Three Oxidation States of Manganese in the Barium Hexaferrite

Article pubs.acs.org/IC

Three Oxidation States of Manganese in the Barium Hexaferrite BaFe12−xMnxO19

Sandra Nemrava,† Denis A. Vinnik,*,‡ Zhiwei Hu,§ Martin Valldor,§ Chang-Yang Kuo,§ Dmitry A. Zherebtsov,‡ Svetlana A. Gudkova,‡,∥ Chien-Te Chen,⊥ Liu Hao Tjeng,§ and Rainer Niewa† †

University of Stuttgart, Stuttgart 70049, Germany South Ural State University, Chelyabinsk 454080, Russia § Max-Planck-Institute for Chemical Physics of Solids, Dresden 01187, Germany ∥ Moscow Institute of Physics and Technology (State University), Dolgoprudny, Moscow Region 141700, Russia ⊥ National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan ‡

S Supporting Information *

ABSTRACT: The coexistence of three valence states of Mn ions, namely, +2, +3, and +4, in substituted magnetoplumbitetype BaFe12−xMnxO19 was observed by soft X-ray absorption spectroscopy at the Mn-L2,3 edge. We infer that the occurrence of multiple valence states of Mn situated in the pristine purely iron(III) compound BaFe12O19 is made possible by the fact that the charge disproportionation of Mn3+ into Mn2+ and Mn4+ requires less energy than that of Fe3+ into Fe2+ and Fe4+, related to the smaller effective Coulomb interaction of Mn3+ (d4) compared to Fe3+ (d5). The different chemical environments determine the location of the differently charged ions: with Mn3+ occupying positions with (distorted) octahedral local symmetry, Mn4+ ions prefer octahedrally coordinated sites in order to optimize their covalent bonding. Larger and more ionic bonded Mn2+ ions with a spherical charge distribution accumulate at tetrahedrally coordinated sites. Simulations of the experimental Mn-L2,3 XAS spectra of two different samples with x = 1.5 and x = 1.7 led to Mn2+:Mn3+:Mn4+ atomic ratios of 0.16:0.51:0.33 and 0.19:0.57:0.24.

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presence of Mn3+, Mn4+, and Mn5+ was indicated by XPS, X-ray absorption spectrometry (XAS), and laser-induced luminescence spectroscopy.5,6 Oxidation of Mn in the original perovskite LaMn3+O3 is induced by substitution of La3+ by Ca2+ and K+. Another example for a triple valence of Mn ions was found in the sorosilicate of the perrierite-type La4Mn5Si4O22, but in contrast to the previously mentioned compounds, Mn exhibits the oxidation states +2, +3, and +4. This assignment was derived according to structure considerations, matching perfectly the nominal composition A3+4B2+C3+2D4+2O8(Si2O7)2 for perrierites, where B, C, and D are represented in this example solely by Mn, while EELS indicated an average oxidation state of only +3.25 of Mn.7 These compounds contain Mn exclusively in octahedral coordination. Other examples were reported, i.e., some substituted spinel manganites MxMn3−xO4 (M = Ni, Cu, Co) and the manganese oxide bromide Mn7.5O10−δBr3, where Mn exists also in the oxidation states +2, +3, and +4, but besides the octahedral surrounding, they show also tetrahedral coordination spheres for the first and cubic ones for the latter compound.8−11 The assignment of the oxidation states for the

INTRODUCTION Simultaneous occurrence of more than two oxidation states of one element in a single compound is rare and hardly unambiguously proven, while reasons for charge disproportionation can be envisioned as manifold. Different oxidation states, implying various electronic configurations, are mostly favored by certain coordination environments due to crystal field splitting, the most popular examples from textbooks probably being 3d-metal spinels typically constituting metal ions in the +2 and +3 oxidation states. Consequently, for such a situation the anionic host lattice has to provide diverse coordination possibilities in order to stabilize more than two oxidation states of a single element within one compound. The magnetoplumbite structure, with its five crystallographically different transition metal ion sites with various coordination spheres by oxygen, provides a rare opportunity to investigate ions in different environments within the same crystal structure and has, i.a., been examined by substitution of iron by different metals of various oxidation states and differing magnetic character, e.g., Al, Ti, or Cr.1−4 Mn is an interesting element for this objective due to its numerous and readily interchangeable oxidation states. Remarkably, Mn was previously indicated to exist in three different valence states within one compound in some Ca- and K-doped lanthanide perovskites, where the © 2017 American Chemical Society

