Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
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CsFe3(SeO3)2F6 with S = 5/2 Cube Tile Lattice Hongcheng Lu† and Hiroshi Kageyama*,†,§ †
Graduate School of Engineering, Kyoto University, Kyoto 615-8510, Japan Japan Science and Technology Agency, CREST, Kawaguchi, Saitama 332-0012, Japan
§
S Supporting Information *
ABSTRACT: A layered iron selenite fluoride CsFe3(SeO3)2F6 1 was hydrothermally synthesized. Single-crystal X-ray diffraction studies show that 1 has a trigonal (P3̅m1) lattice, where [Fe3(SeO3)2F6]− blocks of three iron sublayers are separated by Cs cations. Within the block, only Fe(2)F6 and Fe(1)O3F3 octahedra are magnetically connected via superexchange Fe(1)−F−Fe(2) pathways, giving an S = 5/2 cube tile (dice) lattice. At low magnetic field, 1 exhibits an antiferromagnetic transition at ∼130 K, where ferrimagnetic cube tile layers are arranged in a staggered manner. At low temperatures, we observed a field-induced transition to a ferrimagnetic state with a one-third magnetization plateau.
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INTRODUCTION Magnetic properties of solids are defined by various parameters including the magnitude of spin, magnetic anisotropy, and most importantly the underlying lattices on which magnetic ions reside. So far, a number of magnetic lattices are known, such as zigzag chain1,2 and diamond chain3 for one dimension (1D); triangular lattice,4 square lattice,5 Kagomé lattice,6 honeycomb lattice,7 and Shastry-Sutherland lattice8 for two dimension (2D); and cube lattice,9 pyrochlore lattice,10 and hyper Kagomé lattice11 for three dimension (3D). Discovery of experimental correspondences is crucial to test theoretical predictions or to explore exotic phenomena. Perovskite oxides ABO3 host the 3D cube lattice if the B-site is occupied by magnetic cations. The 2D square lattice is accessible in layered systems as exemplified by single-layered perovskites. Hole doping to the S = 1/2 square lattice results in high-temperature superconductivity,12 while inclusion of second nearest-neighbor interaction brings about spin liquid state.13 Ruddlesden−Popper (A2A′n−1BnO3n+1), Dion−Jacobson (A′An−1BnO3n+1), and Aurivillius ((Bi2O2)An−1BnO3n+1) types of layered perovskites (n = 2, 3,···) provide a series of lattices with intermediate dimensions bridging between 2D and 3D, where n-layers of square lattice are separated along the [001] direction. Another example of intermediate dimensional systems is the cube tile lattice (Figure 1), which is topologically equivalent to the distorted triangular lattice J′ = 0 (see Figure S1 for the definitions of J and J′). A recent theoretical study on the cube tile Heisenberg antiferromagnet revealed an up−up−down spin state without a magnetic field.14 A connection between the ideal triangular lattice and the dice lattice was also examined.15 Unfortunately, experimental realization of the cube tile model has not been reported, which is partly resulting from the lack of layered perovskite oxides aligned along the [111] direction, as © XXXX American Chemical Society
Figure 1. Cube tile lattice.
opposed to the [001] and [110] cases. Here, we hydrothermally synthesized a layered system of iron selenite fluoride CsFe3(SeO3)2F6, where [Fe3(SeO3)2F6]− blocks separated by Cs cations can be viewed as the cube tile lattice. Magnetic measurements show interesting phase transitions as a function of temperature and magnetic field. Received: March 30, 2018
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DOI: 10.1021/acs.inorgchem.8b00859 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
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Magnetic Measurements. A Quantum Design MPMS-XL superconducting quantum interference device (SQUID) magnetometer was used to collect the temperature dependence of directcurrent (DC) magnetic susceptibility data for 1 between 2 and 350 K at various magnetic fields from 0.001 to 1 T and for magnetization curves between −7 and 7 T at a constant temperature of 2, 50, 100, 125, 150, and 300 K.
