Two Charge Ordering Patterns in the Topochemically Synthesized

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Two Charge Ordering Patterns in the Topochemically Synthesized Layer-Structured Perovskite LaCa2Fe3O9 with Unusually High Valence Fe3.67+ Haichuan Guo,† Yoshiteru Hosaka,† Fabio Denis Romero,† Takashi Saito,† Noriya Ichikawa,† and Yuichi Shimakawa*,†,‡ †

Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan Integrated Research Consortium on Chemical Sciences, Uji, Kyoto 611-0011, Japan

‡

S Supporting Information *

ABSTRACT: A-site-ordered layer-structured perovskite LaCa2Fe3O9 with unusually high valence Fe3.67+ was obtained by low-temperature topochemical oxidation of the A-site layerordered LaCa2Fe3O8. The unusually high valence Fe3.67+ in LaCa2Fe3O9 shows charge disproportionation of Fe3+ and Fe5+ first along the layer-stacking ⟨010⟩ direction below 230 K. Fe3+ is located between the La3+ and Ca2+ layers, while Fe5+ is between the Ca2+ layers. The two-dimensional electrostatic potential due to the A-site layered arrangement results in the quasi-stable ⟨010⟩ charge ordering pattern. Below 170 K, the charge ordering pattern changes, and the 2:1 chargedisproportionated Fe3+ and Fe5+ ions are ordered along the ⟨111⟩ direction. The ground-state charge ordering pattern is stabilized primarily by the electrostatic lattice energy, and the Fe5+ ions are arranged to make the distances between the nearest neighboring Fe5+ as large as possible.

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INTRODUCTION Charge ordering is often observed in oxides with mixed-valence transition-metal ions and highlights the electronic instability of these materials. The ordering patterns give information about charge−spin−lattice couplings. The charge-ordering phenomenon was discovered in magnetite (Fe3O4) as an increase in electrical resistivity with decreasing temperature, known as the Verwey transition, where Fe2.5+ changes to Fe2+ and Fe3+ at 125 K.1,2 More than 60 years after its discovery, the charge-ordering pattern in magnetite was fully solved in 2011 to be related to the three-dimensional electron localization over three neighboring Fe sites in the spinel-type crystal structure.3 Some perovskite-structure manganese oxides such as Pr0.5Ca0.5MnO34,5 and Nd0.5Ca0.5MnO36,7 with Mn3.5+ also show charge ordering at low temperatures, and the ordering patterns of Mn3+ and Mn4+ are affected by Jahn−Teller effects and orbital ordering arrangements. Some transition-metal oxides with not only mixed-valence cations but also integervalence cations can also show peculiar charge-ordering phenomena. A typical example is seen in CaFeO3, where Fe4+ shows charge disproportionation (CD) to Fe3+ and Fe5+ with a metal−insulator transition at 290 K.8−11 A similar CD behavior was recently found in the A-site-ordered perovskite CaCu3Fe4O12 with Fe4+ at the octahedral sites.12−14 In both CaFeO3 and CaCu3Fe4O12, the charge-disproportionated Fe3+ and Fe5+ ions are ordered in three-dimensional rock-salt manner at low temperatures, resulting in alternating large Fe3+O6 octahedra and small Fe5+O6 octahedra. The CD of Fe4+ © XXXX American Chemical Society

