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Ferrimagnetic Cage Framework in Ca12Fe10Si4O32Cl6 Soshi Iimura,*,† Yudai Tomota,† Satoru Matsuishi,‡ Ryo Masuda,§ Makoto Seto,§ Haruhiro Hiraka,∥ Kazutaka Ikeda,⊥ Toshiya Otomo,⊥,# and Hideo Hosono*,†,‡ †

Laboratory for Materials and Structures and ‡Materials Research Center for Element Strategy, Tokyo Institute of Technology, Yokohama 226-8503, Japan § Research Reactor Institute, Kyoto University, Kumatori, Osaka 590-0494, Japan ∥ Graduate School of Science and Engineering, Ibaraki University, Mito, Ibaraki 310-8512, Japan ⊥ Institute of Materials Structure Science, High Energy Accelerator Research Organization, Tsukuba 305-0801, Japan # Department of Materials Structure Science, The Graduate University for Advanced Studies (SOKENDAI), Tokai, Ibaraki 319-1106, Japan ABSTRACT: The positively charged cage framework of the natural mineral mayenite, which enables various species with negative charge to be stabilized, is one of the key structures to provide the new functionalities exploited in applications. Here we report the structural and magnetic properties of recently found eltyubyuite, Ca12Fe10Si4O32Cl6, which is the first compound bearing a transition metal oxide as a main constituent in the mayenite-type structure. From neutron powder diffraction measurements at T = 20 K and the low temperature Mössbauer measurement, we determined the magnetic structure of eltyubyuite to be a ferrimagnet with oppositely aligned magnetic moments of +3.17(3) and −3.05(8) μB in two tetrahedral Fe sites with different oxygen ligands, all bridging oxygens or mixed bridging and nonbridging oxygens. As far as is known, this result is likely to be a first example showing ferrimagnetism stemming from only tetrahedral Fe3+ ions. The reduced magnetic moment per Fe3+ and the resultant small net moment per unit cell of 22 μB at μ0H = 5 T and T = 15 K are attributed to strong covalency in much shorter Fe−O bonds in the FeO4 tetrahedra. of electron paramagnetic resonance of Cu2+-doped polycrystalline C12A7 in which the concentration is approximately 5 × 1020 cm−3 (∼0.5Cu/f.u.).9 Ebbinghaus et al. grew single crystals of C12A7 with an Fe concentration of 1 × 1020 cm−3 (∼0.14Fe/f.u.) and revealed the local environment and oxidation state of iron using X-ray absorption near-edge structure spectroscopy.10 Recently, a new mineral, eltyubyuite (Ca12Fe10Si4O32Cl6), was found in Russia, which is the first mayenite-type compound bearing a transition metal oxide as a main component.11 In addition to the novelty as the magnetic ion-incorporated mayenite, the magnetically polarized framework [Ca24Fe20Si8O64]12+ and the loosely bound nature of chloride ion in the cage are also promising to realize a new concept material, magnetic electride, in which the electrons serve as anions occupying the space normally occupied by anions, and the spin of the electron is polarized by interacting with the ferromagnetic or ferrimagnetic ion incorporated in the wall of the cage for the zero-dimensional electride or the layer for the two-dimensional electride. In this article, we examine the structural and magnetic properties of Ca12Fe10Si4O32Cl6 obtained using neutron powder

1. INTRODUCTION Mayenite (12CaO·7Al2O3, abbreviated as C12A7 hereafter) is a natural mineral and is known as a major constituent of commercial alumina cement. Its unit cell is a cubic lattice (Z = 2) composed of 12 cages [Ca24Al28O64]4+ and two oxide ions being accommodated randomly within two of cages as counteranions. The chemically stable and positively charged framework [Ca24Al28O64]4+ can incorporate various active anion species such as O−, O2−, H−, and an electron in addition to O2− and halogen ions.1−3 The electron-incorporated C12A7 shows unique electronic properties, such as metallic conduction, superconductivity, and an extremely low work function of 2.4 eV,3 whereas a strong oxidation capability is found in O−incorporated C12A7.4 Also the hydride or halide ion in the cage of C12A7 can be emitted by heating and/or applying an electric field.2,5,6 Apart from the electronic and ionic properties found in mayenite, the magnetic property remains unexplored to date due to the low solubility limit of magnetic transition metal cations to C12A7. In our survey, only three studies report the substitution of transition metal cations into the C12A7. Specifically, the Ir-doped C12A7 single crystal was grown by the Czochralski method, for which the concentration reached ∼5 × 1017 cm−3 (∼0.0005Ir/f.u.).7,8 Maurelli et al. reported the results © XXXX American Chemical Society