EXPERIMENTAL SECTION
Caution! Hydrof luoric acid is toxic and corrosive. It must be handled with extreme caution and the appropriate protective gear.16−18 Synthesis. Cesium chloride (CsCl, >99.0%, Wako), iron(III) oxide (Fe2O3, 99.9%, Wako), selenium(IV) dioxide (SeO2, >97%, Wako), and aqueous hydrofluoric acid (46.0−48.0% HF by weight, Wako) were used as received. Pale cyan single crystals of compound CsFe3(SeO2)3F6 1 were synthesized by adding 0.135 g (0.8 mmol) of CsCl, 0.04 g (0.5 mmol) of Fe2O3, 0.244 g (2.2 mmol) of SeO2, 0.4 mL (∼11 mmol) of 48% aqueous HF, and 0.4 mL (22.2 mmol) of distilled H2O to a Teflon (fluoro(ethylenepropylene)) pouch made as described previously;3,19−23 all reagents were sealed with a sealer in Teflon pouches and placed into a 125 mL Parr autoclave with a backfill of 45 mL of pure water. The autoclave was quickly heated to 230 °C, held at this temperature for 168 h, and cooled to ambient temperature for 109 h. The single crystals were recovered in air after vacuum filtration. Crystallographic Determination. Single-crystal X-ray diffraction experiment for 1 was conducted at room temperature on a Rigaku RAXIS RAPID image plate diffractometer with Mo Kα radiation (λ = 0.71073 Å). The crystal-to-detector distance was 127 mm, and data integrations were made using Rigaku RAPID-AUTO,24 where multiscan absorption corrections were applied. The structure was determined by direct methods, completed by Fourier difference syntheses with SIR97,25 and refined using SHELXL-2014.26 Additional symmetry elements were checked using the program PLATON.27 Crystallographic data are reported in Table 1.
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RESULTS Phase purity of 1 was examined by powder X-ray diffraction as shown in Figure S2. Compound 1 crystallizes in the space group P3̅m1 with unit cell parameters of a = 5.459(2) Å and c = 10.537(3) Å (Z = 1). There are two distinct iron sites in its asymmetric unit, Fe(1) at the 2d site and Fe(2) at the 1c site. This compound contains [Fe3(SeO3)2F6]− blocks stacking along the c axis that are separated by Cs+ cations, as shown in Figure 2a. The [Fe3(SeO3)2F6]− block is terminated in both sides by (SeO3)2− anions, which provide 12-fold coordination around a Cs+ cation (Figure S3a). Thus, the quasi-2D magnetism is expected in this material. Each [Fe3(SeO3)2F6]− block contains three Fe sublayers, where the middle one with Fe(2) is sandwiched by the outer ones with Fe(1) (see Figure 2a). As shown in Figure 2b, Fe(2) is octahedrally coordinated to form a homoleptic [Fe(2)F6]3− unit with the equidistant Fe(2)−F bond of 1.926(5) Å, while Fe(1) is coordinated by three oxide and three fluoride anions to form a heteroleptic [Fe(1)O3F3]6− unit with an Fe(1)−F and Fe(1)−O distance of 2.015(5) and 1.947(6) Å, respectively. The oxide ligands in the facial-[Fe(1)O3F3]6− octahedron are bonded to Se, while the fluoride ligands are bonded to Fe(2) with a bridging angle of ∠Fe(1)−F−Fe(2) = 176.6°. The formal oxidation states of +3 for both irons, +4 for selenium, and +1 for cesium are confirmed by bond valence sum (BVS) calculations using values reported by Brese and O’Keefe (Table S2).28 Temperature-dependent magnetization M(T) of 1 measured at various magnetic fields is shown in Figure 3. M/H(T) is given in Figure S4. M(T) starts to substantially increase at ∼130 K (= Tc), indicating the onset of magnetic transition. The χT versus T plot (Figure S5) shows an abrupt increase at ∼Tc followed by a decrease, which is a typical behavior for ferrimagnets.29−33 At low fields below 0.2 T, a cusp indicative of an antiferromagnetic ordering is seen, though it is curious that M(T) takes a nearly constant value down to low temperatures. A nonlinear temperature dependence of χ−1(T) (Figure S6) indicates a development of short-range spin−spin correlations well beyond Tc, as expected from low-dimensional
Table 1. Crystal Data, Structure Solutions, and Refinements for Compound 1 compound formula formula weight (g·mol−1) temperature (K) crystal system space group a (Å) c (Å) V (Å3) Z maximum θ (deg) λ (Mo/Cu Kα) (Å) ρcalc (g·cm−3) Rint R1 wR2 goodness-of-fit
CsFe3(SeO3)2F6 668.38 296(2) trigonal P3̅m1 5.459(2) 10.537(3) 271.9(2) 1 26.5 0.710 73 4.082 0.028 0.042 0.105 1.13
Figure 2. Crystal structure of 1 viewed (a) along the axis b; (b) 2D [Fe3(SeO3)2F6]− block along the c axis. B
DOI: 10.1021/acs.inorgchem.8b00859 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 3. Temperature dependence of M for 1 at various magnetic fields.
