Crystal Structures and Magnetic Properties of Nickel Chain

Inorg. Chem. , 2015, 54 (22), pp 10725–10731. DOI: 10.1021/acs.inorgchem.5b01619. Publication Date (Web): October 23, 2015. Copyright © 2015 Americ...
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Crystal Structures and Magnetic Properties of Nickel Chain Compounds PbM2Ni6Te3O18 (M = Mn, Cd) Yoshihiro Doi,*,† Ryo Suzuki,† Yukio Hinatsu,† Katsuaki Kodama,‡ and Naoki Igawa‡ †

Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan Quantum Beam Science Directorate, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan

‡

S Supporting Information *

ABSTRACT: The synthesis, crystal structures, and magnetic properties of the pentanary oxides PbM2Ni6Te3O18 (M = Mn and Cd) were investigated. These compounds crystallize in a hexagonal structure with space group P63/m, in which the Ni2+ ions form a zigzag chain along the c axis. From the magnetic susceptibility and specific heat measurements, we found that the PbCd2Ni6Te3O18 behaves as a low-dimensional magnet due to the intrachain antiferromagnetic interaction between Ni2+ ions. Both compounds show a longrange antiferromagnetic ordering at 25.7 K (M = Cd) and 86.0 K (Mn). The magnetic structure of PbMn2Ni6Te3O18 determined by neutron diffraction measurements is a collinear antiferromagnetic arrangement of Mn2+ ions in the Mn2O9 dimeric unit and Ni2+ ions in the zigzag chain.

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we selected the Cd2+ ion, having a larger ionic radius (0.94 Å) than that of Ni2+ (0.69 Å), to prevent cation disorder. In this paper, we investigated the crystal structures and detailed magnetic properties of PbMn2Ni6Te3O18 and its new analogue PbCd2Ni6Te3O18. The results of their magnetic susceptibility, specific heat, and X-ray and neutron diffraction measurements are reported.

INTRODUCTION Low-dimensional magnetic materials have attracted a great deal of interest because of their anomalous magnetic properties at low temperatures. In them, the subject of one-dimensional (1D) chain systems increases in importance since Haldane’s conjecture:1,2 there is a spin gap of the excitation spectrum for integer-valued spin chains, whereas it is gapless for half-integervalued spin chains. The systems with such a spin gap are called Haldane gap systems3 and have been found in the Ni2+ (S = 1) chain compounds, for example, CsNiCl3,4 Ni(C2H8N2)2NO2ClO4 (NENP),5,6 Y2BaNiO5,7 and PbNi2V2O8.8 In order to find further interesting magnetic behaviors, it is absolutely essential to explore new compounds with lowdimensional structures and study their magnetic properties in detail. Recently, we focused our attention on the magnetic properties of the pentanary oxide PbMn2Ni6Te3O18. It has a hexagonal unit cell with space group P63/m, in which the NiO6 and TeO6 octahedra form a double-chain structure running along the c axis.9 In terms of magnetic properties, this double chain may be regarded as a 1D chain consisting of Ni2+ (S = 1) ions. In addition, this compound has a triangle-based array of chains in the ab plane, reflecting the hexagonal symmetry, which may induce geometrical frustration in the interchain magnetic interaction. Therefore, PbMn2Ni6Te3O18 has possibilities for unique magnetic behavior. However, its magnetic properties are unknown except for a related compound, Na5Co15.5Te6O36, showing a 1D magnetism of the Co2+ chain.10 PbMn2Ni6Te3O18 contains another magnetic ion (Mn2+) located in the trigonal tunnel built by three Ni chains, which makes it complicated to understand the magnetic properties. Thus, we also have tried to synthesize a new compound containing a diamagnetic ion instead of Mn2+. For this purpose, © XXXX American Chemical Society

