Article pubs.acs.org/IC
Structure and Magnetic Behavior of Layered Honeycomb Tellurates, BiM(III)TeO6 (M = Cr, Mn, Fe) Sun Woo Kim,† Zheng Deng,† Zachary Fischer,† Saul H. Lapidus,‡ Peter W. Stephens,§ Man-Rong Li,† and Martha Greenblatt*,† †
Department of Chemistry and Chemical Biology, Rutgers, the State University of New Jersey, 610 Taylor Road, Piscataway, New Jersey 08854, United States ‡ Advanced Photon Source, Argonne National Laboratory, Lemont, Illinois 60439, United States § Department of Physics & Astronomy, State University of New York, Stony Brook, New York 11794, United States S Supporting Information *
ABSTRACT: New layered honeycomb tellurates, BiM(III)TeO6 (M = Cr, Mn, Fe) were synthesized and characterized. BiM(III)TeO6 (M = Cr, Fe) species crystallize in a trigonal space group, P3̅1c (No. 163), of edge-sharing M3+/Te6+O6 octahedra, which form honeycomb-like double layers in the ab plane with Bi 3+ cations located between the layers. Interestingly, the structure of BiMnTeO6 is similar to those of the Cr/Fe analogues, but with monoclinic space group, P21/ c (No. 14), attributed to the strong Jahn−Teller distortion of Mn3+ cations. The crystal structure of BiM(III)TeO6 is a superstructure of PbSb2O6-related materials (ABB′O6). The Cr3+ and Fe3+ cations are ordered 80% and 90%, respectively, while the Mn3+ ions are completely ordered on the B-site of the ABB′O6 structure. BiCrTeO6 shows a broad antiferromagnetic transition (AFM) at ∼17 K with a Weiss temperature (θ) of −59.85 K, while BiFeTeO6 and BiMnTeO6 show sharp AFM transitions at ∼11 K with θ of −27.56 K and at ∼9.5 K with θ of −17.57 K, respectively. These differences in the magnetic behavior are ascribed to the different concentration of magnetic nearest versus next-nearest neighbor interactions of magnetic cations due to the relative differences in the extent of M/Te ordering.
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octahedra separated by A(II) cations (AO6 octahedra).14 ABB′O6-type materials with 3d−5d transition metal cations show interesting magnetic behavior such as an extraordinarily high AFM transition temperature (e.g., 565 K of SrRu2O6).15−23 Previously, we demonstrated similar layered honeycomb structures, SrGeTeO624 with noncentrosymmetric (NCS) space group P312, No. 149, and SrMnTeO625 as well as PbMnTeO613 with NCS space group, P6̅2m, No. 189 form, depending on Bsite cation arrangement: in SrGeTeO6 ordered arrangement of B/B′ leads to NCS, P312 with octahedrally coordinated Bcations, while the B/B′ cations in SrMnTeO6 as well as in PbMnTeO6, P6̅2m, are disordered with trigonal prismatic coordination of the B-cations. Furthermore, for trivalent A cations, such as Y3+, Bi3+, or lanthanides, the crystal structure of PbSb2O6-type ABB′O6 materials is a superstructure of PbSb2O6.26,27 For example, LnCrTeO6 (Ln = Y, La, Tb, Er) is in the CS space group, P3,̅ No. 147, with AFM ordering at ∼10 K,28 and LaFeTeO6 and BiCrTeO6 belong to the CS space group, P3̅1c, No. 163.29,30 These findings suggest that