Received: October 11, 2016

A

DOI: 10.1021/acs.inorgchem.6b02404 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 1. (a) Room-temperature neutron powder diffraction pattern and the result of Rietveld refinement. Orange vertical ticks denote nuclear Bragg positions of eltyubyuite with space group I−43d. A peak between 2.12 and 2.20 Å at 20 K was excluded as belonging to the sample holder. (b) Crystal structure of Ca12Fe10Si4O32Cl6. The black solid line indicates the unit cell. The green, red, blue, brown, and gray atoms denote chlorine, nonbridging oxygen [O(2)], Fe(1)/Si(1) at the 16c site, Fe(2)/Si(2) at the 12a site, and bridging oxygen [O(1)], respectively. (c, d) Side and top views of the cage structure of Ca12Fe10Si4O32Cl6. Brown and blue tetrahedra represent Fe(12a)O4 and Fe(16c)O4. The crystal structure was illustrated using the VESTA program.13 (e) Neutron powder diffraction pattern and result of Rietveld refinement at T = 20 K. Purple vertical tick marks denote ferrimagnetic Bragg positions of eltyubyuite. Two black arrows indicate the position of the strong magnetic peaks.

Table 1. Structural Parameters of Ca12Fe10Si4O32Cl6 Refined from Neutron Powder Diffraction Dataa (a) T = 300 K fractional coordinates site

mult.

x

y

O(1) Ca(1) O(2) Fe(1) Si(1) Fe(2) Si(2) Cl(1)

48e 24d 16c 16c 16c 12a 12a 12b

0.1934(1) 0.1053(2) 0.3184(2) 0.23490(8) 0.23490(8) 3/8 3/8 7/8

site

mult.

x

y

O(1) Ca(1) O(2) Fe(1) Si(1) Fe(2) Si(2) Cl(1)

48e 24d 16c 16c 16c 12a 12a 12b

0.1932(1) 0.1069(4) 0.3195(3) 0.2351(1) 0.2351(1) 3/8 3/8 7/8

0.2824(2) 0 0.3195(3) 0.2351(1) 0.2351(1) 0 0 0

0.2827(1) 0 0.3184(2) 0.23490(8) 0.23490(8) 0 0 0 (b) T = 20 K

z

occ.

Uiso (×100)

0.0974(2) 1/4 0.3184(2) 0.23490(8) 0.23490(8) 1/4 1/4 1/4

1.010(2) 1.010(1) 1.022(1) 0.850(6) 0.150(6) 0.666(6) 0.334(6) 0.900(4)

1.88(4) 0.94(7) 1.38(9) 0.71(3) 0.48(3) 0.69(6) 0.48(7) 2.27(7)

z

Uiso (×100)

μFe/μB

0.09555(3) 1/4 0.3195(3) 0.2351(1) 0.2351(1) 1/4 1/4 1/4

1.87(9) 0.36(1) 1.31(2) 0.712(8) 0.468(8) 0.358(1) 0.490(1) 0.846(1)

fractional coordinates

a

+3.17(3) −3.05(8)

Lattice constants refined at T = 300 and 20 K are 12.2158(8) and 12.2094(1) Å, respectively. Each magnetic moment aligns along the [111] direction. an alumina mortar with ethanol as a solvent. The mixtures were heated at 1000 °C for 2 h and subsequently quenched to room temperature. The NPD measurement was performed using the neutron total scattering spectrometer, NOVA, installed at the Japan Proton Accelerator Research Complex facility. The powder diffraction data were measured at room temperature and 20 K using a vanadium metal holder with a diameter of 3 mm. The resulting data were analyzed using the Rietveld analysis contained in the General Structure Analysis

diffraction (NPD) measurements, Mössbauer spectroscopy, and magnetization measurements and report ferrimagnetism arising from only the FeO4 tetrahedra.