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EXPERIMENTAL SECTION

Sample Preparation. The title compounds PbMn2Ni6Te3O18 and PbCd2Ni6Te3O18 were prepared as polycrystalline samples by a solidstate reaction. As starting materials, PbO (High Purity Chemicals, 99.999%), CdO (Soekawa Chemicals, 99.9%), MnO, NiO, and TeO2 (High Purity Chemicals, 99.9%) were used. The mixture with a stoichiometric metal ratio was pressed into pellets and enclosed in a gold tube, and then it was sealed in an evacuated silica ampule to prevent the loss of reagents by volatilization. To oxidize the tellurium ion from tetravalent to hexavalent under the reaction, the oxidation reagent Ag2O (Mitsuwa Pure Chemicals, 99.9%; equimolar with TeO2) was added in the ampule separated by silica wool. The reaction was carried out in a muffle furnace at 973−1073 K for 12−36 h. Powder X-ray and Neutron Diffraction Measurements. The powder X-ray diffraction (XRD) measurements were performed at room temperature in the range 10° ≤ 2θ ≤ 120° using a 2θ step size of 0.02° with Cu Kα radiation on a Rigaku MultiFlex diffractometer. For PbMn2Ni6Te3O18, powder neutron diffraction (ND) profiles were measured at 18 and 150 K in the range 2.5° ≤ 2θ ≤ 162.4° at intervals of 0.05° with a neutron wavelength λ = 1.823 71 Å. The ND measurements were performed by the high-resolution powder diffractometer, HRPD,11 installed at the JRR-3 M reactor at Japan Atomic Energy Agency (JAEA), Tokai. The XRD and ND data were analyzed by the Rietveld technique. For this purpose the program Received: July 21, 2015

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DOI: 10.1021/acs.inorgchem.5b01619 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry RIETAN-FP12 was used, and the determined crystal and magnetic structures were drawn by the VESTA program.13 For the magnetic reflections observed in the ND data at 18 K, the FullProf program14 was also used to test many magnetic structure models with collinear and noncollinear spin arrangements. Magnetic Susceptibility and Specific Heat Measurements. Magnetic measurements were carried out in the temperature range 1.8−400 K by a SQUID magnetometer (Quantum Design, MPMS5S). These measurements were taken in both zero-field-cooled (ZFC) and field-cooled (FC) conditions, and the applied field was 0.1 T. The field dependence (0−9 T) of magnetization was also performed by a commercial physical property measurement system (Quantum Design, PPMS model). The specific heat measurements were carried out in the temperature range 1.8−300 K by using a relaxation technique with PPMS. The sintered sample in the form of a pellet was mounted on a thin alumina plate with Apiezon N grease for better thermal contact.

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RESULTS AND DISCUSSION Synthesis and Crystal Structure. The pentanary oxide PbMn2Ni6Te3O18 and its new analogue PbCd2Ni6Te3O18 were successfully prepared. They were obtained as dark and light green polycrystalline samples, respectively. They contain a small amount (∼1%) of NiO as an impurity phase. Figure 1

Figure 2. Powder neutron diffraction profiles for PbMn2Ni6Te3O18 at (a) 150 K and (b) 18 K. Some magnetic reflection peaks are indexed (see text). Upper and lower vertical marks mean the reflection positions for main and impurity (NiO) phases, respectively.

both XRD and ND data; however, such evidence has not been found. As a result of the analysis, all the calculated profiles are presented in Figures 1 and 2, and the structural parameters determined from the ND and XRD data are summarized in Table 1 and Tables S1 and S2, respectively. Figure 3 shows the schematic crystal structure of PbM2Ni6Te3O18. In this structure, the smaller Ni and Te ions occupy 12i and 6h sites of the space group P63/m (No. 176), respectively, and both ions are octahedrally coordinated by six oxygen ions. These octahedra connect to each other by edgeTable 1. Structural Parameters for PbMn2Ni6Te3O18 Determined by the Powder Neutron Diffraction Measurement atom