Received: November 8, 2016 Published: March 14, 2017 3861

DOI: 10.1021/acs.inorgchem.6b02688 Inorg. Chem. 2017, 56, 3861−3866

Article

Inorganic Chemistry

compositions BaFe12−xMnxO19 with x = 0.6, 0.8, 1.5, and 1.7 for the single crystals that originated from the different batches. Powder diffractometry proved all samples to have the M-type ferrite structure (Figure 1). Since the crystals are faceted and

spinels was based mostly on electrical conductivity measurements, while for the oxide halide XPS data were presented. The phenomenon of three valence states of Mn was also discussed for Mn-substituted hexaferrites BaFe12−xMnxO19, where neutron diffraction studies indicate Mn to exist in its bi-, tri-, and tetravalent states with certain preference of their occupancy within the crystal structure; however, no direct proof was presented.12 Generally, three valence states of a single element within the same crystal structure are very uncommon and were found, next to Mn, for example, for few V and Co compounds.13−15

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EXPERIMENTAL SECTION

Crystal Growth and Synthesis. Mn-substituted barium hexaferrite crystals with sizes up to 8 mm were grown from sodium oxide flux. Iron oxide (Fe2O3), manganese oxide (MnO), barium carbonate (BaCO3), and sodium carbonate (Na2CO3) were used as starting materials. The compositions of the raw batch providing optimal conditions for crystal growth were chosen according to earlier published data.16 Tables S1 and S2 give more detailed information on the initial batch compositions and the chemical compositions of the resulting crystals. The initial mixture was ground in an agate mortar and filled into a 30 mL platinum crucible, which was then placed in a resistance furnace equipped with an RIF-101 thermoregulator. The furnace was maintained at 1260 °C for 3 h to homogenize the reactants, followed by cooling to 900 °C at a rate of 4.5 K/min. During all heating and cooling segments the sample mixture is in contact with air. After subsequent natural cooling to room temperature, the obtained crystals were separated from the flux by leaching in hot nitric acid. Figure S1 provides a photo of the obtained crystals. Characterization. The chemical composition of ground single crystals was identified using a Jeol JSM7001F scanning electron microscope with an Oxford INCA X-max 80 energy dispersive spectrometer. Powder X-ray diffraction was carried out on a Rigaku Ultima IV diffractometer with Cu Kα radiation (λ = 1.5406 Å) in the angular range from 10° to 90°. The scan time was 80 min with step sizes of 0.02°. Table S3 gives an overview on the unit cell parameters of BaFe12−xMnxO19 (x = 0, 0.6, 0.8, 1.5, 1.7) in comparison with literature data. Single-crystal diffractometry was performed on a NONIUS κ-CCD Bruker AXS four-circle diffractometer at ambient temperature with monochromatic Mo Kα radiation (λ = 0.7107 Å). Structure solution and refinements were carried out by using the SHELX-97 program package,17 and numerical absorption corrections were applied. Due to the limited number of intensity data, oxygen atoms were chosen to be treated with isotropic displacement parameters. Tables S4 and S5 provide more detailed information. The soft X-ray absorption spectra at the Mn-L2,3 and Fe-L2,3 edges were measured at the Dragon beamline of the National Synchrotron Radiation Research Center (NSRRC) in Hsinchu, Taiwan, using the total electron yield method. Clean sample areas were obtained by cleaving the crystals in situ at pressures in the 10−10 mbar range. MnO and Fe2O3 single crystals were also measured simultaneously to serve as energy reference for the Mn-L2,3 and Fe-L2,3 edges, respectively. Single crystals of the reference compound YBaMn3AlO7, containing purely tetrahedrally coordinated Mn2+, were prepared in a mirror furnace (Crystal Systems Corporation, FZ-T-10000-H-VI-VP) by floating zone technique; powders of three precursors (Y2BaO4:BaAl2O4:MnO = 1:1:6) were pressed into polycrystalline bars, which were directly inserted into the mirror furnace. The crystal growth progressed at 5 mm/h in a closed system containing Ar gas.18

Figure 1. Powder diffraction patterns of different samples of BaFe12−xMnxO19 (from bottom to top: calculated pattern, x = 0, 0.6, 0.8, 1.5, 1.7).

show preferred growth orientation, all reflections with h = k = 0 (00l) are found with higher intensity than calculated (especially those at approximately 22.9° and 30.8°). The obtained unit cell parameters are in good agreement with those of pure BaFe12O19 and show only minor changes upon the substitution of Fe by Mn (Table S3). The M-type ferrite structure (Figure 2) was originally derived for the mineral magnetoplumbite with the approximate