can be considered as localization of ligand holes produced by the strong hybridization of low-lying Fe 3d and O 2p orbitals.15 Interestingly, Fe3.67+ in La1/3Sr2/3FeO3 and La1/3Ca2/3FeO3 can also show CD to Fe3+ and Fe5+ in a 2:1 ratio.16−18 In the charge-disproportionated state, the Fe3+ and Fe5+ ions are arranged along the ⟨111⟩ direction of the cubic perovskite cell.19−21 Charge ordering thus usually occurs to maximize the redistribution symmetry in a three-dimensional way. Very recently, CD was also found to occur in compounds adopting two-dimensional layered structures, e.g. the B-siteordered double perovskite Ca2FeMnO6 containing Mn4+ and unusually high valence Fe4+ arranged in alternating layers.22,23 This compound was obtained by low-temperature topochemical oxidation from the layer-structured brownmillerite Ca2FeMnO5,24,25 in which Fe3+ occupies the tetrahedral sites, and Mn3+ occupies the octahedral sites. Treatment with ozone gas at a relatively low temperature oxidizes Fe3+ to Fe4+ and Mn3+ to Mn4+ without disturbing the layered arrangement. Fe4+ in the two-dimensional octahedral layers shows CD below 200 K, and the resultant Fe3+ and Fe5+ are ordered in a checkerboard-type manner. It was concluded that the CD occurs even in the two-dimensional layers. In this study, we are interested in A-site-ordered perovskitestructure oxides. The A-site ordering was found when the difference between the valence states and/or ionic sizes of the Received: January 18, 2017

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DOI: 10.1021/acs.inorgchem.7b00104 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 1. Crystal structures of (a) oxygen-deficient perovskite LaCa2Fe3O8, (b) A-site layer-ordered LaCa2Fe3O9, and (c) A-site-disordered (La1/3Ca2/3)FeO3. CD patterns of (d) A-site layer-ordered LaCa2Fe3O9 with the ⟨010⟩ Fe3+ and Fe5+ arrangement stabilized between 230 and 170 K, (e) A-site layer-ordered LaCa2Fe3O9 with the three-dimensional ⟨111⟩ Fe3+ and Fe5+ arrangement at ground state, and (f) A-site-disordered (La1/3Ca2/3)FeO3 at the ground state. structure model is drawn with VESTA software.35 Electrostatic lattice energy (Madelung energy) was calculated by using the program MADEL34 to sum the attractive and repulsive terms of constituent ions. The oxygen content of LaCa2Fe3O9 was confirmed by thermogravimetric analysis. A thermogravimetry measurement from room temperature to 700 °C was made with a heating rate of 10 °C/ min using a NETZSCH STA/TG-DSC system. The valence states of cations in the compounds were estimated from the bond valence sum (BVS), which is given by the formula BVS = Σiexp[(r0 − ri)/0.37] by using the obtained bond lengths between the cation and coordinated oxygen ions (ri) from the structure analysis. BVS for Fe was calculated with the BVS parameter r0 of 1.759 Å.36,37 57 Fe Mössbauer spectra were measured to confirm the valence and magnetic states of Fe in the compounds. The measurement was performed in transmission geometry with a constant-acceleration spectrometer using a 57Co/Rh radiation source. The velocity scale and the isomer shift (IS) were determined with the relative values of α-Fe at room temperature. The spectrum was fitted to Lorentzian functions using the standard least-squares method. Magnetic properties of the compounds were measured using a Quantum Design MPMS SQUID magnetometer. The magnetic susceptibility was measured at temperatures from 300 to 5 K with an applied field at 10 kOe. Electrical resistivity of the samples was measured by the four-probe method with a Quantum Design PPMS.

A-site ions is significant.26,27 Some of the materials show interesting properties like high electronic conductivity and high oxygen ion diffusion. Thus, the compounds can be used for applications such as membrane oxygen separation and solidoxide fuel cells.28−30 We here focus on the two-dimensional Asite-ordered triple perovskite LaCa2Fe3O9 with unusually high valence Fe. The compound was obtained by low-temperature topochemical oxidation from the A-site-ordered oxygendeficient perovskite LaCa2Fe3O8,31−33 in which the A-site La and Ca ions are ordered in a 1:2 ratio in a layered manner. The resulting fully oxygenated triple perovskite LaCa2Fe 3 O9 contains unusually high valence Fe3.67+ ions in the octahedral sites; two between the La3+ and Ca2+ layers and the other one between the Ca2+ layers. We found that Fe3.67+ in LaCa2Fe3O9 shows CD resulting in a 2:1 ratio of Fe3+ and Fe5+ which order first in a layered manner below 230 K. The ordering pattern then changes below 170 K to a three-dimensional arrangement along the ⟨111⟩ direction in spite of the layered electrostatic potential, although the formation of a three-dimensional charge order would be inhibited. The effects of A-site order/disorder in the CD behaviors will be discussed.