INTRODUCTION Quasi-two-dimensional (2D) transition metal layered oxides with honeycomb lattices have been investigated due to their interesting magnetic behavior including magnetic frustration, spin-glass, or spin-liquid behavior.1−10 Experimental and theoretical works revealed that the interplay between nearest neighbor antiferromagnetic (AFM) and next-nearest neighbor AFM interactions induces exotic magnetic ground states in the honeycomb lattice. For example, the Bi3Mn4O12(NO3) honeycomb lattice antiferromagnet (S = 3/2) shows an exotic magnetic ground state with a relatively large Weiss constant of −257 K, but no long-range magnetic order until 0.4 K.2,4,5,7 Honeycomb lattice alkali-metal iridates, A2IrO3 (A+ = Li, Na) (S = 1/2), have been identified as platforms for anticipated Kitaev quantum spin liquid.3,8−10 Layered honeycomb lattice tellurates have also been reported: for example, Na2M2TeO6 (M = Co, Ni, Zn, Mg) species order as AFMs at low temperature (∼30 K),11,12 and PbMn(IV)TeO6 exhibits a lowdimensional AFM behavior (TN = ∼20 K) with some degree of magnetic frustration (|θ|/TN = ∼2.16).13 The PbSb2O6-type compounds (general formula, ABB′O6) also exhibit a layered honeycomb structure (centrosymmetric (CS) space group, P3̅1m, No.162) with edge-sharing BO6/B′O6 © XXXX American Chemical Society
Received: June 19, 2016
A
DOI: 10.1021/acs.inorgchem.6b01472 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Bi atoms are located at 2b (0, 0, 0), Cr(1)/Te(1) at 2d (2/3, 1/ 3, 1/4), Cr(2)/Te(2) at 2c (1/3, 2/3, 1/4), and O atoms at 12i (x, y, z) positions in P3̅1c. The Rietveld refinement plot of SPXD data for BiCrTeO6 is shown in Figure 1a; atomic coordinates and atomic displacement parameters of BiCrTeO6 are summarized in Table 1. BiCrTeO6 exhibits a 2D crystal structure consisting of edge-sharing Cr(1)/Te(1)O6 (Cr/Te =
compositional modifications of PbSb2O6-related materials could lead to new honeycomb structures. On the basis of these observations, we have investigated PbSb2O6-type ABB′O6 phases to find new materials with interesting properties. Here we report the successful synthesis and characterization of the PbSb2O6-related new layered honeycomb tellurates, BiM(III)TeO6 with M = Cr3+ (d3), Mn3+ Jahn−Teller (high spin, d4), and Fe3+ (high spin, d5). The crystal structure of BiCrTeO6 was previously reported,30 but its physical properties were not investigated.
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EXPERIMENTAL SECTION
Reagents. Bi2O3 (Alfa Aesar, 99.99%), Cr2O3 (Alfa Aesar, 99.97%), Fe2O3 (Alfa Aesar, 99.998%), Mn2O3 (Aldrich, 99.9%), and H2TeO4· 2H2O (Alfa Aeaser, 99+%) were used without any further purification. Synthesis. Polycrystalline BiM(III)TeO6 (M = Cr, Mn, Fe) species were prepared by a conventional solid state reaction. Stoichiometric amounts of Bi2O3 (0.4659 g, 1.0 mmol), Cr2O3 (0.1520 g, 1.0 mmol), Mn2O3 (0.1579 g, 1.0 mmol) or Fe2O3 (0.1597g, 1.0 mmol), and TeO3 (0.3512 g, 2.0 mmol, amorphous TeO3 was prepared by heating H2TeO4·2H2O at 400 °C for 12h in air31) were thoroughly ground and pressed into a pellet. The pellets were placed in alumina crucibles and treated for 24 h at 750 °C in air and then cooled to room temperature (heating and cooling rate is 200 °C/h, respectively). The final products, dark green BiCrTeO6, dark brown BiMnTeO6, and yellowish brown BiFeTeO6 polycrystalline powders, were obtained, respectively. The purity of all materials was confirmed by powder Xray diffraction (PXRD). Laboratory and Synchrotron Powder X-ray Diffraction. BiM(III)TeO6 species (M = Cr, Mn, Fe) were characterized by powder X-ray diffraction (PXRD, Bruker-AXS