Unusual Ferromagnetic Metal: A-Site-Layer-Ordered Double

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Cite This: Chem. Mater. XXXX, XXX, XXX−XXX

Unusual Ferromagnetic Metal: A‑Site-Layer-Ordered Double Perovskite YBaCo2O6 with Unusually High Valence Co3.5+ Masato Goto,*,† Takashi Saito,† 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

‡

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S Supporting Information *

ABSTRACT: A-site-layer-ordered double perovskite YBaCo2O6 with unusually high valence Co3.5+ was found to be an unusual ferromagnetic metal with strong electron correlation. The compound was obtained by topochemically oxidizing the A-site-layer-ordered YBaCo2O5, which contains nominal Co2.5+, in ozone at a low temperature. YBaCo2O6 crystallizes as a fully oxygenated A-site-layer-ordered double perovskite with a tetragonal structure at room temperature and shows an orthorhombic distortion below approximately 140 K. This anisotropic structural distortion induces ferromagnetic ordering of the mixed-valence Co spins with intermediate spin states, suggesting strong spin−lattice coupling in this system. In the ferromagnetic state, the up-spin band mainly consisting of hybridized Co 3d and O 2p crosses the Fermi level, giving the metallic behavior, regardless of the strong electron correlation.

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INTRODUCTION

A-site cation order in the double perovskites is much less common than B-cation order and a few compounds with the A-site-layered order were reported.1−3 Among them, layerordered oxygen-deficient perovskites, RBaCo2O5 (R: Y or lanthanoid) with mixed-valence Co2.5+, are structurally interesting compounds.4−7 The layer ordering of R3+ and Ba2+ induces the layer ordering of oxygen vacancies along the stacking direction, giving an apically connected double layer of corner-sharing CoO5 square pyramids (see structure inset in Figure 1a). In this class of compounds, extra oxygen δ can be introduced into the R-layer, changing them into RBaCo2O5+δ with CoO5 pyramids and CoO6 octahedra.8−12 For some specific values of δ, superlattice structures of CoO5/CoO6 are formed by the ordering of the oxygen vacancies in the layered plane.8−11 On the other hand, oxygen can also be removed from RBaCo2O5 by topochemical reduction. For example, YBaCo2O4.5 and YBaCo2O4.25 with CoO5 and CoO4 were reported to be stabilized.13 The extra oxygen δ not only modifies the local oxygen coordination but also increases the valence state of the Co ions. As a result, the electronic structures of the compounds and spin states of the Co ions are significantly changed, giving rise to a wide variety of physical properties.4−11,14,15 In particular, when the difference between the ionic radii of R3+ and Ba2+ is large, various exotic electronic phases appear owing to strong electron-lattice coupling.4,5,8,9,11,16,17 In YBaCo2O5+δ, with the nonmagnetic small Y3+ ions at the R site, for example, the δ = 0 (Co2.5+) compound shows a G-type antiferromagnetic ordering below 330 K and a charge ordering of Co2+/ © XXXX American Chemical Society

Figure 1. Results of the Rietveld refinement of the X-ray diffraction data at 300 K for (a) YBaCo2O5 and (b) YBaCo2O6. The dots and solid line represent observed and calculated patterns, respectively. The plots below the diffraction patterns are the difference between the observed and calculated intensities. Vertical marks below the profiles are Bragg reflection positions. Each crystal structure is shown in the inset.

Co3+ below 220 K.4,8 The compounds with 0.35 ≤ δ ≤ 0.4 (≈Co2.9+) show successive magnetic transitions,8 and those with 0.5 ≤ δ ≤ 0.52 (≈Co3+) also show complicated transitions in the transport and magnetic properties accompanied by a change of the spin state.8,11 On the other hand, it was difficult to obtain a fully oxygenated YBaCo2O6 with the unusually high valence Co3.5+. Received: October 3, 2018 Revised: November 8, 2018

A

DOI: 10.1021/acs.chemmater.8b04203 Chem. Mater. XXXX, XXX, XXX−XXX

Article

Chemistry of Materials The maximum δ reported so far is 0.52 and was obtained in a compound synthesized by a solid-state reaction under high oxygen pressure (100 atm).8 Although RBaCo2O6 (R = La, Pr, Nd) perovskites with relatively large R-site ions were reported,18−20 there were some arguments about degree of the ordering of the R and Ba ions at the A site because of their close ionic radii. We thus considered using low-temperature topochemical ozone oxidation of YBaCo2O5 to obtain fully oxidized YBaCo2O6. This technique is also useful for obtaining oxides with unusually high valence cations such as Fe4+ and Co4+, as we saw in the synthesis of the B-site-layer-ordered perovskite Ca2FeMnO6 with Fe4+ from the brownmillerite Ca2FeMnO5.21−23 In this work, we have synthesized a fully oxygenated polycrystalline sample of YBaCo2O6 by low-temperature topochemical ozone oxidation of the precursor YBaCo2O5. The obtained A-site-ordered perovskite YBaCo2O6 contains unusually high valence Co3.5+ and shows a ferromagnetic transition at 140 K, where a structural transition also occurs. We found that the compound is an unusual ferromagnetic metal with strong electron correlation. We report the structural and physical properties of YBaCo2O6 and discuss the structure−property relations.