2. EXPERIMENTAL SECTION Ca12Fe10Si4O32Cl6 was synthesized by solid state reaction of CaCO3, Fe2O3, SiO2, and excess CaCl2. These starting materials were mixed in B


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Inorganic Chemistry

Figure 2. Mössbauer spectra of Ca12Fe10Si4O32Cl6 at various temperatures. Blue and green solid lines represent nonmagnetic doublet components of Fe(1) and Fe(2), respectively. Orange and purple denote magnetic sextet components of Fe(1) and Fe(2), respectively. System (GSAS) software package.11,12 Elemental composition of the samples was determined using an electron-probe microanalyzer (EPMA) (JXA-8530F, JEOL Inc.) equipped with a field-emissiontype electron gun and wavelength dispersive X-ray detector. The DC magnetic susceptibility and the M−H curve measurements were conducted in a static field of 0.01 T and ±5 T sweep field, respectively, using a vibrating sample magnetometer (VSM, Quantum Design). 57Fe Mössbauer spectra were recorded using a conventional radiation source, 57Co(Rh) RI, and a Mössbauer velocity transducer. The zero velocity of the spectrum was taken as the center of the six-line profile of an α-Fe foil at room temperature.

The vertical (dCav) and horizontal (dCah) Ca−Ca distance, calculated as dCav = a × (2xCa + 1/4) and dCah = a × sqrt[(−2xCa + 1/2)2 + 1/4], are 5.63 and 7.06 Å, respectively (Figure 1c).14 This result shows that the cage has a distinct anisotropic structure in contrast with the almost isotropic cage in C12A7 with dCav = 6.34 and dCah = 6.56 Å. The short dCav distance results from ionic bonds forming between hexahedrally coordinated Ca2+ and Cl− ions with ionic diameters of 2.00 and 3.62 Å, respectively.15,16 The chemical composition, taking into account occupancy, is Ca12.12(2)Fe10.80(8)Si3.20(8)O32.4(2)Cl5.40(2), which is close to Ca12.0(3)Fe9.7(4)Si3.6(2)O32.4(7)Cl5.5(3) determined using the EPMA. The valence state of Fe was estimated to be 3.3(2)+ from the EPMA-measured chemical composition based on the assumption that the valence states of other ions take their formal charge. Crystallographically, the Fe and Si atoms occupy 16c and 12a-sites. The 16c site [Fe(1) and Si(1)] is coordinated by three O(1) and one O(2), whereas the 12a [Fe(2) and Si(2)] site is coordinated by the four O(1). The O(1), referred to as the bridging oxygen (Ob), connects [Fe(1)/Si(1)]O4 and [Fe(2)/Si(2)]O4, whereas the O(2) is referred to as the nonbridging oxygen (Onb).17 We determined the occupancy ratio between Fe and Si at each 12a or 16c-site

3. RESULTS AND DISCUSSION 3.1. Crystal Structure and Chemical State in Eltyubyuite. Figure 1a shows a collected NPD pattern of the sample at room temperature along with the Rietveld fitting result using the space group I−43d. The resultant fractional coordinates, occupancy, and isotropic atomic displacement parameter (Uiso) are summarized in Table 1a. The (Fe/Si)O4 tetrahedral network within the embedded Ca2+ ions forms positively charged cages such as mayenite and wadalite (Figure 1b−d). The center of the cage is almost fully occupied by a Cl− ion with occupancy of 0.900(4) that acts as a counteranion to maintain electroneutrality. C


