A Layered Double Perovskite Ca2FeMnO6 with Unusually High Valence Fe4+ Obtained by Low-Temperature Topotactic Oxidation

Yoshiteru HOSAKA; Noriya ICHIKAWA; Takashi SAITO; Masaichiro MIZUMAKI; Yuichi SHIMAKAWA

doi:10.2497/jjspm.63.605

Abstract

A new double perovskite Ca₂FeMnO₆ with a layered arrangement of Mn⁴⁺ and the unusually high valence Fe⁴⁺ was obtained by oxidizing the brownmillerite Ca₂FeMnO₅ with ozone at 200 °C. The low-temperature topotactic reaction kept the layered cation arrangement of the brownmillerite but oxidized Mn³⁺ to Mn⁴⁺ and Fe³⁺ to Fe⁴⁺. The crystal structure with a P2₁/c space group was analyzed with the synchrotron X-ray diffraction data. The changes in the valence states of Fe and Mn were confirmed by bond valence sum calculations, X-ray absorption spectroscopy, and Mössbauer spectra measurements.

1 Introduction

The stable valence states of Fe ions in oxides are 2+ and 3+. Unusually high valence states of Fe, however, can be stabilized by synthesis under strong oxidizing atmosphere. CaFeO₃ is one of the oxides containing such unusually high valence Fe ions^1–3). The compound shows a characteristic behavior, i.e. charge disproportionation (2Fe⁴⁺ → Fe³⁺ + Fe⁵⁺) at 290 K with the structural and metal-to-insulator transitions. In the charge disproportionated state below the temperature Fe³⁺ and Fe⁵⁺ ions are arranged in a rock-salt type manner⁴⁾. LaCu₃Fe₄O₁₂ is another compound containing the unusually high valence Fe⁵⁾. The compound with high valence state Fe^3.75+ shows intersite charge transfer (3Cu²⁺ + 4Fe^3.75+ → 3Cu³⁺ + 4Fe³⁺) at 393 K with paramagnetism-to-antiferromagnetism and metal-to-insulator transitions. Both charge disproportionation and intersite charge transfer can be considered as the results of localization of oxygen p holes, which are produced by strong hybridization of Fe-d and O-p orbitals, to relieve the instability of the unusually high valence states. In CaFeO₃ and LaCu₃Fe₄O₁₂ the Fe ions are arranged in the three-dimensional perovskite-type structures, and the localization of p holes at the Fe sites results in the charge disproportionation whereas that at the Cu sites gives the intersite charge transfer.

There have been so far no reports on compounds with unusually high valence Fe ions arranged in a two-dimensional way. We thus attempt to make such a new compound with Fe⁴⁺ and another transition-metal ion in a layered arranged manner. Because highly oxidizing atmosphere at high temperature is needed to stabilize the unusually high valence state of Fe in the oxides^1,5), it is usually difficult to keep the layered arrangement of the two kinds of transition-metal ions. In this study, we focus on a brownmillerite-type structure compound, Ca₂FeMnO₅ with the layered arrangement of Fe³⁺ and Mn³⁺ ^6,7), and modify it to a layered double perovskite Ca₂FeMnO₆ with Fe⁴⁺ and Mn⁴⁺ (see the crystal structures in Fig. 1) by low temperature topochemical oxidation with ozone. An advantage by using an ozone strong oxidizing agent is that we can increase the Fe valence state in the oxide at a relatively low temperature with keeping the ion ordered arrangement.

Fig. 1

Crystal structures of brownmillerite Ca₂FeMnO₅ (left) and double perovskite Ca₂FeMnO₆ (right). Ca₂FeMnO₆ is obtained from Ca₂FeMnO₅ by low-temperature topotactic oxidation by ozone.