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EXPERIMENTAL SECTION

RESULTS AND DISCUSSION We first made LaCa2Fe3O8 by solid-state reaction. Structure analysis with SXRD data confirmed that the compound crystallizes in an A-site-and-oxygen-vacancy-ordered perovskite in the orthorhombic Pmc2 1 space group (Supporting Information, Figure S1 and Table S1). The crystal structure consists of double layers of FeO6 octahedra alternating with single layers of FeO4 tetrahedra along the b axis. The A sites between the FeO6 octahedron layers are mainly occupied by La3+, and those between the FeO6 octahedron and FeO4 tetrahedron layers are mainly occupied by Ca2+ (Figure 1a). The refined crystal structure is essentially the same as that reported by Hudspeth et al.32 BVS for Fe with octahedral and tetrahedral oxygen coordinations give 3.08 and 3.20, respectively. The 57Fe Mössbauer spectrum of LaCa2Fe3O8 measured at room temperature (Supporting Information, Figure S2 and Table S2) consists of two sextet components with an abundance ratio of approximately 2:1, with ISs 0.36 and

LaCa2Fe3O9 was made by low-temperature topochemical oxidation of the layer-structured oxygen-deficient perovskite LaCa2Fe3O8. The LaCa2Fe3O8 precursor was prepared by a solid-state reaction of La2O3, CaCO3, and Fe2O3. A mixture of raw materials was calcined at 1000 °C for 12 h in air and fired at 1100 °C for 48 h in argon with intermediate grindings. The LaCa2Fe3O8 thus obtained was then mixed with the oxidizing agent KClO4, packed into a gold capsule, placed in a cubic-anvil-type high-pressure apparatus, and treated at 4 GPa and 500 °C for 30 min. The pressure was released after the sample was quenched to room temperature. The obtained structural phase was identified with X-ray diffraction measurements. Detailed crystal structures of the obtained phases were analyzed using synchrotron X-ray diffraction (SXRD) data obtained at BL02B2 in SPring-8. The powder sample was put into a 0.1 mm glass capillary tube to minimize absorption and rotated during the measurement. The SXRD patterns obtained with a 0.773633(7) Å X-ray at temperatures from room temperature down to 90 K were recorded. The crystal structure parameters were refined by the Rietveld method using the program RIETAN-FP.34 The crystal B


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Inorganic Chemistry 0.20 mm/sec, respectively. The values are typical for Fe3+ with octahedral and tetrahedral oxygen coordinations. Thus, all the results confirm that we successfully obtained the layerstructured oxygen-deficient perovskite La3+Ca2+2Fe3+3O8 with layered ordering of the oxygen vacancies as well as the A-site cations. LaCa2Fe3O8 thus obtained was then mixed with the oxidizing agent KClO4, packed into a gold capsule, placed in a cubicanvil-type high-pressure apparatus, and treated at 4 GPa and 500 °C for 30 min. This temperature is low enough to maintain the cation arrangement while oxidizing Fe in the sample. The oxygen content of the resulting material was confirmed by thermogravimetric analysis. The weight decrease of about 3% from the oxidized sample corresponds to 1.0 mol oxygen per formula unit (Figure 2), indicating the sample has the

Figure 3. SXRD pattern and the result of Rietveld structure refinement for oxidized LaCa2Fe3O9. The observed (red + marks) and calculated (black solid line) patterns are shown along with the difference between them (bottom blue line). The ticks indicate the allowed Bragg reflection positions.