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Inorganic Chemistry from the NPD data rather than the X-ray diffraction data because the neutron scattering cross section of Fe is ∼6 times as large as that of Si. As commonly found in iron-bearing silicates, Si4+ prefers to occupy bridging sites, the networkforming 12a site, whereas the Fe3+ occupies nonbridging sites, the network-modifying 16c site.17,18 3.2. Ferrimagnetic Properties of Eltyubyuite. Figure 1e and Table 1b present a NPD profile collected at T = 20 K and the structural parameters obtained, respectively. Comparing the profile with that at room temperature, an additional diffraction peak at d ≈ 3.86 Å appears, and an increase in peak intensity at d ≈ 4.32 Å is observed. These changes are attributed to an evolving ferrimagnetic structure with oppositely aligned magnetic moments of +3.17(3) and −3.05(8) μB at the Fe(1) and Fe(2) sites, respectively. Each size of the magnetic moment per Fe (μFe) is smaller than that of the tetrahedral Fe3+ site in magnetite Fe3O4 (4.44 μB at 90 K by NPD measurement) and yttrium garnet Y3Fe5O12, abbreviated as YIG (3.95 μB at 10 K by NPD measurement).19,20 Figure 2 summarizes the Mössbauer spectra at various temperatures from 300 to 15 K. At T = 300 K the spectrum shows strong two doublet-components and weak peaks from hematite (α-Fe2O3) as the impurity phase. The estimated isomer shifts δ of 0.19−0.21 mm/s from the two doublet are close to the value from Fe3+ in the tetrahedral site of Fe3O4, indicating that the two Fe sites in this compound have valence 3+ (0.15 ≤ δ ≤ 0.30 mm/s against α-Fe21). For an isotope with a I = 3/2 excited state, such as 57Fe or 119Sn, the quadrupole splitting (Δ) is expressed as 1/2 ⎛ η2 ⎞ Δ = e Qq/2⎜1 + ⎟ 3⎠ ⎝

Figure 3. (a) Area ratio of paramagnetic sites to magnetic ordering sites estimated by Mössbauer spectra as a function of temperature. (b) Temperature dependence of the internal magnetic field (Bhf). The error bar expresses a standard deviation of asymmetric Gaussian function used to fit the distribution of Bhf. The solid lines are to guide the eye.

Figure 3b shows the temperature dependences of Bhf for the Fe(1) and Fe(2) sites. Similarly for μFe, Bhf of both sites are smaller than those of magnetite (46−49 T at T = 300 K) and YIG (40−50 T at T = 295 K).25,26 The large error in Bhf expressing a broad distribution for Bhf suggests that there is a variation in the local magnetic structure arising from the nonmagnetic Si4+ substituted in the 12a and 16c sites. From the M−H curves at various temperatures (Figure 4a), the magnetization at T = 400 K changes linearly with magnetic field, whereas the others show a saturation at very low field,

2

(1)

where Q is the nuclear quadrupole moment, and η describes the asymmetry in the electric field gradient (η = |Vxx−Vyy|/Vzz). The splitting Δfor the two doublets are Δ1 = 1.12(3) and Δ2 = 2.13(3) mm/s. The relative areas between the hematite, one doublet with Δ1, and the other with Δ2 are calculated to be 5.8(5)/37(3)/58(5)%, respectively, by fitting at T = 300 K without constraint (the results are not shown in Figure 2). We assigned the second and third spectral components to Fe(2) and Fe(1), respectively, on the following bases. First, the relative area ratios of the second and third spectral components, 37/58 = 0.63, is in good agreement with the relative population ratio of the Fe(2) and Fe(1) sites determined by NPD, 12 × 0.666/16 × 0.850 = 0.588. Second, 27Al NMR studies on C12A7 reported the asymmetric parameter η, the nuclear quadrupole coupling constant CQ = e2qQ/h, and the nuclear quadrupole frequency νQ = (3/20)e2qQ of Al(1) at 16c site and Al(2) at 12a site.22,23 If we substitute these values into eq 1, the splitting Δ at the 16c site is 2−3 times larger than that at the 12a site, irrespective of Q. Third, because Fe(1) occupies the nonbridging 16c site coordinated with three Ob and one Onb, Δ of Fe(1) is expected to be larger than that of Fe(2). Next, we consider the temperature dependence of the Mössbauer spectra (the constraints for fittings are summarized in ref 24). As temperature decreases, there is a pronounced increase in the intensity, especially around the doublet peaks. These broad components were fitted well by assuming two magnetic sites with a distribution of internal magnetic field (Bhf). Below T = 100 K, the relative area of the doublet arising from Fe(2) becomes zero, whereas that from Fe(1) with Δ of 2.6 mm/s remains even at the lowest temperature of 15 K (Figure 3a).