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indexed with an orthorhombic ap × ap × 2ap cell (ap: a lattice parameter for the corresponding simple perovskite structure) with the space group Pmmm. The refined lattice constants are a = 3.89263(2) Å, b = 3.88654(2) Å, and c = 7.48456(3) Å, which are in good agreement with those reported in the previous papers.4,8 As shown in Figure 1b, the diffraction peak positions for the oxidized sample slightly change from those for the precursor YBaCo2O5, with preserving a superlattice reflection (d = 7.56 Å) originating from the layered ordering of Y/Ba. All of the peaks can be well indexed with a tetragonal ap × ap × 2ap cell with the space group P4/mmm and a = 3.85707(2) Å and c = 7.55931(5) Å. The lattice constant c is significantly greater than that of YBaCo2O5, confirming the incorporation of oxygen ions into the structure. Although the precise oxygen content is difficult to determine from the structural analysis with SXRD data, it is estimated to be approximately 6.0 from the TGA measurement as shown in Figure 2. Thus, lowtemperature topochemical oxidation with ozone introduces extra oxygen (δ ≈ 1) into the precursor YBaCo2O5.

EXPERIMENTAL SECTION

A polycrystalline sample of YBaCo2O6 was obtained by annealing the precursor YBaCo2O5 topochemically with ozone. The precursor YBaCo2O5 was first synthesized by a solid-state reaction of raw oxide materials under conditions similar to those reported by Akahoshi and Ueda.8 The synthesized precursor was then heated at 280 °C with a heating rate 35 °C/min under flowing ozone gas. The ozone gas was generated from oxygen gas in a dielectric barrier discharge reactor. Phase identification and structure analysis were conducted with synchrotron X-ray diffraction (SXRD). The data of YBaCo2O5 at 300 K were collected using TPS09A beamline at National Synchrotron Radiation Research Center (NSRRC) (λ = 0.82656 Å), and those of YBaCo2O6 in a temperature range of 100−300 K were collected using BL02B2 beamline at SPring-8 (λ = 0.60116 Å). Each sample was packed into a glass capillary that was rotated during the measurement. The low-temperature diffraction data were collected by cooling the sample with a nitrogen-gas-flow-type refrigerator. The obtained data were analyzed by the Rietveld method using the program RIETANFP.24,25 The oxygen content of the oxidized sample was determined by thermogravimetric analysis (TGA) from room temperature to 1000 °C in a flow of 5% H2−95% Ar mixture with a heating rate 10 °C/min using a NETZSCH STA 449 F3. The dc magnetization measurement was performed in a temperature range of 5−300 K using a Quantum Design MPMS-XL. The heat capacity measurement in a temperature range of 5−300 K was performed using a Quantum Design PPMS-9LHS. The electronic structure of YBaCo2O6 was calculated by fullpotential linearized augmented plane-wave (FLAPW) first-principle calculations with the WIEN2k code. The lattice constant and atomic position parameters obtained by the structural refinement were used in this calculation. The FLAPW sphere radii for Y, Ba, Co, and O were, respectively, 2.45, 2.50, 1.89, and 1.63 a.u. An effective Ueff (U = U − J) was introduced for Co. Self-consistency was carried out on 1000 k-point meshes in the whole Brillouin zone.

Figure 2. TGA curve of the oxygenated sample YBaCo2O5+δ. Corresponding Co valence states are given on the right-side scale.

Given the full oxygen occupancy for the O3 site, the crystal structure of the oxidized sample was analyzed with the SXRD data (Figure 1b). The refinement of the site occupancy gives Ba:Y = 1.007:−0.007 for Ba site and −0.007:1.007 for Y site, confirming the A-site-ordered double perovskite YBaCo2O6. The refined structural parameters of YBaCo2O6 are listed in Table 1. Note that c is much shorter (2%) than 2a, giving that the CoO6 octahedra are compressed along the c axis. In the octahedron, as shown in the inset of Figure 1b, Co and the inplane O2 are slightly shifted to the YO layer owing to the large chemical pressure from the BaO layer. The apical distance dCo−O1 (1.901 Å) is longer than dCo−O3 (1.879 Å), producing Table 1. Refined Structural Parameters of YBaCo2O6 at 300 K with Rwp = 4.402, the Space Group P4/mmm, a = 3.85707(2) Å, and c = 7.55931(5) Åa