2 Experimental details

The precursor material Ca₂FeMnO₅ was prepared by solid-phase reaction. Stoichiometric amount of Ca₂CO₃, Fe₂O₃, and MnCO₃ were mixed, calcined at 1000 °C in air, ground, and fired at 1250 °C in air. The obtained powder sample was then pressed into a pellet and annealed at 1100 °C in vacuum. The prepared Ca₂FeMnO₅ sample was oxidized to Ca₂FeMnO₆ at 200 °C for 6 hours in ozone containing oxygen flow.

The crystal structures of the compounds were analyzed with powder X-ray diffraction (XRD) data taken at room temperature at the beamline BL02B2 in SPring-8 with a wave length 0.775 Å. Rietveld refinement was performed using the RIETAN-FP program⁸⁾. The valence states of Fe and Mn were estimated by the bond valence sum (BVS) calculations from the refined bond lengths^9,10), ⁵⁷Fe Mössbauer spectroscopy, and X-ray absorption spectroscopy (XAS). The ⁵⁷Fe Mössbauer spectra were obtained in transmission geometry in combination with a constant-acceleration spectrometer using ⁵⁷Co/Rh as a radiation source. α-Fe was used as a control for velocity calibration and isomer shift (IS). The obtained spectra were fitted by a least-squares method with Lorentzian functions. XAS spectra near K-edges of Fe and Mn were measured at the beam line BL01B1 in SPring-8. The spectra at 300 K were obtained by a transmission method.

3 Results and discussion

Fig. 2 (a) shows the XRD pattern of the precursor Ca₂FeMnO₅ and the result of Rietveld refinement. The observed pattern is well fitted with the brownmillerite structure with the Pnma space group. The refinement results listed in Table 1 are essentially the same as those reported previously^11,12). The Fe and Mn ions respectively are located at the tetrahedral and octahedral sites in a layered manner along the b axis. The refined Fe-O bond lengths, 1.805(5), 1.805(5), 1.967(1), and 1.869(1) Å give the BVS value of 3.1 (Table 2), indicating Fe³⁺ at the tetrahedral site. The Mn-O bond lengths, 2.244(5) Å, along the b axis and 1.922(1) and 1.903(1) Å along the a and c axes, form distorted octahedra and give the BVS value of 3.2 (Table 2), suggesting Mn³⁺. This highly anisotropic distortion of the octahedra is thus due to the Jahn-Teller active Mn³⁺ ions. The structure refinement result confirms that Ca₂FeMnO₅ crystallizes in the brownmillerite structure with the layered arrangement of the Fe³⁺O₄ tetrahedra and the Mn³⁺O₆ octahedra.

Fig. 2

Synchrotron X-ray diffraction patterns at room temperature and the results of Rietveld analyses for brownmillerite Ca₂FeMnO₅ and double perovskite Ca₂FeMnO₆. The dots and solid line represent observed and calculated patterns, respectively. The plots below the diffraction patterns are the differences between the observed and calculated intensities. Vertical marks below the profiles are Bragg reflection positions of main phase (green) and a small amount of Ca₂MnO₄ impurity (purple).

Table 1 Refined structural parameters of Ca₂FeMnO₅. Space group, Pnma; a = 5.3027(3) Å, b = 15.3489(8) Å, c = 5.4523(3) Å, and R_wp = 7.48 %.

Atom	x	y	z	B (Å²)	Occupancy
Ca1	0.9862(5)	0.1095(1)	0.4779(4)	0.79(5)	1.0
Fe1	0.9547(5)	0.25	0.9399(5)	0.86(6)	1.0
Mn1	0.0	0.0	0.0	0.35(5)	1.0
O1	0.260(3)	0.9872(4)	0.244(1)	0.9(1)	1.0
O2	0.019(1)	0.14411(3)	0.068(1)	0.9(1)	1.0
O3	0.090 (2)	0.25	0.623(2)	0.9(1)	1.0

Table 2 Bond valence sum (BVS) values of Ca₂FeMnO₅ and Ca₂FeMnO₆ calculated from the refined structures.