D8-Advanced diffractometer with Cu Kα, λ = 1.5406 Å, 40 kV, 35 mA) for purity and phase identification. Synchrotron powder X-ray diffraction (SPXD) data were collected at ambient temperature at the 11-BM beamline of the Advanced Photon Source (APS), Argonne National Laboratory, with the following X-ray wavelengths: λ = 0.413981 Å for M = Mn, Fe; λ = 0.414223 Å for M = Cr. Diffraction data analysis and Rietveld refinement for the Cr and Fe materials were performed with the GSAS-EXPGUI software package32 and with TOPAS-Academic33 for the Mn compound. Small amounts of impurities were detected in all samples. We could identify about 3% Bi2Te2O8 in the BiMnTeO6 sample and less than 1% in the BiFeTeO6, which were included in the Rietveld refinements. There were several unidentified impurity peaks seen, principally in the BiMnTeO6, with an estimated weight fraction of a fraction of 1%. Magnetic Measurements. The magnetic measurements for BiM(III)TeO6 (M = Cr, Mn, Fe) were performed with a commercial Quantum Design SQUID magnetometer. The dc magnetic susceptibility data were collected in the range 2 ≤ T ≤ 300 K under an applied magnetic field of 1000 and 10000 Oe. Isothermal magnetization curves were obtained for magnetic fields − 5 T ≤ H ≤ 5 T at 2 and 300 K.
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RESULTS AND DISCUSSION Structure. The crystal structure of BiCrTeO 6 was previously reported (P31̅ c, No. 163, unit cell: a = b = 5.16268(7) Å, c = 9.91861(17) Å, V = 228.945(6) Å3, Z = 2);30 however, its magnetic properties were not investigated. In order to elucidate its structure−physical properties relationship, the structural refinement of BiCrTeO6 was performed on SPXD data on the basis of the reported BiCrTeO6 structural model. From the Rietveld refinement, the following apply: Rp = 6.57%, Rwp = 9.47%, χ2 = 2.21 with lattice parameters of a = b = 5.16451(1) Å, c = 9.91680(1) Å, V = 229.065(1) Å3, and Z = 2. The only significant difference from the previously published structure (Cr/Te = 68/32 on the B site) is a significantly lower degree of Cr/Te site disorder (Cr/Te = 80/20; vide inf ra). The
Figure 1. (a) Rietveld refinement plot from synchrotron XRD data for BiCrTeO6, (b) Rietveld refinement plot from synchrotron XRD data for BiFeTeO6, and (c) Rietveld refinement plot from synchrotron XRD data for BiMnTeO6. B
DOI: 10.1021/acs.inorgchem.6b01472 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry Table 1. Atomic Coordinates and Atomic Displacement Parameters for BiM(III)TeO6 (M = Cr, Mn, Fe)a atom
Wyckoff
x
y
z
Uiso (Å2)
SOF
0 1/4 1/4 0.1414(1)
0.0055(3) 0.0099(1) 0.0055(1) 0.0085(4)
1 0.8/0.2 0.2/0.8 1
0 1/4 1/4 0.1417(4)
0.0038(3) 0.0053(5) 0.0037(1) 0.0026(5)
1 0.9/0.1 0.1/0.9 1
−0.0006(1) 0.7430(2) 0.7348(1) 0.6557(7) 0.8629(7) 0.8721(7) 0.8449(7) 0.8480(7) 0.8797(7)
0.0054(1) 0.0040(2) 0.0033(4) 0.0042(6) 0.0042(6) 0.0042(6) 0.0042(6) 0.0042(6) 0.0042(6)
1 1 1 1 1 1 1 1 1
b
Bi(1) Cr(1)/Te(1) Cr(2)/Te(2) O(1)
2b 2d 2c 12i
0 2/3 1/3 0.0077(4)
Bi(1) Fe(1)/Te(1) Fe(2)/Te(2) O(1)
2b 2d 2c 12i
0 2/3 1/3 0.0117(2)
Bi(1) Mn(1) Te(1) O(1) O(2) O(3) O(4) O(5) O(6)
4e 4e 4e 4e 4e 4e 4e 4e 4e
0.2591(1) 0.7761(4) 0.2642(2) 0.6256(13) −0.0992(13) −0.0958(14) 0.4655(14) −0.0508(15) 0.4474(15)
BiCrTeO6 0 1/3 2/3 0.3698(4) BiFeTeO6c 0 1/3 2/3 0.3750(4) BiMnTeO6d 0.7611(1) 0.9149(2) 0.0837(1) 0.7562(9) 0.4299(9) 0.7341(8) −0.0479(8) 0.0597(9) 0.5906(9)
