Hexagonal Perovskite Ba4Fe3NiO12 Containing Tetravalent Fe and

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Hexagonal Perovskite Ba4Fe3NiO12 Containing Tetravalent Fe and Ni Ions Zhenhong Tan,† Takashi Saito,† Fabio Denis Romero,†,‡ Midori Amano Patino,† Masato Goto,† Wei-Tin Chen,§ Yu-Chun Chuang,∥ Hwo-Shuenn Sheu,∥ and Yuichi Shimakawa*,⊥ †

Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan Hakubi Center for Advanced Research, Kyoto University, Yoshida-honmachi, Sakyo-ku, Kyoto 606-8501, Japan § Center for Condensed Matter Sciences, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan ∥ National Synchrotron Radiation Research Center, 101 Hsin-Ann Road, Hsinchu Science Park, Hsinchu 30076, Taiwan ⊥ Integrated Research Consortium on Chemical Sciences, Uji, Kyoto 611-0011, Japan

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‡

S Supporting Information *

ABSTRACT: BaFexNi1−xO3 with end members of BaNiO3 (x = 0) and BaFeO3 (x = 1), which, respectively, adopt the 2H and 6H hexagonal perovskite structures, were synthesized, and their crystal structures were investigated. A new single phase, Ba4Fe3NiO12 (x = 0.75), that adopts the 12R perovskite structure with the space group R3̅m (a = 5.66564(7) Å and c = 27.8416(3) Å), was found to be stabilized. Mössbauer spectroscopy results and structure analysis using synchrotron and neutron powder diffraction data revealed that nominal Fe3+ occupies the corner-sharing octahedral site while the unusually high valence Fe4+ and Ni4+ occupy the face-sharing octahedral sites in the trimers, giving a charge formula of Ba4Fe3+Fe4+2Ni4+O11.5. The magnetic properties of the compound are also discussed.

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INTRODUCTION Some 3d-transition-metal ions in oxides can show unusually high valence states, in addition to the conventional valence states. For example, Fe ions, whose conventional valence states are 2+ and 3+, can be Fe4+ when they are in oxides synthesized under a strongly oxidizing atmosphere. Fe4+ in the simple perovskite CaFeO3 shows a characteristic behavior, i.e., charge disproportionation (2Fe4+ → Fe3+ + Fe5+) below 290 K with structural and metal-to-insulator transitions to relieve the electronic instability.1,2 Fe4+ was also found to be stabilized in the A-site ordered perovskite structure oxide CaCu3Fe4O12 and shows a similar charge disproportionation transition at 210 K.3−5 Also, although the conventional valence states of Ni ions are 1+ and 2+, Ni ions in oxides can show the unusually high valence state Ni4+. So far, however, only a few compounds containing Ni4+ have been reported, and the properties originating from a characteristic electronic structure of Ni4+ have not been explored in detail.6−8 It has been reported that both Fe4+ and Ni4+ are stabilized in hexagonal-type perovskite structure oxides, whose chemical formula is ABO3. The hexagonal-type perovskite structure consists of close-packed layers with the A′B′ stacking (hereafter referred to as “h”), while the simple perovskite structure consists of cubic close-packed layers with the A′B′C′ stacking (referred to as “c”). Fe4+ in BaFeO3 (BFO) is stabilized with the 6H hexagonal perovskite structure, which consists of the hexagonal/cubic layers ...cch..., with cornersharing octahedra and face-sharing dimerized octahedra, but © XXXX American Chemical Society

does not show charge disproportionation at temperatures between room temperature and 4 K.9,10 The magnetic properties of Fe4+ in the compound are complicated, and the competing ferro- and antiferromagnetic interactions give rise to a metamagnetic transition at 178 K.11 On the other hand, unusually high valence Ni4+ is stabilized in the 2H hexagonal perovskite BaNiO3 (BNO), which consists of the hexagonal layers ...hhh...6,7 Because the Ni4+ in BNO has a low-spin 3d6 electron configuration, the compound is diamagnetic.12 This is in contrast to the Ni4+ ions in the simple cubic perovskite SrFe0.5Ni0.5O3 having substantial magnetic moments due to the strong hybridization of Ni4+, Fe4+, and O2− ions.13 In this study, we are interested in the structural changes between the above hexagonal perovskites, 6H BFO and 2H BNO, and are curious about the properties that arise when they are stabilized in the system. We have therefore synthesized BaFexNi1−xO3 (BFNO) by using a high-pressure and high-temperature technique and have investigated their structural and magnetic properties of BFNO.