Figure 4. (a) M−H curves at various temperatures. Sweeping range of applied magnetic field is ±5 T. (b) An expanded view of the M−H curve at T = 15 K. Black and red dashed lines indicate H = 0 and a linear fitting result in the range of 2 ≤ H ≤ 4 T, respectively. (c) An expanded view of the hysteresis loop at T = 15 K. (d) Magnetic susceptibility at μ0H = 0.01 T from 400 to 2 K. D


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Inorganic Chemistry

is expected to stem from superexchange interaction in the −Fe(2)−Ob−Fe(1)−Ob−Fe(2)− chain. For temperatures between 100 K < T < TC, both MFe1 and MFe2 gradually develop as temperature decreases. However, in the case of the Fe(1) that is bonded to −Ob−Si(2) and Onb, the spin on Fe(1) does not interact with that of Fe(2) and behaves paramagnetic at an even lower temperature. In contrast, since the Fe(2) occupies the bridging site (12a), the spins can be fully ordered at 100 K, though the ferrimagnetism does not grow smoothly on cooling due to the Si-substitution. Finally, we compare the magnetic properties of Ca12Fe10Si4O32Cl6 with those of YIG, which is a typical soft ferrimagnet. Although a recent debate surrounds whether the garnet crystallizes in space group Ia3d̅ or R3,̅ the crystal structures of mayenite and garnet are closely related.20,26,31,32 Figure 6 provides a

indicating small magneto-crystalline anisotropy in this compound. At T = 15 K, the magnetic moment at zero field estimated by extrapolating the fitted linear line at high field to H = 0 (T) reaches ∼19.1(4) μB per unit cell, which is in good accordance with 19 μB calculated based on the ferrimagnetic structure and μFe at each site determined using NPD (Figure 4b). The weak magnetic hysteresis loop, with its small coercive field of Hc = 20 Oe at T = 15 K (Figure 4c), indicates that Ca12Fe10Si4O32Cl6 is a soft ferrimagnet. Figure 4d shows the M/H−T curves with zero-field-cooling (ZFC) and field-cooling (FC) at μ0H = 0.01 T. The magnetic susceptibility gradually increases below 400 K, and a small jump is observed at T = 100 K. A slight drop with ZFC below 10 K may be due to a spin glass transition. The temperature dependencies of the M−H curves and Mossbauer spectra suggest that the Curie temperature (TC) is located between 300 K < T < 400 K. In Figure 4d, a small difference between ZFC and FC curves is observed. Strong irreversible thermomagnetic behavior is frequently observed for highly anisotropic compounds, e.g., the ferromagnetic perovskite-oxide SrRuO3 and spinel-oxide SrFe12O19, in which the ZFC curve shows a sharp peak around TC and a sudden decrease below TC.27,28 On the other hand, soft magnetic materials, such as ferrimagnetic spinel Ni0.8Zn0.2Fe2O4 and ferromagnetic perovskite La0.7Ca0.3MnO3 exhibit much less irreversibility.29 Therefore, the observation of weak thermomagnetic irreversibility also suggests that Ca12Fe10Si4O32Cl6 is a soft magnet. According to molecular field theory for ferrimagnets,30 the net magnetization below TC is expressed by the summation of the magnetizations of two antiferromagnetically coupled sublattices. Figure 5 shows the temperature dependences of the

