO2 and 3.04 Å for Sr3Fe2O5,27 implying that the Fe−Fe distance is not the sole factor determining Pc. The present result suggests that a shortened in-plane Fe−O distance, which destabilizes the dx2−y2 orbitals,28 also plays a role in reducing Pc, since the Ho-substituted sample has an a axis shorter than that of SrFeO2.

4. CONCLUSION We have investigated the effect of A-site aliovalent substitution on the structural and physical properties in SrFeO2. The hydride reduction of the perovskite precursors results in the incomplete removal of apical oxygen atoms to maintain the divalent iron state. This result indicates in part the failed carrier doping as originally planned. However, the surviving oxygen is found to induce incoherent buckling of the FeO2 plane, leading to a significant reduction in TN. The present high-pressure E


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Inorganic Chemistry

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study on the Ho-substituted system suggests that that not only the out-of-plane but also the in-plane distance is a crucial factor for the spin state transition. The substitution chemistry shown in this study can be viewed as the isovalent conversion of block layers from [Sr]2+ to [Sr1−xLnxOx/2]2+. Since insertion of apical oxygen changes the coordination geometry of the metal in neighboring [MO2]2− layers, such substitution in general can give a new direction of material design that eventually leads to different physical properties.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02513. Powder XRD patterns of (Sr1−xLnx)FeO3−δ and reduced samples, mass spectrometry during heating, 57 Fe Mössbauer spectra for (Sr0.7Ho0.3)FeO2.15, fitting parameters of 57Fe and Mössbauer spectra for (Sr1−xHox)FeO2+x/2, PDF patterns of SrFeO2 and (Sr0.8Ho0.2)FeO2.1, refined parameters of the PDF analysis, and powder synchrotron XRD patterns of (Sr0.75Ho0.25)FeO2.125 under high pressure (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail for H.K.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Japan Science and Technology Agency, CREST, and by a Grant-in-Aid for Challenging Exploratory Research (No. 26620044). Neutron experiments at Echidna in ANSTO were supported by the Japanese Society for Neutron Science. The synchrotron radiation experiments were performed at the BL02B2 of SPring-8, with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal Nos. 2011B1231, 2011B1461, and 2013A1230). High-pressure synchrotron radiation experiments waswere performed under the approval of the Photon Factory Program Advisory Committee (Proposal No. 2012G512 and 2015G012).

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REFERENCES

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