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EXPERIMENTAL METHODS

Precursors for the synthesis of BFNO were first prepared by solidstate reactions of BaCO3 and NiO for BaNiOy and of BaCO3 and Fe2O3 for BaFeOz. The oxygen contents of the prepared precursors were determined by thermogravimetric analysis and iodometric Received: June 10, 2018

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DOI: 10.1021/acs.inorgchem.8b01618 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry titration to be y = 2.81 and z = 2.67. The precursors were then mixed according to the substitution index x (x = 0, 0.1, 0.2, 0.5, 0.75, 0.8, 0.9, and 1) with an oxidizing agent KClO4, packed into a platinum capsule, placed in a cubic-anvil-type high-pressure apparatus, and treated at 8 GPa and 1273 K for 30 min. The pressure was released after the sample was quenched to room temperature. The sample was then washed with distilled water to remove the remaining KCl. The obtained powder samples BaFexNi1−xO3 (x = 0, 0.1, 0.2, 0.5, 0.75, 0.8, 0.9, and 1) were characterized with powder X-ray diffraction (XRD, AXS Advance D8 diffractometer, Bruker). Detailed crystal structures of BaFexNi1−xO3 (x = 0, 0.75, and 1) were analyzed by using synchrotron X-ray diffraction (SXRD) data collected at TPS09A in NSRRC (λ = 0.82656 and 0.61992 Å) and BL02B2 in SPring-8 (λ = 0.5996871 Å). The powder sample was filled into a 0.1 mm glass capillary tube to minimize absorption and rotated during the measurement. The crystal structure parameters were refined by the Rietveld method using the computer program RIETAN-FP,14,15 and the crystal structure model was drawn by the VESTA software.16 Neutron powder diffraction (NPD) data for BaFexNi1−xO3 (x = 0.75) was collected at room temperature by using the GEM diffractometer at the ISIS Neutron Source. The powder sample was sealed in a vanadium can, and the data was refined by Rietveld method using the GSAS suite of programs.17 The valence states of cations in the BFNO samples were estimated from the refinement results by using bond valence sum (BVS) given by the formula BVS = ∑iexp[(r0 − ri)/0.37], where ri represents the bond lengths between the cation and the coordinated oxygen ions.18 BVS values for Fe and Ni were calculated using r0 values of 1.780 and 1.750 Å, respectively.19,20 57Fe Mössbauer spectra were measured to confirm the valence and magnetic states of Fe in the compounds at room temperature and 5 K. 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 by using the standard least-squares method. Magnetic properties of the samples were measured by using a Quantum Design MPMS SQUID magnetometer. The magnetic susceptibility data was collected at temperatures from 400 to 5 K with an applied field of 100 Oe. Magnetic hysteresis was measured at 5 and 300 K with applied fields ranging from −10 to +10 kOe.

1.888(1) Å is 4.13, suggesting that the Ni valence state is 4+. On the other hand, for BFO, the BVSs for Fe at the two different crystallographic sites are calculated to be 2.83 (2a site) and 4.24 (4f site), indicating that Fe3+ is included in addition to unusually high valence Fe4+. This is consistent with the 57Fe Mössbauer result reported by Gallagher et al.10 With increasing x (0 < x < 0.75) in the system, the peak intensity of the 2H BNO phase decreases and a new phase characterized by diffraction peaks at 2θ ≈ 24.1° and 29.0° appears. The observed diffraction peaks of this new phase can be indexed with the 12R hexagonal-type perovskite structure, and a single phase of the sample is obtained at x = 0.75 [BaFe0.75Ni0.25O3 (Ba4Fe3NiO12, BFNO75)]. With further increasing x (0.75 < x < 1.0), the fraction of the BFNO75 phase decreases and that of the 6H BFO phase increases. The changes in lattice parameters, cell volume, and mass fraction of 2H BNO, 12R BFNO75, and 6H BFO, which were obtained from refinements by using XRD data (Supporting Information), are plotted in Figure 2. Note that when the phase

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RESULTS AND DISCUSSION Figure 1 shows the XRD patterns of BaFexNi1−xO3 (x = 0, 0.1, 0.2, 0.5, 0.75, 0.8, 0.9, and 1). They confirm that the end

Figure 2. Changes in (a) lattice parameters, (b) lattice volumes, and (c) mass fractions of 2H BaNiO3 (red), 12R BFNO75 (green), and 6H BaFeO3 (purple) phases, as a function of x in BaFexNi1−xO3.