Figure 6. Comparison of the crystal structures of Y3Fe5O12 with the corresponding cage structure of eltyubyuite. (a) Blue and brown polyhedra denote FeO6 and FeO4, respectively. In the cubic lattice, Fe in each FeO6 occupies the 16a site, whereas that in FeO4 occupies the 24d site. The Y3+ ions colored pink are located at the 24c site. (b) Half of the FeO4 tetrahedra in YIG correspond to the Fe(2)O4 tetrahedra in eltyubyuite (colored brown), whereas the other half correspond to the chlorines (green). The FeO6 octahedra in YIG correspond to the Fe(1)O4 tetrahedra in eltyubyuite (colored brown). The Ca2+ ions colored yellow are located at the Y3+ site in YIG. The black solid line marks the unit cell of the cubic lattice.

comparison of the crystal structures for YIG and eltyubyuite. The framework of YIG consists of vertex-shared FeO6 octahedra and FeO4 tetrahedra. Its ferrimagnetism is described as oppositely aligned magnetic moments of Fe3+ at the octahedral and tetrahedral sites. Half of the FeO4 tetrahedra in YIG correspond to the Fe(2)O4 tetrahedra in eltyubyuite, whereas the other half corresponds to the Cl− ions. The FeO6 octahedra in YIG correspond to the Fe(1)O4 tetrahedra in eltyubyuite. Indeed, in our survey for the magnetism of various ferric oxides, including the four polymorphs of Fe2O3 (α-, β-, γ-, and εtypes), three ferrite structures (spinel-, garnet-, and hexagonaltypes), and the other ternary alkali-metal or alkaline-earth-metal iron oxides, the eltyubyuite is the first example in which ferrimagnetism stems from only Fe3+O4 tetrahedra. Comparing with the Fe3+ ion in FeO6 octahedra, the lower coordination number around Fe3+ ion in FeO4 acts to reduce TC [or Néel temperature (TN) for an antiferromagnet], whereas the shorter bond length between Fe3+ and O2− (dFe−O) increase TC (or TN) through large kinetic superexchange interaction.33,34 The latter results from strong covalency effects with neighboring O2−, which transfers a certain amount of moment from Fe to O and then reduces μFe.35,36 In Table 2, we summarize the dFe−O, μFe, and TC (or TN) of the Ca12Fe10Si4O32Cl6 and Y3Fe5O12.20 Those of KFeO2 and LaFeO3 are also shown as examples of

Figure 5. Temperature dependences of magnetization for the Fe(1)-, Fe(2)-sublattices (MFe(1) and MFe(2), respectively), and the summation (Mtot). Filled circles, open triangles, and open inverted triangles denote MFe(1), MFe(2), and Mtot data, respectively.

magnetizations for the Fe(1)- and Fe(2)-sublattices, and their summation (MFe(1), MFe(2), and Mtot, respectively). These values were calculated using i=1

M tot(T ) =

i=1

sext. ∑ MFe(i)(T ) = ∑ μFe(i)AFe( i)(T )nFe(i) 2

2

(2)

μFe(i), AFe(i)sext,

where and nFe(i) are the magnetic moment of Fe(i), the relative area of a Fe(i)-sextet, and the number of Fe(i) per unit cell, respectively. Because of the saturated behavior of AFe(2)sext below T = 100 K, Mtot shows a small hump at 100 K as observed in the M/H−T curve. The complex temperature variations of MFe1 and MFe2 are reasonably explained as follows; the ferrimagnetic ordering with TC of 300−400 K E


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Inorganic Chemistry Table 2. Bond Lengths between Fe3+ and O2− Ions (dFe−O), Sizes of Magnetic Moment on Fe, and TC (or TN) of the Ca12Fe10Si4O32Cl6, Y3Fe5O12, KFeO2, and LaFeO320,37,38a magnetism Ca12Fe10Si4O32Cl6 ferri. Y3Fe5O12

ferri.

KFeO2 LaFeO3

antiferro. antiferro.

dFe−O/Å

μFe/μB

TC or TN/K

Fe(1)−O(1) Fe(1)−O(2) Fe(2)−O(1) Fetet.−O Feoct.−O Fetet.−O Feoct.−O

1.85 1.77 1.78 1.86 2.03 1.86 2.01

3.17 at Fe(1) 3.05 at Fe(2)

300−400

3.95 at Fetet. 4.01 at Feoct.