Uncertainties quoted in parentheses are estimates from the Rietveld standard uncentainties of the first type; as such, they reflect only the propagation of statistical errors from the experimental data. Plausible accuracies are probably several times larger than the numbers quotes, but this is the common practice of the community. bFrom SPXD Rietveld refinement using space group P31̅ c (No. 163): Rp = 6.57%, Rwp = 9.47% and χ2 = 2.21, unit cell: a = b = 5.16451(1) Å, c = 9.91680(1) Å, V = 229.065(1) Å3, Z = 2. cFrom SPXD Rietveld refinement using space group P3̅1c (No. 163): Rp = 5.28%, Rwp = 8.49% and χ2 = 1.52, unit cell: a = b = 5.21194(1) Å, c = 9.88177(1) Å, V = 232.468(1) Å3, Z = 2. dFrom SPXD Rietveld refinement using space group P21/c (No. 14): Rp = 7.37%, Rwp = 10.74% and χ2 = 4.77, unit cell: a = 5.17263(1) Å, b = 9.06019(2) Å, c = 9.91407(3) Å, β = 90.1784(2)°, V = 464.621(2) Å3, Z = 4. a
80/20) and Cr(2)/Te(2)O6 (Cr/Te = 20/80) octahedra, which form honeycomb-like double layers in the ab plane with Bi3+ cations (BiO6 octahedra) located between the layers (see Figure 2a). The bond distances of Cr(1)/Te(1)−O(1) and Cr(2)/Te(1)−O(1) are 1.991(1) Å and 1.938(1) Å with six equivalent distances, respectively. The bond angle Cr(1)/ Te(1)−O(1)−Cr(2)/Te(2) is 98.67(1)°. It is noteworthy that there are six equivalent Bi(1)−O(1) bond distances of 2.353(1) Å in the BiO6 octahedra, which indicates that the 6s2 lone pair on the Bi3+ cation is not stereoactive as also observed in other Bi3+-containing layered oxides.30 The structural refinement of BiFeTeO6 was also performed on SPXD data based on the same structural model. From the Le Bail fit, Rp = 5.34%, Rwp = 8.47%, χ2 = 1.48, and from Rietveld refinement, Rp = 5.28%, Rwp = 8.49%, χ2 = 1.52, with lattice parameters of a = b = 5.21194(1) Å, c = 9.88177(1) Å, V = 232.468(1) Å3, and Z = 2. The Bi atoms are located at 2b (0, 0, 0), Fe(1)/Te(1) at 2d (2/3, 1/3, 1/4), Fe(2)/Te(2) at 2c (1/3, 2/3, 1/4), and O atoms at 12i (x, y, z) positions in P3̅1c. The Rietveld refinement plot of SPXD data for BiFeTeO6 is shown in Figure 1b; atomic coordinates and atomic displacement parameters of BiFeTeO6 are summarized in Table 1. BiFeTeO6 also exhibits a 2D crystal structure consisting of edge-sharing Fe(1)/Te(1)O6 (Fe/Te = 90/10) and Fe(2)/ Te(2)O6 (Fe/Te = 10/90) octahedra, which form honeycomblike double layers in the ab plane with Bi3+ cations (BiO6 octahedra) located between the layers (see Figure 2b). The bond distances of Fe(1)/Te(1)−O(1) and Fe(2)/Te(1)−O(1) are 2.008(1) and 1.927(1) Å with six equivalent distances, respectively. The bond angle Fe(1)/Te(1)−O(1)−Fe(2)/ Te(2) is 99.68(1)°. Six equivalent Bi(1)−O(1) bond distances
in the BiO6 octahedra are 2.380(1) Å, which indicates again that the 6s2 lone pair electrons on the Bi3+ cation are not stereoactive as discussed above.30 In contrast, the powder pattern of BiMnTeO6 indexed to a monoclinic cell of the following (refined) dimensions: a = 5.17263(1) Å, b = 9.06019(2) Å, c = 9.91407(3) Å, β = 90.1784(2)°, V = 464.621(2) Å3, probable space group P21/c. Figure S1 is a magnified view of this pattern, to illustrate the lower symmetry. This lattice is closely related to the trigonal lattices of the Cr and Fe compounds, viz. aMn ≈ atrig, bMn ≈ 