A-site-disordered perovskite La1/3Ca2/3FeO3.17 Both perovskites contain Fe ions with unusually high valence states of 3.67+ in oxygen octahedral coordination environments. However, the A-site ordering in LaCa2Fe3O9 results in two crystallographically distinct FeO6 octahedra, one between the La3+ and Ca2+ ions and the other between two layers of Ca2+ ions (Figure 1b). La1/3Ca2/3FeO3 has only a single FeO6 octahedral layer between the (La1/3Ca2/3)2.33+ ions (Figure 1c). The order/disorder in the A-site layers can thus produce a difference in electrostatic potential at the Fe sites in the perovskite structures. When LaCa2Fe3O9 is cooled to low temperatures, structural transitions evidenced by discontinuities in the temperature dependence of lattice parameters and unit cell volume occur at 230 and 170 K (Figure 5). At 230 K, there is a significant increase in the b lattice parameter (along the layer-stacking direction), and there are decreases in the a and c lattice parameters and the unit cell volume. The reverse is observed at 170 K, where there are sharp increases in the volume and the a and c lattice parameters and a decrease in the b lattice parameter. In the SXRD patterns obtained with decreasing temperature, development of an additional diffraction peak at 2θ ≈ 13.4° is observed below 150 K (Figure 6). This peak can be indexed as the (2/3 2/3 2/3) reflection of the cubic perovskite cell. The first-order type of volume increase at 170 K and the development of the superstructure reflection below this temperature are behaviors similar to those observed in the Asite-disordered La1/3Ca2/3FeO3,17 but the transition at 230 K was not seen in the A-site-disordered La1/3Ca2/3FeO3. This transition at 230 K and the phase stabilization between 230 and 170 K are thus characteristic for the A-site layer-ordered compound. The Mössbauer spectrum obtained at 200 K consists of three components: a magnetically ordered sextet and two paramagnetic singlets (Figure 4b). One of the paramagnetic component with an IS = 0.14 mm/sec originates from the singlet observed at room temperature, which suggests that some portion of the room temperature paramagnetic phase still remains below the transition temperature at 230 K. CD in this compound is a gradual progress, and about one-third of Fe3.67+

Figure 2. Changes in the sample weight during thermogravimetric measurement in air. The weight decrease of about 3% from the oxidized LaCa2Fe3O9 corresponds to 1.0 mol oxygen per formula unit.

composition of LaCa2Fe3O9. The SXRD data collected from the oxidized sample includes a superstructure diffraction peak at 2θ ≈ 3.8°, indicating that the A-site La and Ca ordering remains after the high-pressure treatment. A model based on an A-site-ordered triple perovskite structure [√2a × 6a × √2a unit cell in the space group Pnma (Figure 1b)] was constructed and refined against the SXRD data with good fits, as shown in Figure 3. Details of the refined structure are listed in Table 1. The refined occupancies for the A-site La and Ca are the same as those in LaCa2Fe3O8. The absence of vacancies at any of the oxygen sites is also confirmed in the structure analysis. The BVS values calculated from the structure refinement results are 3.66 for Fe(1) and 3.59 for Fe(2). The Mössbauer spectrum at room temperature (Figure 4a and Table 2) consists of a single paramagnetic component with the IS of 0.14 mm/sec, which is much smaller than the ISs in the precursor LaCa2Fe3O8, indicating that the valence states of Fe in LaCa2Fe3O9 are higher than those in LaCa2Fe3O8. Given the fully oxygenated perovskite composition LaCa2Fe3O9, a simple charge-neutral ionic model also gives an average Fe valence state of 3.67+. All these results are consistent with the presence of unusually high valence Fe. We therefore conclude that the high-pressure annealing at 500 °C in an oxidizing atmosphere changed the Asite layer-ordered LaCu2Fe3+3O8 to LaCa2Fe3.67+3O9 topochemically without changing the cation arrangement. Note that the chemical composition of the obtained A-siteordered triple perovskite LaCa2Fe3O9 is identical to that of the C


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Inorganic Chemistry Table 1. Refined Structure Parameters of LaCa2Fe3O9 at Room Temperaturea atom

site

x

y

z

Biso (Å2)

occupancy

La(1)/Ca(1) Ca(2)/La(2) Fe(1) Fe(2) O(1) O(2) O(3) O(4) O(5)

4c 8d 4b 8d 8d 4c 8d 8d 8d

0.0092(5) 0.9707(6) 0.5 0.5055(9) 0.528(2) 0.521(4) 0.211(2) 0.239(3) 0.280(3)