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RESULTS AND DISCUSSION A single-phase sample of the precursor YBaCo2O5 was obtained. As shown in the SXRD pattern at 300 K (Figure 1a), a superlattice reflection at d = 7.48 Å, which indicates the layered orderings of the A-site Y/Ba and the oxygen vacancies, is observed. All of the observed diffraction peaks can be

atom

Wyckoff position

x

y

z

B (Å2)

Ba Y Co O1 O2 O3

1a 1b 2h 1c 4i 1d

0 0 0.5 0.5 0.5 0.5

0 0 0.5 0.5 0 0.5

0 0.5 0.7485(2) 0 0.2871(5) 0.5

0.43(2) 1.62(4) 0.43(2) 1.85(7) 1.85(7) 1.85(7)

a

The numbers in parentheses are the standard deviations of the least significant figures.

B

DOI: 10.1021/acs.chemmater.8b04203 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials asymmetrically distorted CoO6 octahedra with a site symmetry 4mm at the Co site. With decreasing temperature, YBaCo2O6 is found to show a structural transition. As shown in the temperature dependence of the SXRD data in Figure 3a, the 200 reflection peak of the

Figure 4. Temperature dependence of the magnetization M and the inverse magnetic susceptibility H/M for YBaCo2O6 under a fieldcooled condition. The inset shows the M−H curve at 5 K between −5 and 5 T.

typical ferromagnetic behavior, and the saturation magnetization under 5 T is approximately 0.8 μB/Co, as shown in the inset of Figure 4. Considering the mixed ionic state of Co3.5+ (Co3+/Co4+) in YBaCo2O6, the observed spin quantum number S seems to indicate that both Co3+ and Co4+ are in the intermediate spin (IS) states 1(t52ge1g) and 3/2(t42ge1g), respectively. Although the observed saturation magnetization (0.8 μB/Co) is smaller than that expected from the ferromagnetic ordering of IS states of both Co3+ and Co4+, the strong hybridization of Co d orbitals to oxygen p orbitals could reduce the moments. At the orthorhombic structural transition, in-plane anisotropic hybridization can induce the magnetic ordering of YBaCo2O6, which is considered to be closely related to in-plane magnetic anisotropy observed in the thin-film sample.26 The heat capacity Cp was also measured. As shown in the Cp/T−T plot between 2 and 300 K (Figure 5), only one

Figure 3. (a) Temperature dependence of the 200 peak of SXRD data for YBaCo2O6. (b) Temperature dependence of lattice constants a, b, and c, volume V, the bond distances between Co and O, and the distortion of CoO6 for YBaCo2O6.

tetragonal structure broadens below 140 K and apparently splits into two peaks below 110 K, suggesting a symmetry lowering from a high-temperature tetragonal to low-temperature orthorhombic cell. In the low-temperature phase, the observed diffraction peaks can be reproduced with the space group Pmmm (see the Supporting Information). The temperature dependences of the lattice constants, the unit-cell volume, and the Co−O bond distances are shown in Figure 3b. The unit-cell volume decreases monotonically and shows no anomaly at the structural phase transition temperature Ts (≈140 K). In addition, neither the compressibility defined as 2dCo−O(inplane)/(dCo−O1 + dCo−O3) with the average of four inplane Co−O distances dCo−O(inplane) and the apical distances dCo−O1 and dCo−O3 nor the asymmetric distortion of the octahedra defined as dCo−O1/dCo−O3 show an anomaly at Ts. These behaviors are different from those observed in the Asite-layer-ordered LaBaCo2O6, where the asymmetric distortion of the CoO6 octahedra is released without any structural transition.18 In the magnetization measurement of YBaCo2O6, a ferromagnetic-like increase is observed at the structural transition temperature Ts. This is in sharp contrast to the antiferromagnetic properties of YBaCo2O5+δ (0 ≤ δ ≤ 0.52).4,8,11 Although a similar behavior has been reported recently for a thin-film sample of YBaCo2O6,26 neither the structural transition nor its relation to the magnetic anomaly has been reported. Figure 4 shows the temperature dependence of the magnetization M and the inverse magnetic susceptibility χ−1 = H/M measured under 0.1 T of the present bulk compound. Above Ts, the χ−1−T plot is linear and is well fitted with the Cure−Weiss law χ = C/(T − θ), where C = peff2/8 is the Curie constant, θ is the Weiss temperature, and peff is the effective Bohr magneton number. The resulting θ = 139 K is very close to Ts, indicating that the transition is ferromagnetic and that the dominant interaction of magnetic Co ions is ferromagnetic as well. The peff = 2.97 roughly corresponds to S = 1 for g = 2. The M−H curve at 5 K shows a