Figure 2. Section of the magnetoplumbite crystal structure with assignment of the transition metal sites (tetrahedrally coordinated M(3) in 4f1, trigonal bipyramidally surrounded M(2) in 4e, and three octahedrally surrounded positions: M(1) in 2a, M(4) in 4f2, and M(5) in 12k).

composition PbFe12O19.19 The structure with space group P63/mmc (No. 194) and Z = 2 can be described with alternating hexagonal and cubic close-packed oxygen layers ...BAB′ABCAC′AC..., while the primes mark that one-fourth of the oxygen atoms in the layer are replaced by a large cation, such as Ba2+. The hexagonal close-packed layers are called an Rblock and the cubic close packed ones an S-block, derived from

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RESULTS AND DISCUSSION Large single crystals with sizes up to 8 mm were obtained from Na2CO3 flux. EDX measurements led to the chemical 3862


Article

Inorganic Chemistry the spinel structure. With these blocks, the structure can also be described as RSR*S* (the star indicates that the block is rotated by 180°, with respect to the 63 screw axis). There are five crystallographically different Fe-containing sites, of which one is tetrahedrally surrounded (4f1), one trigonal bipyramidally (4e), and three octahedrally (2a (single octahedra), 4f2 (octahedra doubles), 12k (edge-sharing octahedra layers in the ab-plane)). It was shown that the Fe atom at the trigonal bipyramidally surrounded site does not occupy an ideal position, but is moved out of the mirror plane, creating a split position, leading from the ideal site 2b to half occupied 4e.20−22 Except from this ideal structure, there are also distorted derivative structures known, e.g., for BaFe12−xMnxO19, with high Mn3+ (d4) concentrations x > 8 at sites with octahedral coordination due to Jahn−Teller distortion.23−25 X-ray diffraction on selected single crystals confirmed the magnetoplumbite structure (Tables S4, S5). No indications for a symmetry reduction of the hexagonal space group P63/mmc were found for the crystals with a relatively low x in this study. Previous studies on the oxidation states of Mn disagree; for example, on one hand, it was found that Fe3+ is simply substituted by Mn3+,26,27 while on the other hand, a partial disproportionation of Mn3+ to Mn2+ and Mn4+ was discussed based on neutron diffraction experiments.12 To answer this question, XAS measurements on two different single crystals with compositions BaFe10.5Mn1.5O19 and BaFe10.3Mn1.7O19 with comparably high Mn contents were carried out at the Fe- and Mn-L2,3 edges (Figures 3, 4). XAS spectra at the 3d transition

Figure 4. Mn-L2,3 XAS spectra of (a) BaFe12−xMnxO19 crystals with x = 1.5 (black) and x = 1.7 (red). (b) Simulations for x = 1.5 (black) and x = 1.7 (red), as well as an additional simulation (cyan). (c) SrMnO3 and (d) LaMnO3 spectra are presented as Mn3+ and Mn4+ references with octahedral local symmetry. (e) YBaMn3AlO7 and (f) MnO spectra were used as Mn2+ references with tetrahedral and octahedral local symmetry, respectively.

trigonal prismatic and tetrahedral coordination also contribute to this spectrum, although their coordination features are not directly detectable. We can rule out the presence of Fe2+ or Fe4+ species. For example, the dominant peak in the spectrum of BaFe10.5Mn1.5O19 is shifted by about 2 eV at both the Fe-L2 and -L3 edges relative to those of the Fe2+ reference Fe0.04Mg0.96O,31 thus proving the absence of any significant contribution of Fe2+. Similarly, the presence of Fe4+ would have shown up as features at about 1−2 eV higher energies than those due to Fe3+. Figure 4 presents Mn-L2,3 XAS spectra of (a) BaFe12−xMnxO19 with x = 1.5 (black) and x = 1.7 (red). Spectra of four different substances serve as references for Mn in various valence states and coordination environments by oxygen: (c) SrMnO3 with Mn4+ and (d) LaMnO3 with Mn3+ in octahedral coordination, (e) YBaMn3AlO7 with Mn2+ in tetrahedral coordination, and (f) MnO with Mn2+ in octahedral coordination. From this comparison, it was possible to prove the presence of Mn in three different valence states, namely, +2, +3, and +4: the lowest sharp peak at 640.8 eV can also be found in the spectrum of YBaMn3AlO7, which serves as a reference for Mn2+ ions in tetrahedral local symmetry, while the peaks at 642.5 and 643.7 eV arise from Mn3+ and Mn4+ ions, respectively, as shown in their reference spectra from LaMnO3 and SrMnO3.29 For further confirmation of the mixed Mn valence states in BaFe12−xMnxO19, a simple simulation was carried out by superposition of the as-measured spectra of YBaMn3AlO7, LaMnO3, and SrMnO3 as seen in Figure 4b. All experimental spectral features of BaFe12−xMnxO19 could be well reproduced with Mn 2+ :Mn 3+ :Mn 4+ ratios of 0.16:0.51:0.33 and 0.19:0.57:0.24 for x = 1.5 (black) and x = 1.7 (red), respectively. Additionally, another simulation considering Mn2+ in an octahedral surrounding was performed, using (f) MnO as a reference (Figure 4b, cyan). In this case, a pre-edge peak at 639.7 eV can be seen, which is nonexistent in the experimental spectra of BaFe12−xMnxO19. Hence, Mn2+ in octahedral local symmetry can be excluded. We would like to note that using these XAS spectra we cannot prove or disprove