31/2atrig, cMn ≈ ctrig, suggesting that the Mn compound is distorted from the higher-symmetry parent phase. This significantly increases the complexity of structure determination, with all atoms in general Wyckoff positions, and six nonequivalent oxygen atoms. Distortion mode analysis, via the ISODISTORT software, is a very powerful tool for such problems; it recasts the 27-dimensional configuration space into local distortions, each with relatively clear interpretation as shifts, rotations, and distortions of polyhedra.34 In such problems, it is frequently observed that only a few distortion modes are active, and a random search over distortion modes is more fruitful than searching individual atomic x, y, z fractional coordinates. Starting with the parent structure and the presumed lattice and space group of the distorted structure, the ISODISTORT furnished a transformation from the trigonal parent to the distorted monoclinic superlattice, and a convenient parametrization of irreducible representations (distortion modes), which ultimately led to the correct solution. ISODISTORT produces macros suitable for using TOPAS-Academic software33 for the solution and Rietveld refinement of the powder diffraction data, which led to the C
DOI: 10.1021/acs.inorgchem.6b01472 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 2. Crystal structures of BiM(III)TeO6 (M = Cr, Mn, Fe): (a) BiCrTeO6 in the bc (top) and ab (bottom) planes, (b) BiFeTeO6 in the bc (top) and ab (bottom) planes and (c) BiMnTeO6 in the bc (top) and ab (bottom left) planes and the illustration of JT distortion of Mn3+O6 octahedra in BiMnTeO6 (bottom right). D
DOI: 10.1021/acs.inorgchem.6b01472 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry Table 2. Selected Bond Distances, Angles, and BVS for BiM(III)TeO6 (M = Cr, Mn, Fe) cation
anion
bond length (Å)
Bi(1) Cr(1)/Te(1) Cr(2)/Te(2)
O(1) O(1) O(1)
2.353(1) × 6 1.991(1) × 6 1.938(1) × 6
Bi(1) Fe(1)/Te(1) Fe(2)/Te(2)
O(1) O(1) O(1)
2.380(1) × 6 2.008(1) × 6 1.927(1) × 6
Bi(1)
O(1) O(2) O(3) O(4) O(5) O(6) O(1) O(2) O(3) O(4) O(5) O(6) O(1) O(2) O(3) O(4) O(5) O(6)
2.449(7) 2.355(8) 2.238(7) 2.548(7) 2.469(8) 2.180(8) 1.853(8) 1.981(7) 2.181(7) 1.930(7) 1.898(8) 2.312(8) 1.982(8) 1.899(8) 1.932(7) 1.921(7) 1.993(8) 1.878(7)
Mn(1)
Te(1)
× × × × × × × × × × × × × × × × × ×
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
BVS BiCrTeO6 2.98 (Bi3+) 2.92 (Cr3+) 5.67 (Te6+) BiFeTeO6 2.77 (Bi3+) 3.06 (Fe3+) 5.85 (Te6+) BiMnTeO6 3.00 (Bi3+)
atoms
angle (deg)
Cr(1)/Te(1)−O(1)−Cr(2)/Te(2)
98.67(1)
Fe(1)/Te(1)−O(1)−Fe(2)/Te(2)
99.68(1)
Mn(1)−O(1)−Te(1) Mn(1)−O(3)−Te(1) Mn(1)−O(4)−Te(1) Mn(1)−O(6)−Te(1)
103.9(2) 94.0(2) 105.3(2) 93.0(2)
2.96 (Mn3+)
5.76 (Te6+)
Magnetic Behavior. The dc magnetic susceptibility of BiCrTeO6 was measured under 1000 and 10000 Oe in the temperature range 2−300 K and is shown as χ and 1/χ versus T plots in Figure 3a,b, respectively. BiCrTeO6 exhibits AFM behavior with a broad Néel transition temperature (TN) at ∼17 K. No significant divergence between ZFC (zero field cooling) and FC (field cooling) magnetization curves is observed. From the 1/χ versus temperature shown in Figure 3b, the susceptibility data were fit to the Curie−Weiss (CW) law, χ = C/(T − θ) for T > 100 K, where C is the Curie constant and θ is the Weiss constant: C = 1.57 emu K mol−1 and θ = −59.85 K were extracted from the CW fit of the data. On the basis of the CW fit, the effective magnetic moment, μeff = 3.54 μB/Cr, is in good agreement with the theoretical spin only value for Cr3+ (S = 3/2, 3.87 μB). The negative Weiss constant indicates AFM interactions, which could arise from superexchange interaction of nearest (Cr3+−O2−−Cr3+) and next-nearest neighbor (Cr3+− O2−−Te6+−O2−−Cr3+) interactions.38−40 In Figure S2, the isothermal magnetization data of BiCrTeO6 measured at 2 and 300 K as a function of applied field H at both temperatures are linear, which indicate that no ferromagnetic interactions are involved. The magnetic interactions of Cr3+ in adjacent layers should be negligible because the layers are well-separated by the Bi3+ ions with distances longer than 4.9 Å. The dc magnetic susceptibility of BiFeTeO6 was measured under 1000 and 10000 Oe in the temperature range 2−300 K and is shown as χ and 1/χ versus T plots in Figure 4a,b, respectively. BiFeTeO6 shows AFM behavior with a sharp transition at TN of ∼11 K, which is likely due to long-range magnetic order. No significant divergence between ZFC and FC magnetization curves is observed, corroborating the near ordering of the Fe/Te cations. From the 1/χ versus
refinement presented here. Figure 1c shows the completed Rietveld refinement, and Table 1 includes the refinement details and structural parameters. The refinement statistics of BiMnTeO6 are poorer than the other two compounds, due to the presence of small amounts of unidentified impurities. Nevertheless, the quality of refinement in Figure 1c gives strong support to the structure described here. Similar to the Cr and Fe analogues, BiMnTeO6 also exhibits a 2D crystal structure consisting of edge-sharing MnO6 and TeO6 distorted octahedra, which form honeycomb-like double layers in the ab plane with Bi3+ cations (BiO6 distorted octahedra) located between the layers (see Figure 2c). The Mn and Te cations are completely ordered, with an upper limit of approximately 2% site disorder. The more complicated and lower-symmetry structure of BiMnTeO6 is caused by a Jahn− Teller distortion of the Mn3+O6 octahedra (see Figure 2c). We therefore see a larger range of metal−oxygen distances throughout the structure: Mn(1)−O of 1.853(8)−2.312(8) Å, Bi(1)−O of 2.175(8)−2.548(7) Å, Te(1)−O of 1.878(7)− 1.993(8) Å. The maximum Mn−O distance is unusually long, but not unheard of; approximately 10% of the entries in Pearson’s Crystal Database with MnO6 octahedra have at least one Mn−O distance greater than or equal to 2.31 Å.35 The octahedral distortion parameter (Δd = 66.35 × 10−4) of Mn3+O6 for BiMnTeO6 also indicates the presence of strong JT distortion. Selected bond distances and angles for BiMTeO6 (M = Cr, Mn, Fe) are summarized in Table 2. Bond valence sum (BVS) calculations36,37 resulted in values of 2.77−3.00, 2.92, 2.96, 3.06, and 5.67−5.85 for Bi3+, Cr3+, Mn3+, Fe3+, and Te6+, respectively (see Table 2). E
DOI: 10.1021/acs.inorgchem.6b01472 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 3. (a) Temperature dependence of the magnetic susceptibility of BiCrTeO6 measured at 1000 and 10000 Oe. The inset shows a close up of the low-temperature region revealing the ∼17 K Néel temperature. (b) The inverse magnetic susceptibility of BiCrTeO6 with a Curie−Weiss fit (solid line).