Figure 1. XRD patterns of BaFexNi1−xO3 (x = 0, 0.1, 0.2, 0.5, 0.75, 0.8, 0.9, and 1).

changes from 2H to 12R to 6H, the lattice parameters a are comparable but the lattice parameters c decrease, leading to the significant decrease in the cell volume V. Because shared octahedral faces increase the electrostatic repulsion between the B-site cations, the observed decrease in V is consistent with the decrease in the fraction of face-sharing octahedra.

member samples (x = 0 and 1) are single phases and crystallize, respectively, with the 2H- and 6H-type hexagonal perovskite structures in the space group P63/mmc, as previously reported.7,9 The structure refinement results for BNO and BFO are shown in the Supporting Information. For BNO, the Ni BVS, calculated from the refined bond distance of B


Article

Inorganic Chemistry

Figure 3. (a) SXRD pattern and Rietveld refinement result for 12R BFNO75 with Ni at the 3b site and Fe at the 3a and 6c sites. The green, black, orange, and red vertical marks indicate the Bragg peak positions of BFNO75 (wt = 90.14%), BaCO3 (wt = 4.45%), NiO (wt = 1.55%), and 6H BFO (wt = 3.85%), respectively. (b) NPD pattern and Rietveld refinement result for 12R BFNO75 at room temperature. The green vertical marks indicate the peak positions of BFNO75. Structural data follow: space group R3̅m; a = 5.6700(2) Å; c = 27.8588(9) Å; Rwp = 3.69%; Rp = 4.77%; χ2 = 2.28%. The red dots and green line represent the observed data and fitted pattern, respectively. The bottom blue solid line represents the difference between observed and calculated intensities.

by the 57Fe Mössbauer measurement result described later. The fact that the single phase for 12R structure is obtained only at x = 0.75 (Fe:Ni = 3:1) in the system appears to be related to the selective Ni and Fe occupations. The final refinement result for BFNO75 gives the rhombohedral (R3̅m) crystal structure with the lattice parameters a = 5.66564(7) and c = 27.8416(3) Å. The obtained BVS values for Fe1 (3a), Fe2 (6c), and Ni1 (3b) calculated from the refinement results are 3.11, 4.07, and 3.95, respectively. From the above BVS values, we can assign nominal Fe3+, Fe4+, and Ni4+, respectively, to the 3a, 6c, and 3b sites. Given oxygen nonstoichiometry by considering charge neutrality of the compound, we can deduce a possible charge formula for BFNO75 as Ba4Fe3+Fe4+2Ni4+O11.5. Because the corner-sharing octahedra with Fe at the 3a site are sandwiched by the facesharing octahedral trimers under significant tensile strain, the Fe ions in the sites are difficult to oxidize to the unusually high valence state and are, therefore, readily accompanied by a small amount of oxygen vacancies. Indeed, in the structure refinement results for NPD at room temperature shown in Figure 3b and Supporting Information, the occupancies for O1 in the face-sharing octahedra and O2 in the corner-sharing octahedra are 0.988(4) and 0.958(5), respectively, giving an oxygen content of 11.64, which is close to the oxygen content in the proposed formula Ba4Fe3+Fe4+2Ni4+O11.5. The refined occupancy for O2 also indicates that the site in the cornersharing octahedra includes vacancies, keeping the Fe at the 3a site in the 3+ state. 57 Fe Mössbauer spectroscopy results for BFNO75 also give information on the valence states of Fe with the preferential site occupations. The spectrum at 5 K (Figure 5a) consists of two sets of magnetically ordered sextets with IS value of +0.473 and −0.145 mm s−1 and the area ratio of approximately 1:2, respectively (Table 2). The former IS value is typical for octahedrally coordinated Fe3+ as seen in LaFe3+O3 (IS = 0.48 mm s−1) at 15 K,21 while the latter (negative) suggests an unusually high valence state of Fe.22,23 Although the observed area ratio of the two components is slightly different from the

The crystal structure of the 12R BFNO75 phase is analyzed in detail with the SXRD data, and the result is shown in Figure 3a. The structure is described by the ...cchh... stacking sequence with three octahedral sites: 3a with corner-sharing oxygens, 3b with face-sharing oxygens, and 6c with three corner-sharing and three face-sharing oxygens (Figure 4c). In

Figure 4. Crystal structures of (a) 2H BaNiO3, (b) 6H BaFeO3, and (c) 12R BFNO75.

the refinement it is difficult to distinguish between Fe and Ni because the atomic scattering factors of Fe and Ni are very close. Nevertheless, we can conclude that there is selective site occupation, with Ni occupying the 3b site and Fe occupying the 3a and 6c sites. Indeed, this structure model with the selective Ni and Fe occupations gives a better fitting result (Rwp = 4.93% and Rp = 3.58%, Table 1) in the refinement to the observed SXRD data than does a model with Ni(3a) and Fe(3b and 6c) occupations (Rwp = 5.17% and Rp = 3.78%, Supporting Information). Another possible model with random occupation of Ni and Fe for the three sites is excluded C


Article

Inorganic Chemistry Table 1. Crystallographic Parameters Refined from SXRD Data Collected from BFNO75 at Room Temperature atom

site

occupancya

x

y

z

Biso (Å2)

Ba1 Ba2 Fe1 Ni1 Fe2 O1 O2

6c 6c 3a 3b 6c 18h 18h

1.0 1.0 1.0 1.0 1.0 1.0 1.0

0 0 0 0 0 0.1505(3) 0.1639(4)

0 0 0 0 0 −0.1505(3) −0.1639(4)

0.28781(2) 0.12715(2) 0 0.5 0.40836(5) 0.4568(1) 0.6252(1)

0.42(1) 0.35(1) 0.24(4) 0.52(3) 0.28(3) 0.71(7) 0.19(6)

All site occupancies were fixed during the refinements. Structural data follow: space group R3̅m; a = 5.66564(7) Å; c = 27.8416(3) Å; Rwp = 4.93%; Rp = 3.58%. a

not seem to be reasonable due to insulating behavior of BFNO75. Temperature dependence of the magnetic susceptibility of BFNO75 is shown in Figure 6, and the observed behavior is

Figure 6. Temperature dependence of magnetic susceptibility for BFNO75 measured at an applied magnetic field of 100 Oe after zerofield cooling (ZFC) and field cooling (FC). Figure 5. Mössbauer spectra of BFNO75 at (a) 5 K and (b) room temperature. Components shown in red and blue, respectively, correspond to the spectra of Fe4+ or Fe3+.

different from those of 2H BNO and 6H BFO (Supporting Information). The very small anomaly seen at about 180 K is due to the magnetic transition of a tiny amount of 6H BFO impurity.11 Ferromagnetic-like increase and divergence between FC and ZFC data in the magnetic susceptibility below about 200 K suggest a magnetic transition at that temperature. The extrapolated magnetic moment at zero applied field of about 1.0 μB/f.u., observed at 5 K (Figure 7), suggests ferrimagnetic or weak ferromagnetic behavior in BFNO75. It is difficult, however, to construct a magnetic structure model. Because the observed magnetic moment increases with increasing applied field, the saturation magnetic moments of BFNO75 cannot be determined. Nevertheless, it is interesting to consider the role of the unusual Ni4+ ion in the magnetic properties of BFNO75 from the structural features. The volume of the [Ni4+O 6 ]8− octahedron (V = 9.089 Å3) is quite small, and approximates that of the [Ni4+O6]8− octahedron in diamagnetic BaNiO3 (V = 8.824 Å3). This small-sized octahedron increases the crystalfield splitting energy between the t2g and eg orbitals of Ni and stabilizes the low-spin 3d6 electron configuration. Actually, the S = 0 low-spin state Ni4+ in the face-sharing octahedra was recently reported in a similar 12R hexagonal perovskite Ba4Ni2Ir2O12.24 Therefore, the observed weak ferromagnetism seems to originate from antiferromagnetic (ferrimagnetic) coupling between Fe3+ and Fe4+ spins, and Ni4+ with a low-spin