560

3.91 4.6

1003 750

For KFeO2 and LaFeO3, we show the averaged dFe−O, because their unit cell contain the six Fetet−O bonds (1.84, 1.85, 1.86, 1.86, 1.86, and 1.87 Å) and two Feoct.-O bonds (2.00 and 2.02 Å), respectively.

a

antiferromagnetism originating from only Fe3+O4 tetrahedra and Fe3+O6 octahedra, respectively.37,38 Obviously, from the table Ca12Fe10Si4O32Cl6 shows that the strong covalency of the much shorter dFe−O bond does reduce μFe. The lower TC of Ca12Fe10Si4O32Cl6 than that of YIG is attributed to the presence of the nonbridging oxygen [O(2)] and to Si(2) reducing the number of Fe(1)−O−Fe(2) channels for exchange interaction.

and Technology Agency in Japan. A part of this research was supported by MEXT Element Strategy Initiative to form a research core and the Japan Society for the Promotion of Science (JSPS) KAKENHI Grant No. 24221005. The neutron powder diffraction experiment was approved by the Neutron Scattering Program Advisory Committee of IMSS, KEK (Proposal No. 2014S06).

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4. CONCLUSIONS We examined the structural and magnetic properties of Fesubstituted mayenite-analogue, Ca12Fe10Si4O32Cl6, using NPD, Mössbauer spectroscopy, and magnetization measurements. The primary results obtained are summarized as follows (i) The Rietveld analysis of NPD revealed that Fe3+ ions preferentially occupy the 16c sites (coordinated with a nonbridging oxygen) over Si4+ in eltyubyuite. (ii) The low temperature NPD pattern at T = 20 K demonstrated that Ca12Fe10Si4O32Cl6 has unique ferrimagnetic properties stemming from only the tetrahedral Fe3+ ions. The magnetic moment μFe of Fe(1) with mixed ligands of three bridging oxygen and a nonbridging oxygen and Fe(2) with four bridging oxygens were determined to 3.17(3) and −3.05(8) μB, respectively. (iii) The saturated magnetic moment per unit cell is 22 μB at μ0H = 5 T and T = 15 K, which is half as small as that of YIG. The smaller magnetic moments are attributed to strong covalency in the much shorter Fe−O bonds in the FeO4 tetrahedra. The positively charged cage [Ca24Fe20Si8O64]12+ will stabilize various negatively charged species as well as Cl− in analogy with C12A7. We expect that the magnetically polarized cage in eltyubyuite gives an opportunity to realize magnetic semiconductors if a fraction of the Cl ions can be replaced by electrons.