Figure 5. A proposed spin structure with the cube tile lattice in 1.
With the antiferromagnetic interaction of J, the spin structure (below Tc) within the cube tile lattice would be the one depicted in Figure 6b, where Fe(1) and Fe(2) moments align
spin systems. Under higher magnetic fields, M(T) shows a typical temperature dependence for a ferrimagnet. The field dependence of magnetization M(H) measured at 300, 150, 125, 50, and 2 K is shown in Figure 4. The M−H data
Figure 6. Proposed magnetic structure for 1: (a) the antiferromagnetic ground state and (b) the field-induced ferrimagnetic state with onethird magnetization plateau.
antiparallel to each other. Given the Fe(1)/Fe(2) ratio of 2:1, each cube tile layer will be ferrimagnetically arranged as [Fe(1)↑−Fe(2)↓−Fe(1)↑]. If the ferrimagnetic blocks stack in a uniform manner along the c axis, a ferrimagnetic state will result, with a spontaneous magnetization of 5 μB per formula unit. This spin structure well explains the observed one-third plateau under magnetic field at low temperatures. The spin structure is shown in Figure 6b. Among possible antiferromagnetic structures, the simplest is where ferrimagnetic [Fe3(SeO3)2F6]− layers are arranged in a staggered manner, with a stacking sequence of·−[Fe(1)↑−Fe(2)↓−Fe(1)↑]− [Fe(1)↓−Fe(2)↑−Fe(1)↓]−; see Figure 6a.
Figure 4. Field dependence of M for 1 at various temperatures. (inset) M(H) at 2 K in a low-field region.
at 2 K exhibits a plateau corresponding to almost one-third of saturated magnetization. A closer look at the low-field region (inset of Figure 4 and Figure S7) revealed an obvious change in slope at ∼0.05 T, indicating a meta-magnetic transition from the antiferromagnetic ground state to the field-induced ferrimagnetic state. As shown in Figure 2a, neighboring Fe(1)O3F3 and Fe(2)F6 octahedra in the [Fe3(SeO3)2F6]− block are connected by corner sharing through Fe(1)−F−Fe(2); see Figure S8. The dominant magnetic interaction J is provided through Fe(1)− F−Fe(2) superexchange pathway with Fe(1)−Fe(2) distance of 3.939 Å. The Fe(1)−F−Fe(2) bridging angle of 176.6° is close to 180°, so that one can expect a fairly strong antiferromagnetic interaction according to the Goodenough−Kanamori rule.34,35 By contrast, neighboring Fe(1) ions and neighboring Fe(2) ions with distances of 5.459 and 5.459 Å, respectively, do not possess bridging ligands, indicating much smaller magnetic interactions. As a result, the Fe sublattice within the block serves as the cube tile lattice (Figure 5). Note though that there is a slight rhomboidal deformation with ∠Fe−Fe−Fe = 87.7° or 92.3°.
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CONCLUSION We have synthesized CsFe3(SeO3)2F6 with Fe spins on the cube tile lattice. CsFe3(SeO3)2F6 exhibits at low fields an antiferromagnetic transition around 130 K. The magnetic field also induces a transition to the ferrimagnetic state with the onethird magnetization plateau. We hope that our experimental finding of the cube tile lattice will stimulate extensive search for the relevant compounds. As mentioned earlier, conventional layered perovskite materials cannot host this lattice, so mixedanion compounds may provide such playgrounds.36 Alternatively, epitaxial thin films can open an effective platform to access various magnetic lattice. Recently, a (111)-oriented superlattice with alternating two-layers of Ca0.5Sr0.5IrO3 and two layers of SrTiO3 on SrTiO3(111) substrates has been shown to represent a honeycomb lattice by Ir.37 The cube tile C
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lattice can be obtained by fabricating a superlattice of nonmagnetic B′ layer(s) and three magnetic B layers along [111].
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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.8b00859. Characterization including PXRD and magnetic properties data (PDF) Accession Codes
CCDC 1409698 contains 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 data_
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Fax: +81-75-383-2510. ORCID
Hongcheng Lu: 0000-0003-0414-4768 Hiroshi Kageyama: 0000-0002-3911-9864 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by CREST (JPMJCR1421) and Grant-in-Aid for Scientific Research on Innovative Areas “Mixed anion” (JP16H06439) from MEXT.
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DOI: 10.1021/acs.inorgchem.8b00859 Inorg. Chem. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.inorgchem.8b00859 Inorg. Chem. XXXX, XXX, XXX−XXX