0.25 0.08190(7) 0 0.16338(6) 0.0820(2) 0.25 0.0116(4) 0.1578(3) 0.1780(3)

0.9959(9) 0.005(1) 0 0.002(2) 0.933(2) 0.075(2) 0.224(3) 0.240(3) 0.720(3)

0.07(2) 0.20(4) 0.32(3) 0.12(2) 0.36(6) 0.36(6) 0.36(6) 0.36(6) 0.36(6)

0.753(1)/0.247(1) 0.8766(7)/0.1234(7) 1 1 1 1 1 1 1

a

The Wyckoff positions in space group Pnma, coordinates, isotropic atomic displacement parameter Biso, site occupancy, and lattice parameters are listed together with the refinement reliability factors. Numbers in parentheses are standard deviations of the last significant digit. a = 5.4289(1); b = 22.9852(4); c = 5.4223(1) Å; V = 676.62(2) Å3; Rwp = 6.77%; and RB = 2.77%.

Table 2. Mössbauer Parameters of the Precursor LaCa2Fe3O8, the A-Site Layer-ordered LaCa2Fe3O9, and the A-Site Disordered La1/3Ca2/3FeO3a temperature (K) RT

RT 200

5

RT 5

component

IS (mm/sec)

HF (T)

QS (mm/sec)

precursor LaCa2Fe3O8 Fe3+ (oct): S 0.36 51.65 −0.31 Fe3+ (tet): S 0.20 42.57 0.62 A-site ordered LaCa2Fe3O9 Fe3.67+: D 0.14 0.17 Fe3.67+: Sg 0.14 Fe3+: S 0.32 38.08 −0.04 Fe5+: Sg 0.05 Fe3+: S 0.40 45.19 −0.01 Fe5+: S −0.04 26.58 −0.02 A-site disordered (La1/3Ca2/3)FeO317 Fe3.67+: D 0.14 0.18 Fe3+: S 0.39 45.81 −0.02 Fe5+: S −0.03 26.45 −0.03

area ratio (%) 67 33 100 36 41 23 68 32 100 69 31

a

S, sextet spectrum; D, doublet spectrum; Sg, singlet spectrum; IS, isomer shift; HF, hyperfine field; QS, quadruple splitting.

for Fe3+ octahedrally coordinated by oxygens, and the other singlet with an IS of 0.05 mm/sec appears to originate from paramagnetic high-valence Fe (Table 2). The area ratio of the latter two components is about 2:1, which suggests that the unusually high valence Fe3.67+ shows 2:1 CD to Fe3+ and Fe5+. Because no apparent superstructure peaks appear in the SXRD patterns obtained at temperatures between 230 and 170 K, a commensurate arrangement where Fe3+ are located in the octahedra between the La3+ and Ca2+ layers and Fe5+ in the octahedra between the Ca2+ layers (Figure 1d) is likely to be stabilized. In the structure analysis with the SXRD data obtained at 185 K (Supporting Information, Figure S3 and Table S3), we indeed see that the BVS value for Fe(1), which is located between the Ca2+ layers, increased to 3.73 (from 3.66 at room temperature), while that for Fe(2), which is located between the La3+ and Ca2+ layers, decreased slightly to 3.58 (from 3.59 at room temperature). The Mössbauer spectrum obtained at 5 K consists of two sextets consistent with charge-disproportionated Fe3+ and Fe5+ with an area ratio of 2:1 (Figure 4c). Their ISs are 0.40 and −0.04 mm/sec, which are typical values of Fe3+ and Fe5+, respectively, indicating complete CD from Fe3.67+ to Fe3+ and Fe5+. The observed 2:1 spectral components are very close to those observed in the charge-disproportionated La1/3Ca2/3FeO317 and La1/3Sr2/3FeO318 (Table 2). The (2/3

Figure 4. Mössbauer spectra of LaCa2Fe3O9 at (a) room temperature, (b) 200 K, and (c) 5 K. The circles show experimental data, and the lines show Lorentzian fits.

does not show CD at this temperature. The sextet with an IS of 0.32 mm/sec and a hyperfine field (HF) of 38.08 T is typical D