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REFERENCES

(1) Jeevaratnam, J.; Glasser, F. P.; Glasser, L. S. D. Anion Substitution and Structure of 12CaO·7A12O3. J. Am. Ceram. Soc. 1964, 47, 105. (2) Hayashi, K.; Matsuishi, S.; Kamiya, T.; Hirano, M.; Hosono, H. Light-induced conversion of an insulating refractory oxide into a persistent electronic conductor. Nature 2002, 419, 462. (3) Matsuishi, S.; Toda, Y.; Miyakawa, M.; Hayashi, K.; Kamiya, T.; Hirano, M.; Tanaka, I.; Hosono, H. High-Density Electron Anions in a Nanoporous Single Crystal: [Ca24Al28O64]4+(4e−). Science 2003, 301, 626. (4) Hayashi, K.; Hirano, M.; Matsuishi, M.; Hosono, H. Microporous Crystal 12CaO·7Al2O3 Encaging Abundant O− Radicals. J. Am. Chem. Soc. 2002, 124, 738. (5) Huang, F.; Li, J.; Xian, H.; Tu, J.; Sun, J. Q.; Yu, S. Q.; Li, Q. X.; Torimoto, Y.; Sadakata, M. Substitution of H− for O− and H− emissions of 12CaO·7Al2O3. Appl. Phys. Lett. 2005, 86, 114101. (6) Song, C.; Sun, J.; Qiu, S.; Yuan, L.; Tu, J.; Torimoto, Y.; Sadakata, M.; Li, Q. Atomic Fluorine Anion Storage Emission Material C12A7F−and Etching of Si and SiO2 by Atomic Fluorine Anions. Chem. Mater. 2008, 20, 3473−3479. (7) Kurashige, K.; Toda, Y.; Matstuishi, S.; Hayashi, K.; Ueda, K.; Kamiya, T.; Hirano, M.; Hosono, H. Czochralski growth of 12CaO· 7Al2O3 Crystals. Cryst. Growth Des. 2006, 6, 1602. (8) Kim, S. W.; Matsuishi, S.; Nomura, T.; Kubota, Y.; Takata, M.; Hayashi, K.; Kamiya, T.; Hirano, M.; Hosono, H. Metallic State in a Lime−Alumina Compound with Nanoporous Structure. Nano Lett. 2007, 7, 1138. (9) Maurelli, S.; Ruszak, M.; Witkowski, S.; Pietrzyk, P.; Chiesa, M.; Sojka, Z. Spectroscopic CW-EPR and HYSCORE investigations of Cu2+ and O2− species in copper doped nanoporous calcium aluminate (12CaO·7Al2O3). Phys. Chem. Chem. Phys. 2010, 12, 10933. (10) Ebbinghaus, S. G.; Krause, H.; Lee, D. K.; Janek, J. Single Crystals of C12A7 (Ca12Al14O33) Substituted with 1 mol % Iron. Cryst. Growth Des. 2014, 14, 2240. (11) Larson, A. C.; Von Dreele, R. B. GSAS-General Structure Analysis System. Los Alamos Natl. Lab., Rep. 2000, 86. (12) Toby, B. H. EXPGUI, a graphical user interface for GSAS. J. Appl. Crystallogr. 2001, 34, 210. (13) Momma, K.; Izumi, F. VESTA 3 for three-dimensional visualization of crystal, volumetric and morphology data. J. Appl. Crystallogr. 2011, 44, 1272−1276. (14) Sakakura, T.; Tanaka, K.; Takenaka, Y.; Matsuishi, S.; Hosono, H.; Kishimoto, S. Determination of the local structure of a cage with

AUTHOR INFORMATION

Corresponding Authors

*(S.I.) E-mail: [email protected]. *(H.H.) E-mail: [email protected]. ORCID

Soshi Iimura: 0000-0003-3270-155X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a fund from Accelerated Innovation Research Initiative Turning Top Science and Ideas into High-Impact Values (ACCEL) of Japan Science F


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(33) Anderson, P. W. New Approach to the Theory of Superexchange Interactions. Phys. Rev. 1959, 115, 2. (34) Kim, M.; Kim, B. H.; Choi, H. C.; Min, B. I. Origin of high Néel temperature in the low coordination number system AFeO2 (A = K and Rb). Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 81, 212405. (35) Plakhty, V. P.; Gukasov, A. G.; Papoular, R. J.; Smirnov, O. P. Spin density on ligands O2− and covalency of Fe3+ ions in octahedral sites of the Ca3Fe2Ge3O12 garnet: A polarised neutron diffraction study. Europhys. Lett. 1999, 48, 233. (36) Walters, A. C.; Perring, T. G.; Caux, J.; Savici, A. T.; Gu, G. D.; Lee, C.; Ku, W.; Zaliznyak, I. A. Effect of covalent bonding on magnetism and the missing neutron intensity in copper oxide compounds. Nat. Phys. 2009, 5, 867. (37) Sheptyakov, D.; Ali, N. Z.; Jansen, M. A neutron diffraction study of structural and magnetic transformations in AFeO2 (A = K, Rb and Cs). J. Phys.: Condens. Matter 2010, 22, 926001. (38) Koehler, W. C.; Wollan, E. O. Neutron-diffraction study of the magnetic properties of perovskite-like compounds LaBO3. J. Phys. Chem. Solids 1957, 2, 100.

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