Figure 5. Temperature dependence of the heat capacity divided by a temperature, Cp/T, for YBaCo2O6. The inset shows the Cp/T−T2 plot below 20 K.

anomaly at approximately 140 K is observed. This too is evidence that both orthorhombic distortion and ferromagnetic transitions occur simultaneously at 140 K. In this system, therefore, the lattice and spin properties are strongly coupled. Note also that, from the Cp/T−T2 plot below 20 K, YBaCo2O6 is suggested to be metallic. The data below 20 K show a typical behavior of metallic compounds and is well fitted to Cp = γT + βT3, where γ and β are, respectively, the electronic and lattice heat capacity coefficients. Actually, the metallic transport property of YBaCo2O6 has been observed in the thin-film sample.26 Interestingly, the obtained γ, which is proportional to the density of states (DOS) at the Fermi surface, is about 50 mJ mol−1 K−2, which is much larger than that expected from C

DOI: 10.1021/acs.chemmater.8b04203 Chem. Mater. XXXX, XXX, XXX−XXX

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the free electron model. The results thus suggest that the oxidized YBaCo2O6 is an unusual ferromagnetic metal with strong electronic correlation. The unusual ferromagnetic and metallic state of YBaCo2O6 is also suggested by density functional theory (DFT) electronic structure calculation. As indicated by the unusually large γ values obtained in the heat capacity measurement, strong electron correlation, for instance 6 eV for the Co ions, is included in the calculation. As shown in the calculated DOS in Figure 6, a spin polarized metallic state is obtained. The

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b04203. Result of the Rietveld refinement of the X-ray diffraction data at 100 K for YBaCo2O6 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: (+81) 774-383115 (M.G.). *E-mail: [email protected]. Phone: (+81) 774-383110. Fax: (+81) 774-38-3118 (Y.S.). ORCID

Masato Goto: 0000-0002-8198-7622 Yuichi Shimakawa: 0000-0003-1019-2512 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank S. Kawaguchi in SPring-8, Y.-C. Chuang and H.-S. Sheu in NSRRC, and W.-T. Chen in National Taiwan University for assistance with the SXRD experiments. The synchrotron radiation experiments were performed at the Japan Synchrotron Radiation Research Institute, Japan (proposal no. 2017B1557), and the National Synchrotron Radiation Research Center, Taiwan (proposal no. 2017-1125). This work was partly supported by Grants-in-Aid for Scientific Research (nos. 16H00888, 16H02266, and 17K19177) 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. This work was also supported by the Japan Society for the Promotion of Science (JSPS) Core-to-Core Program (A) Advanced Research Networks.

Figure 6. Calculated DOS of ferromagnetic YBaCo2O6.

calculated total magnetic moment is 1.5 μB/Co, which is larger than the ferromagnetic moment observed in the magnetization measurement. This would be due to the strong hybridization of Co 3d orbitals to O 2p orbitals. Interestingly, the up-spin DOS, which mainly consists of hybridized Co 3d and O 2p states, crosses the Fermi level, contributing to the metallic behavior, whereas the down-spin DOS has a gap, producing a nearly halfmetallic electronic structure. Therefore, even with the strong electron correlation, the ferromagnetic spin-polarized metallic state is stabilized in the present YBaCo2O6.

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REFERENCES

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CONCLUSION We succeeded in obtaining the fully oxidized A-site-layerordered double perovskite YBaCo2O6 topochemically from the precursor YBaCo2O5 by low-temperature ozone oxidation. Structure analysis with SXRD data revealed that YBaCo2O6 crystallizes in a tetragonal structure at room temperature and shows an orthorhombic distortion below approximately 140 K. At the structural transition temperature, a ferromagnetic transition is induced, which suggests strong spin-lattice coupling. YBaCo2O6 contains unusual high valence Co3.5+ and the spins order ferromagnetically below 140 K. Lowtemperature electronic heat capacity data revealed that the compound is a metal with strong electronic correlation. The DFT calculation with large Ueff clarified that the up-spin band mainly contributes the metallic property in the spin polarized electronic structure. Therefore, the present A-site-layerordered double perovskite YBaCo2O6 with unusual high valence Co3.5+ is concluded to be an unusual ferromagnetic metal with strong electron correlation. D

DOI: 10.1021/acs.chemmater.8b04203 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials

Chemical Topotactic Oxidation. J. Mater. Chem. C 2018, 6, 3445− 3450.

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DOI: 10.1021/acs.chemmater.8b04203 Chem. Mater. XXXX, XXX, XXX−XXX