Figure 3. Fe-L2,3 XAS spectra of BaFe12−xMnxO19 crystals with x = 1.5 (black) and x = 1.7 (red) with spectra of α-Fe2O3 (blue) and Fe0.04Mg0.96O31 (pink) as Fe3+ and Fe2+ references, respectively.

metal L2,3 edges are highly sensitive to the valence state: an increase of the valence state of the metal ion causes a shift of the XAS L2,3 spectra toward higher energies.28,29 Furthermore, the dipole selection rules are very effective in determining which of the 2p53dn+1 final states can be reached and with what probability, which makes the technique extremely sensitive to the local symmetry of 3d transition metal atoms in the solid state.30 The same energy position and very similar multiplet spectral features in the Fe-L2,3 spectra of BaFe12−xMnxO19 and α-Fe2O3 in Figure 3 indicate Fe to exclusively remain in the Fe3+ valence state, like in the pristine BaFe3+12O19, with predominantly octahedral local symmetry. The smaller amounts of iron in 3863


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Inorganic Chemistry

Table 1. M−O Distances (Å) of BaFe12−xMnxO19 with x = 1.5 from Single-Crystal X-ray Diffraction in Comparison with the Respective Bond Lengths of the Pure Fe Compound (M = Fe, Mn)a

the presence of a minor fraction of Mn3+ ions occupying the trigonal bipyramidal coordinated site. The excess of Mn4+ compared to the Mn2+ content particularly in the crystal with x = 1.5 may mostly reflect high uncertainties of this simple fitting procedure. However, barium hexaferrite is known to be highly susceptible to isovalent substitution (e.g., by Al3+ or Cr3+)2,4 and, with somewhat reduced x, to substitution by ions with higher charge (e.g., Ti4+).3 Divalent ions, in contrast, lead only to small degrees of substitution.32,33 The finding of three different valence states for Mn is in strong contrast to the single valency of Fe in this compound. We attribute this very different behavior to the fact that the charge disproportionation of two Mn3+ ions into Mn2+ and Mn4+ requires less energy than that of two Fe3+ ions into Fe2+ and Fe4+. This is directly related to the considerably smaller effective Coulomb interaction (Ueff) of Mn3+ (d4) compared to Fe3+ (d5). Following van der Marel and Sawatzky,34 Ueff for a d4 ion can be written as F0 − J − C, while for a d5 ion it is given by F0 + 4J, where J = (F2 + F4)/14, C = (9F2 − 5F4)/98, and F0, F2, and F4 represent the Slater integrals. F0 is about 4−5 eV,35 while J and C can be estimated to be about 0.7 and 0.4 eV, respectively.34 We thus can estimate Ueff for d4-Mn3+ to be about 3−4 eV, while it amounts to 7−8 eV for d5-Fe3+. The direct disproportionation energy is thus 4 eV larger for Fe3+ than that for Mn3+. The high energy cost for the Fe3+ case can then no longer be compensated by the gain in Madelung energy and the adjustment of the metal−oxygen bond lengths once the disproportionation has taken place. Moreover, it is much more difficult to stabilize an Fe4+ ion than a Mn4+ in an oxide; in other words, an Fe4+ oxide prefers formation of oxygen deficiency rather than holes in the oxygen band. In comparison to the Mn2+ ion, the orbitals of Mn4+ with its higher oxidation state are much closer in energy to those of the oxide ions. Thus, the bonding of Mn2+ is rather ionic, while that of Mn4+ is highly covalent. In order to maximize the gain in hybridization energy, the high-spin Mn4+ (d3) ion prefers an octahedral coordination so that its all-empty eg orbitals can participate in the charge transfer process with the σ-bonded oxygen orbitals. It was previously found by Mößbauer spectroscopy that Mn enters all sites except for the 5-fold surrounded one with a trigonal bipyramidal environment until a substitution level of x < 8.12 For x ≥ 8 in BaFe12−xMnxO19, a certain amount of Mn occupying this site was detected by neutron diffractometry.23,36 In our crystals with x ≤ 1.7, no occupancy of the bipyramidally coordinated site can be assumed. Taking a closer look at the tetrahedrally surrounded site, Mn2+ with its d5 configuration is the most reasonable species to occupy it according to crystal field splitting.24 This aspect harmonizes perfectly with the notably enlarged M(3)−O distances in comparison with the pure compound BaFe12O19, since the radius of Mn2+ in a 4-fold surrounding is larger than the radius of Fe3+ in the same surrounding; namely r(Mn2+) = 0.80 Å and r(Fe3+) = 0.63 Å (Table 1).33 Other M−O distances are not affected, so it can be supposed that most Mn2+ cations accumulate at this site; at the same time no hints at the remaining cation distribution could be found. The nonexistent diminishment of the corresponding M−O bond lengths (r(Mn4+) = 0.67 Å, r(Fe3+) = 0.785 Å in an octahedral surrounding)37 can be understood from the number of positions within the crystal structure: the single tetrahedrally surrounded site with Wyckoff position 4f1 has obviously a