Compound	Ion	BVS
Ca₂FeMnO₅	Fe	3.1
Ca₂FeMnO₅	Mn	3.2
Ca₂FeMnO₆	Fe	4.0
Ca₂FeMnO₆	Mn	4.1

The valence state of Fe in Ca₂FeMnO₅ is also examined by Mössbauer spectroscopy. The spectrum at room temperature shown in Fig. 3 (a) is fitted with a magnetically ordered sextet component, and its IS and hyperfine field respectively are 0.18 mm/sec 35.0 T. These values are typical for magnetic Fe³⁺ and they are also close to the values reported in the previous reports¹³⁾. The result thus also confirms that the Fe ion in the brownmillerite Ca₂FeMnO₅ has 3+ valence state at the single tetrahedral site.

Fig. 3

Mössbauer spectra of Ca₂FeMnO₅ (a) and Ca₂FeMnO₆ (b) at room temperature. The dots are observed data and solid lines are fittings.

Fig. 2 (b) shows the XRD pattern of the ozone oxidized Ca₂FeMnO₆. The pattern is well reproduced with a perovskite structure model with the space group of P2₁/c and √2a_PC × 2a_PC × √2a_PC unit cell (a_PC represents a perovskite unit cell). The details of refined parameters are listed in Table 3. All the oxygen sites are fully occupied making the FeO₆ and MnO₆ octahedra, and the crystal structure of Ca₂FeMnO₆ is the double perovskite. Although the refined crystal structure has the monoclinic space group P2₁/c symmetry, the refined β angle converges to 90.00(1) °. The BVS values are 4.0 for Fe and 4.1 for Mn (Table 2), confirming that both Fe and Mn are oxidized to the high valence states. By the low-temperature topotactic reaction, oxygen ions are introduced into the Ca₂FeMnO₅, changing the Fe³⁺O₄ tetrahedra to the Fe⁴⁺O₆ octahedra in Ca₂FeMnO₆ with keeping the two-dimensional layered arrangement of Fe and Mn ions. Since the scattering factors of X-ray for Fe and Mn are close each other, it was difficult to confirm the layered ordering by X-ray diffraction. We thus analyzed the detailed structure with neutron diffraction, and the refined B-site occupancies of Fe/Mn were 92.2(4)/7.8(4) and 14.4(2)/85.6(2) for each layer confirming that the layered ordering remains in Ca₂FeMnO₆ ⁶⁾.

Table 3 Refined structural parameters of Ca₂FeMnO₆. Space group; P2₁/c, a = 7.5037(3) Å, b = 5.3037(3) Å, c = 5.3155(2) Å, and β = 90.00(1) °. R_WP = 4.78 %.

Atom	x	y	z	B (Å²)	Occupancy
Ca1	0.2470(7)	0.9918(8)	0.0307(3)	0.46(3)	1.0
Fe1	0.0	0.5	0.0	0.4(1)	1.0
Mn1	0.5	0.5	0.0	0.1(1)	1.0
O1	0.253(2)	0.081(2)	0.4879(9)	0.1(1)	1.0
O2	0.16(2)	0.722(4)	0.272(4)	0.3(2)	1.0
O3	0.531(2)	0.303(4)	0.726(4)	0.5	1.0

The Mössbauer spectroscopy for Ca₂FeMnO₆ also confirms the unusually high valence state of Fe. The spectrum at room temperature, shown in Fig. 3 (b), consists of a singlet component with IS of 0.02 mm/sec, which is close to that observed for Fe⁴⁺ in CaFeO₃ ¹⁾. Because no other spectra observed, all Fe³⁺ ions in the precursor brownmillerite are fully oxidized to Fe⁴⁺ in the double perovskite Ca₂FeMnO₆.