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Inorganic Chemistry

along the ⟨111⟩ direction. This is surprising because the charge order pattern along the three-dimensional ⟨111⟩ direction is stabilized in spite of the two-dimensional electrostatic potential due to the ⟨010⟩ layered arrangement of the A-site La3+ and Ca2+ ions. We here discuss the relative stability of a few possible charge arrangement patterns by evaluating their electrostatic lattice energies (Table 3). It is clear that the charge-disproportionated Table 3. Electrostatic Lattice Energies (eV per LaCa2Fe3O9) of A-Site-Ordered/Disordered LaCa2Fe3O9 with Fe3+/Fe5+ CD Arrangements along the ⟨010⟩ and ⟨111⟩ Directions lattice energy (eV) A-site layer-ordered LaCa2Fe3.67+3O9 Fe uniform distribution −535.7 Fe3+/Fe5+ ⟨010⟩ layer −541.7 arrangement Fe3+/Fe5+ ⟨111⟩ 3D arrangement −543.1 A-site-disordered (LaCa2)Fe3.67+3O9 Fe3.67+ uniform distribution −533.3 Fe3+/Fe5+ ⟨010⟩ layer −529.4 arrangement Fe3+/Fe5+ ⟨111⟩ 3D arrangement −540.3

energy difference (eV)

3.67+

−6.0 −7.4

+3.9 −7.0

structures give lattice energies lower than those of the Fe3.67+mixed-valence room-temperature structures, confirming that the charge-disproportionated states are electrostatically stable at low temperatures. Importantly, in the A-site-layer-ordered LaCa2Fe3O9 the 2:1 CD pattern along the ⟨111⟩ direction (−543.1 eV/f.u.) is more stable than the layered CD pattern along the ⟨010⟩ direction (−541.7 eV/f.u.). Therefore, the three-dimensional Fe3+ and Fe5+ arrangement along the ⟨111⟩ direction is the ground state even in the two-dimensional layered crystal structure. Because both A-site-ordered and Asite-disordered LaCa2Fe3O9 show the same three-dimensional CD patterns at low temperatures, it is concluded that the electrostatic lattice energy primarily determines the CD pattern to minimize the electrostatic lattice energy. The Fe5+ ions are arranged to make the distances between the nearest neighboring Fe5+ as large as possible. It is also noted that the ⟨010⟩ layered CD pattern in the A-site disordered La1/3Ca2/3FeO3 is unstable, and this may explain why this CD pattern does not appear in this compound. Although the charge-disproportionated ground states of the A-site-ordered LaCa 2 Fe 3 O 9 and the A-site-disordered La1/3Ca2/3FeO3 are similar, the CD transition behaviors of these compounds are different, and we see the different charge−spin−lattice couplings at the CD transition temperatures. A plot of the magnetic susceptibility of the A-site layerordered LaCa2Fe3O9 as a function of temperature (Figure 7a) contains a sharp increase at the ⟨010⟩ CD transition temperature of 230 K followed by a decrease before reaching an essentially temperature-independent value below the ⟨111⟩ transition temperature of 170 K. The small magnetization observed between 230 and 170 K is presumably caused by the canting of antiferromagnetically coupled Fe3+, while the Fe5+ in the octahedra between the Ca2+ layers are paramagnetic. The decrease in the magnetic susceptibility is ascribed to the adoption of a three-dimensional antiferromagnetic arrangement of the charge-disproportionated Fe3+ and Fe5+ spins, which was proposed in the La1/3Sr2/3FeO3 system.19 The electrical resistivity increases in the whole measured range. The slope

Figure 5. Temperature dependence of (a) lattice parameters and (b) unit cell volume of LaCa2Fe3O9.

Figure 6. SXRD patterns of LaCa2Fe3O9 at 300, 200, 150, and 100 K. A superstructure diffraction peak at 2θ ≈ 13.4° is observed below 150 K.