temperature shown in Figure 4b, the susceptibility data were fit to the CW law, χ = C/(T − θ) for T > 100 K: C = 4.21 emu K mol−1 and θ = −27.56 K were extracted from the CW fit of the data. From the CW fit, the effective magnetic moment, μeff = 5.80 μB/Fe, is in good agreement with the theoretical spin only value for Fe3+ (S = 5/2, 5.92 μB). The negative Weiss constant is consistent with AFM interactions, which could arise from the super-superexchange interaction of next-nearest neighbor Fe3+−O2−−Te6+−O2−−Fe3+.38−40 In Figure S3, the isothermal magnetization of BiFeTeO6 measured at 2 and 300 K as a function of applied field H indicates that some degree of spin orientations is present below TN (∼11 K) although an interlayer magnetic interaction between Fe3+ and Fe3+ should be negligible due to the good separation by the Bi3+ ions with distances longer than 4.9 Å. The dc magnetic susceptibility of BiMnTeO6 was measured under 1000 and 10000 Oe in the temperature range 2−300 K and is shown as χ and 1/χ versus T plots in Figure 5a,b, respectively. BiMnTeO6 shows AFM behavior with a sharp transition at TN of ∼9.5 K, which is likely due to long-range
magnetic order. No significant divergence between ZFC and FC magnetization curves is observed. From the 1/χ versus temperature shown in Figure 5b, the susceptibility data were fit to the CW law, χ = C/(T − θ) for T > 100 K: C = 3.13 emu K mol−1 and θ = −17.57 K were extracted from the CW fit of the data. From the CW fit, the effective magnetic moment, μeff = 5.00 μB/Mn, is in good agreement with the theoretical spin only value for Mn3+ (S = 2, 4.90 μB). The negative Weiss constant is consistent with AFM interactions, which could arise from the super-superexchange interaction of next-nearest neighbor Mn3+−O2−−Te6+−O2−−Mn3+.38−40 In Figure S4, the isothermal magnetization of BiMnTeO6 measured at 2 and 300 K as a function of applied field H indicates that some degree of spin orientations is present below TN (∼9.5 K) although interlayer magnetic interaction between Mn3+ and Mn3+ should be negligible due to the good separation by the Bi3+ ions with distances longer than 5.0 Å. Although the BiMTeO6 (M = Cr, Fe) species are isotypic, the Cr3+ (d3) phase is partially ordered (Cr:Te = 80:20), while the Fe3+ (d5) material is nearly completely ordered (Fe:Te = F
DOI: 10.1021/acs.inorgchem.6b01472 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 4. (a) Temperature dependence of the magnetic susceptibility of BiFeTeO6 measured in 1000 and 10000 Oe. The inset shows a close up of the low-temperature region revealing the ∼11 K Néel temperature. (b) The inverse magnetic susceptibility of BiFeTeO6 with a Curie−Weiss fit (solid line).