Table 2. Mössbauer Parameters for BFNO75 at Room Temperature and 5 K T (K) 298 5

component Fe1 Fe2 Fe1 Fe2

(Fe3+) (Fe4+) (Fe3+) (Fe4+)

IS (mm s−1) 0.354 −0.220 0.473 −0.145

Hhf (T)

Q (mm s−1)

area ratio (%)

48.7 23.9

0.532 0 0.182 0.0024

46.4 53.6 41.1 58.9

one expected from the site multiplicities, it seems to be reasonable to assign the first component Fe to the 3a site and the second to the 6c site. The observation of the two components in the Mössbauer spectrum also rules out the possibility of random occupation of Fe and Ni at the three distinct crystallographic sites. Note that the Mö ssbauer spectrum at room temperature can also be reproduced by the same two (but paramagnetic doublet) components (Figure 5b). This implies that Fe4+ in BFNO75 does not show charge disproportionation between room temperature and 5 K. Another charge formula Ba4Fe(4−2δ)+Fe(4+δ)+2Ni4+O12 (δ ≈ 0.5) with oxygen stoichiometry is possible, but the model does D


Inorganic Chemistry

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Article

AUTHOR INFORMATION

Corresponding Author

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

Fabio Denis Romero: 0000-0002-0402-6983 Yuichi Shimakawa: 0000-0003-1019-2512 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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Figure 7. Magnetic field dependence of magnetic moment at 5 and 300 K for BFNO75.

ACKNOWLEDGMENTS We thank D. Kan and Á . M. Arévalo-López for helpful suggestions and discussions and thank S. Kawaguchi for assistance with the SXRD experiments at SPring-8, and thank J. P. Attfield for the help with the NPD experiments at ISIS Neutron Source. The synchrotron radiation experiments were performed at the Japan Synchrotron Radiation Research Institute, Japan (proposal nos. 2016A1353 and 2016A1533), and the National Synchrotron Radiation Research Center, Taiwan (proposal nos. 2017-1-125). 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.

electron configuration does not seem to contribute to the magnetism in BFNO75.

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CONCLUSIONS A new 12R hexagonal perovskite was found to be stabilized in the BaFexNi1−xO3 system prepared by using a high-pressure and high-temperature technique. Its crystal structure consists of corner-sharing octahedra and face-sharing octahedral trimers with a ...cchh... stacking sequence of close-packed layers. The structure analysis from SXRD and NPD data, along with Mössbauer spectroscopy results, revealed that nominal Fe3+ occupies the corner-sharing octahedral site while the unusually high valence Fe4+ and Ni4+ occupy the face-sharing octahedral sites in the trimers, giving a charge formula of Ba4Fe3+Fe4+2Ni4+O11.5. A small amount of oxygen vacancies are included in the corner-sharing octahedra. At temperatures below about 200 K, this compound shows a ferromagnetic-like behavior with small magnetization. Ni4+ is suggested to have a low-spin electron configuration and not contribute to the magnetic properties of the compound, and thus, the observed weak ferromagnetism of this compound seems to originate from ferrimagnetic coupling between Fe3+ and Fe4+ spins.

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REFERENCES

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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.8b01618. SXRD patterns and Rietveld refinement results for 2H BNO and 6H BFO; XRD patterns and Rietveld refinement results for BaFexNi1−xO3 (x = 0, 0.1, 0.2, 0.5, 0.75, 0.8, 0.9, and 1); Rietveld refinement result of NPD for 12R BFNO75; SXRD pattern, Rietveld refinement result, and structure for 12R BFNO75 with a model where Ni at 3a site and Fe at 3b and 6c sites; and magnetic susceptibility data for 2H BNO and 6H BFO (PDF) Accession Codes

CCDC 1850278−1850280 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. E


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Hexagonal Perovskite Ba4Fe3NiO12 Containing Tetravalent Fe and

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