M(1) M(2) M(3) M(4) M(5)

−O(4) −O(1) −O(3) −O(2) −O(4) −O(3) −O(5) −O(1) −O(2) −O(4) −O(5)

×6 ×2 ×3 ×3 ×3 ×3

×2 ×2

BaFe12−xMnxO19

BaFe12O19

1.994(5) 2.313(8) 1.866(7) 1.925(8) 1.921(5) 2.065(5) 1.980(5) 1.975(4) 2.080(5) 2.089(3) 1.931(3)

2.001(3) 2.311(5) 1.858(4) 1.905(5) 1.895(3) 2.073(3) 1.971(3) 1.979(2) 2.088(3) 2.111(2) 1.928(2)

a The trigonal bipyramidally surrounded site M(2) was refined as ideal position 2b (Table S5).

smaller number of positions than the three octahedrally surrounded sites 2a, 4f2, and 12k. With respect to electroneutrality, which means that the number of Mn2+ ions equals those of the Mn4+ ions, the same quantity of Mn4+ ions is distributed over more sites, so the smaller radius has a reduced impact on the M−O distances. In addition, Mn3+ has virtually the same size as Fe3+ in an octahedral surrounding (r(Mn3+) = 0.785 Å).37

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CONCLUSIONS Partial substitution of Fe by Mn in the iron(III) compound BaFe12O19 leads to charge disproportionation into Mn2+, Mn3+, and Mn4+. Mn2+ accumulates at the tetrahedrally coordinated site, while the Mn4+ favors the octahedral coordination, with the majority of manganese substituting Fe3+ and residing in the oxidation state +3 in (distorted) octahedral coordination. Hence, in a simplified formula the compound BaFe12−xMnxO19 can be written according to Ba(Fe3+)tbp(Fe3+2−mMn2+m)tet(Fe3+9−m−nMn3+nMn4+m)octO19 with 2m + n = x and tbp = trigonal bipyramidal, tet = tetrahedral, and oct = octahedral.

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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.6b02688. Batch compositions for Mn-substituted barium hexaferrite crystal growth and resulting compositions from EDX data with photograph of the crystals; unit cell parameters from powder X-ray diffraction; single-crystal X-ray diffraction data collection and refinement parameters for BaFe10.5Mn1.5O19 and pure BaFe12O19 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Denis A. Vinnik: 0000-0002-5190-9834 Martin Valldor: 0000-0001-7061-3492 3864


Article

Inorganic Chemistry

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Funding

The work was supported by the Ministry of Education and Science of the Russian Federation (no. 4.1346.2017/PP). Additionally the work was supported by Act 211 Government of the Russian Federation, contract no. 02.A03.21.0011, and by the Russian Foundation for Basic Research (no. 16-08-01043). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Falk Lissner for collecting single-crystal X-ray diffraction data.

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REFERENCES

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Three Oxidation States of Manganese in the Barium Hexaferrite

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