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90:10) on the octahedral B sites (see Figure 2a,b); this degree of ordering of B-site cations affects long-range ordering of M3+ (M= Cr, Fe) cations in the honeycomb layer, because of the different concentrations of magnetic nearest versus next-nearest neighbor interactions of magnetic cations. On the other hand, in the Mn compound, Mn/Te are ordered completely on the octahedral B site (see Figure 2c); thus, only next-nearest neighbors interactions of magnetic cations are possible. BiCrTeO6 shows a broad AFM transition (TN = ∼17 K and θ = −59.85 K), while BiFeTeO6 and BiMnTeO6 show a sharp AFM transition (TN = ∼11 K and θ = −27.56 K; TN = ∼9.5 K and θ = −17.57 K, respectively). This suggests that in partially Cr/Te-ordered BiCrTeO6 both nearest neighbor and nextnearest neighbor AFM interactions coexist, which are relatively stronger than the next-nearest neighbor AFM interactions in nearly completely Fe/Te-ordered BiFeTeO6 and completely ordered BiMnTeO6.
CONCLUSION
Three new layered honeycomb tellurates, BiM(III)TeO6 (M = Cr, Mn, Fe) were successfully synthesized via solid state reaction. Interestingly, the crystal structure of BiMnTeO6 is highly distorted (monoclinic space group, P21/c, No. 14) due to the strong Jahn−Teller effect of Mn3+ cations compared to the trigonal lattice of Cr and Fe analogues (trigonal space group, P3̅1 c, No. 163); all the compounds exhibit a superstructure of PbSb2O6-type. The Cr phase is partially ordered (Cr:Te = 80:20), the Fe phase is nearly completely ordered (Fe:Te = 90:10) and the Mn phase is ordered on the octahedral B site; these ordering tendencies affect long-range ordering of M3+ (M = Cr, Mn, Fe) cations in the honeycomb layer, because of the different concentration of magnetic nearest versus next-nearest neighbor superexchange interactions of magnetic cations. The difference between the degree of Cr/Te order in our measurements and previous work suggests the G
DOI: 10.1021/acs.inorgchem.6b01472 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 5. (a) Temperature dependence of the magnetic susceptibility of BiMnTeO6 measured in 1000 and 10000 Oe. The inset shows a close up of the low-temperature region revealing the ∼9.5 K Néel temperature. (b) The inverse magnetic susceptibility of BiMnTeO6 with a Curie−Weiss fit (solid line).
interesting possibility that such ordering may be controllable by annealing temperature and time, which would allow direct experimental access to the relationship between cation order and magnetic properties. All the compounds are insulating and exhibit antiferromagnetic ordering (AFM) at relatively low temperature. BiCrTeO6 shows a broad AFM transition at ∼17 K with a Weiss temperature (θ) of −59.85 K, while BiFeTeO6 and BiMnTeO6 present sharp AFM transitions at ∼11 K, with θ = −27.56 K, and at ∼9.5 K with θ = −17.57 K, respectively. Near-complete, or complete cation ordering and octahedral coordination of the B/B′ cations of these centrosymmetric PbSb2O6-type ABB′O6 super structure lattices, further corroborate the hypothesis that noncentrosymmetric structures are possible in the parent PbSb2O6-type ABB′O6, when the B/B′ site cations are ordered and octahedrally coordinated, or completely disordered and trigonal prismatically coordinated.
Further examples in these systems are in progress to confirm the correlations between B/B′ cation composition and arrangement and noncentrosymmetric property.
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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.6b01472. Comparison of the powder X-ray diffraction patterns of BiMnTeO6 and BiFeTeO6 and isothermal magnetization of BiM(III)TeO6 as a function of applied field H (PDF) BiM(III)TeO6 (M = Cr) (CIF) BiM(III)TeO6 (M = Mn) (CIF) BiM(III)TeO6 (M = Fe) (CIF) H
DOI: 10.1021/acs.inorgchem.6b01472 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
The manuscript was written through contributions of all authors. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS S.W.K., Z.D., Z.F., M.-R.L., and M.G. gratefully acknowledge support from NSF-DMR-1507252 grant. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract DE-AC02-06CH11357.
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DOI: 10.1021/acs.inorgchem.6b01472 Inorg. Chem. XXXX, XXX, XXX−XXX