The changes in the valence states of both Fe and Mn are also clearly seen in the XAS results. Fig. 4 (a) shows the XAS spectra near the Fe K-edge for Ca₂FeMnO₅ and Ca₂FeMnO₆. The figure also includes the spectrum for Fe₂O₃ as a reference of Fe³⁺. The Fe absorption edge of Ca₂FeMnO₅ at about 7120 eV is quite similar to that of Fe₂O₃, confirming Fe in Ca₂FeMnO₅ is in the Fe³⁺ valence state. The absorption edge of Ca₂FeMnO₆ is shifted to a higher energy than those of Fe₂O₃ and Ca₂FeMnO₅, indicating that Fe in Ca₂FeMnO₆ becomes in the unusually high valence state. It is also interesting to see the change in the peak shape of the pre-edge at about 7109 eV. This pre-edge peak mainly reflects the Fe-3d electronic states through the 1s→3d quadrupole transition, but the 1s→4p dipole transition also contributes the intensity when the Fe is located at the tetrahedral site. The observed decrease in the intensity of the pre-edge peaks clearly indicates that the oxygen coordination of Fe changes from the tetrahedral one in Ca₂FeMnO₅ to the octahedral one in Ca₂FeMnO₆. The Mn K-XAS spectra also show the clear shift by the low-temperature oxidation, as shown in Fig. 4 (b). The absorption edge of Ca₂FeMnO₆, which is located at about 6550 eV, is higher than that of Ca₂FeMnO₅ at 6547 eV, indicating the increase in the valence state of Mn.

Fig. 4

X-ray absorption spectra of Ca₂FeMnO₅ and Ca₂FeMnO₆ near Fe-K (a) and Mn-K (b) edges The spectrum of Fe₂O₃ is also shown in (a) as a reference of Fe³⁺.

All the results of the crystal structure analysis, the Mössbauer spectroscopy, and the XAS measurements, show that the precursor brownmillerite Ca₂FeMnO₅ changes to the fully oxygenated double perovskite Ca₂FeMnO₆ by low-temperature topotactic reaction. The Fe³⁺ in the tetrahedra and Mn³⁺ in the octahedra in Ca₂FeMnO₅ change to the unusually high valence Fe⁴⁺ and Mn⁴⁺ both in the octahedra in Ca₂FeMnO₆. An important finding in the present study is that the two-dimensional layered arrangement of Fe and Mn are kept, and the obtained Ca₂FeMnO₆ has the two-dimensional arranged unusual Fe⁴⁺ ions in the crystal structure, which is the first compound made. Interestingly, we recently found that Fe⁴⁺ in Ca₂FeMnO₆ shows charge disproportionation in the two-dimensional layers^6,7).

4 Summary

We prepared the brownmillerite-structure oxide Ca₂FeMnO₅ and oxidized it to a layered double perovskite Ca₂FeMnO₆ by low-temperature topotactic oxidation with ozone. The precursor Ca₂FeMnO₅ had the layered arrangement of Fe³⁺O₄ tetrahedra and highly distorted Mn³⁺O₆ octahedra, while the oxidized Ca₂FeMnO₆ had the layered arrangement of Fe⁴⁺O₆ and Mn⁴⁺O₆ octahedra. The changes in the valence states of Fe and Mn ions were confirmed by BVS calculations, XAS measurement and Mössbauer spectroscopy. The obtained Ca₂FeMnO₆ had the two-dimensional layered arrangement of the unusually high valence Fe⁴⁺.

5 Acknowledgement

We thank D. Kan and H. Seki in ICR Kyoto Univ. (Japan) for useful discussions. The synchrotron radiation experiments at SPring-8 were performed with the approval of the Japan Synchrotron Radiation Research Institute (proposal Nos: 2014B1770, 2013A1008, 2013A1009, 2013B1017, and 2012B1978). This work was supported by Grants-in-Aid for Scientific Research (Nos. 19GS0207, 22740227, and 24540346) and by a grant for the Joint Project of Chemical Synthesis Core Research Institutions from the Ministry of Education, Culture, Sports, Science and Technology of Japan. The work was also supported by Japan Science and Technology Agency, CREST.

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