2/3 2/3) reflection observed in the SXRD patterns obtained below 150 K (Figure 6) indicates that the charge-disproportionated Fe3+ and Fe5+ are ordered along the ⟨111⟩ direction (Figure 1e). The results strongly suggest that the chargedisproportionated ground state of the present A-site-ordered LaCa2Fe3O9 is very similar to those of the A-site-disordered La1/3Ca2/3FeO3 and La1/3Sr2/3FeO3 and that both A-siteordered and A-site-disordered compounds show the same CD patterns in the ground states. The results also imply that the CD pattern changes at 170 K from along the ⟨010⟩ direction to E


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Inorganic Chemistry

dimensional CD pattern is stabilized primarily by the electrostatic lattice energy, and the Fe5+ ions are arranged to make the distances between the nearest neighboring Fe5+ as large as possible. The observed successive transitions are characteristic for the A-site layer-ordered compound, and the three-dimensional electrostatic lattice energy overcomes the two-dimensional layered electrostatic potential at the ground state.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00104. (1) Structure analysis result of the precursor oxygendeficient perovskite LaCa2Fe3O8, (2) Mössbauer spectrum data of LaCa2Fe3O8, and (3) structure analysis result of LaCa2Fe3O9 at 185 K, including additional figures and tables (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Email: [email protected]. ORCID

Yuichi Shimakawa: 0000-0003-1019-2512

Figure 7. Temperature dependences of magnetic susceptibility and resistivity of (a) A-site layer-ordered LaCa2Fe3O9 and (b) A-sitedisordered La1/3Ca2/3FeO3.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank H. Seki, P. Xiong, and K. Manabe at Kyoto University for help during the experiments and D. Kan for fruitful discussion. The SPring-8 experiments were performed with the approval of the Japan Synchrotron Radiation Research Institute (Proposal 2015B1757). This work was partly supported by Grants-in-Aid for Scientific Research (Grants 22740227, 24540346, 16H00888, and 16H02266) and by a grant for the Integrated Research Consortium on Chemical Sciences from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. The work was also supported by Japan Society for the Promotion of Science (JSPS) Core-to-Core Program (A) Advanced Research Networks and Japan Science and Technology Agency (JST), CREST.

in the temperature dependence of resistivity changes at about 170 K but not at 230 K (Figure 7a). Only the complete threedimensional ⟨111⟩ CD transition is accompanied by the change in the transport properties. Note again that the changes in charge, spin, and lattice observed at 170 K are quite similar to those observed at the CD transition in the A-site-disordered La1/3Ca2/3FeO3 at 217 K (Figure 7b), where the 2:1 CD transition to Fe3+ and Fe5+ along the ⟨111⟩ direction occurs.

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CONCLUSIONS The two-dimensional A-site-ordered layer-structured perovskite LaCa2Fe3O9 was obtained by low-temperature topochemical oxidation of the A-site layer-ordered LaCa2Fe3O8. The results of SXRD crystal structure analysis and Mössbauer spectroscopy confirmed that, at room temperature, LaCa2Fe3O9 contains unusually high valence Fe3.67+ ions in the octahedral sites: two between the La3+ and Ca2+ layers and the other one between the Ca2+ layers. The unusually high valence Fe3.67+ in LaCa2Fe3O9 shows CD of Fe3+ and Fe5+ first along the layerstacking ⟨010⟩ direction below 230 K. Fe3+ is located between the La3+ and Ca2+ layers, while Fe5+ is between the Ca2+ layers, although this ordering may be incomplete. At this transition temperature, the lattice parameter b increases significantly; the magnetically ordered Fe3+ results in an increase in the magnetic susceptibility, but the resistivity shows no anomaly. The twodimensional electrostatic potential due to the A-site layered arrangement results in the quasi-stable ⟨010⟩ CD ordering pattern. Below 170 K, the CD pattern changes, and the 2:1 completely charge-disproportionated Fe3+ and Fe5+ ions are ordered along the ⟨111⟩ direction. The first-order-type transition is confirmed by the abrupt changes in unit-cell volume, magnetic susceptibility, and resistivity. The three-

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REFERENCES

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Two Charge Ordering Patterns in the Topochemically Synthesized

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