site

x

y

z

B/Å2

T = 150 K, space group P63/m; a = 9.2789(1) Å, c = 8.8124(1) Å, Rwp = 4.21%, Rp = 3.25%, Re = 4.09%, RB = 0.94%, RF = 0.22%, NiO(impurity): 1.22%, μNi(NiO) = 1.2(3) μB Pb 2b 0 0 0 1.03(5) Mn 4f 1/3 2/3 0.0620(5) 0.31(8) Ni 12i 0.9887(1) 0.3483(1) 0.9111(1) 0.27(2) Te 6h 0.3622(3) 0.0254(4) 1/4 0.35(6) O1 12i 0.1022(2) 0.3067(2) 0.0898(3) 0.18(4) O2 12i 0.1295(2) 0.6073(2) 0.9119(2) 0.19(3) O3 6h 0.4200(3) 0.5524(4) 1/4 0.29(5) O4 6h 0.2908(3) 0.1848(3) 1/4 0.42(7) T = 18 K, space group P63/m; a = 9.2741(2) Å, c = 8.8070(1) Å, Rwp = 4.03%, Rp = 3.11%, Re = 2.90%, RB = 0.89%, RF = 0.65%, μMn = 4.71(3) μB, μNi = 1.96(2) μB, NiO(impurity): 1.19%, μNi(NiO) = 1.7(2) μB Pb 2b 0 0 0 0.40(4) Mn 4f 1/3 2/3 0.0626(5) 0.25(7) Ni 12i 0.9883(1) 0.3482(1) 0.9108(1) 0.14(1) Te 6h 0.3626(3) 0.0257(3) 1/4 0.17(5) O1 12i 0.1017(2) 0.3065(2) 0.0903(2) 0.06(4) O2 12i 0.1299(2) 0.6070(2) 0.9117(2) 0.09(3) O3 6h 0.4191(3) 0.5521(4) 1/4 0.18(4) O4 6h 0.2911(3) 0.1842(3) 1/4 0.25(6)

Figure 1. Powder X-ray diffraction profiles for (a) PbMn2Ni6Te3O18 and (b) PbCd2Ni6Te3O18. Upper and lower vertical marks means the reflection positions for main and impurity (NiO) phases, respectively.

shows the XRD profiles for the PbM2Ni6Te3O18 compounds. The observed Bragg peaks were indexed on a hexagonal unit cell (a = 9.3−9.4 Å and c = 8.8−8.9 Å) with space group P63/m (No. 176). These data could be explained by using a structural model for PbMn2Ni6Te3O18 determined by Wedel et al.9 For PbMn2Ni6Te3O18, the ND measurements were also carried out at 18 and 150 K, as shown in Figure 2. The structural phase transition was not observed in these profiles; that is, this compound maintains its hexagonal structure even at lower temperatures. The profile at 18 K contains some additional reflections due to the magnetic ordering of Mn and Ni ions, which will be discussed later. In the structural model applied for the Rietveld analysis, there are four crystallographic sites occupied by the cations (Pb2+, M2+, Ni2+, and Te6+).9 We have checked the possibility of disordering among them by analyzing B

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Inorganic Chemistry

Magnetic Properties of PbCd2Ni6Te3O18. Figure 4 shows the temperature dependence of magnetic susceptibility (χM) for

Figure 3. Schematic crystal structure of PbM2Ni6Te3O18 (M = Mn and Cd).

sharing and form the double-chain structure extending along the c axis. By linking three double chains, a trigonal tunnel is built; its center is on the 6̅ axis at (1/3, 2/3, z) or (2/3, 1/3, z). The larger M (Mn or Cd) ion occupies the 4f site on this axis and is in a MO6 trigonal prism. Two MO6 prisms form a M2O9 dimeric unit by face-sharing. The largest Pb ion is located at the origin and (0, 0, 1/2) (2b site) of the unit cell, which is surrounded by 12 oxygen ions, forming a distorted icosahedron. For these cation sites, the bond lengths and bond valence sums (BVSs) are calculated from the structural data and listed in Table 2. The BVS values indicate that the charge distribution of these compounds is represented as Pb2+M2+2Ni2+6Te6+3O18. This crystal structure is quite similar to those for the painite CaZrBAl9O1815 and fluoborite Mg3(BO3)3(OH, F)316 (Figure S1). As compared to PbM2Ni6Te3O18, their chemical formulas can be represented as Ca(ZrB)Al6 Al3 O18 and □(B3)Mg6Mg3(O, OH, F)18 (□: vacancy), respectively. The PbM2Ni6Te3O18 can be considered as their derivative if the difference in coordination arrangement of the M site and the vacancy in the Pb site is ignored.

Figure 4. Temperature dependence of the magnetic susceptibility for PbCd2Ni6Te3O18. The red and green lines are the fitting curves by an alternating 1D chain and Curie−Weiss models, respectively.

PbCd2Ni6Te3O18, in which only the Ni2+ ion is magnetic and the other ions are diamagnetic. The contribution to χM from the small impurity phase NiO (antiferromagnet; TN = 525 K) is ignored. The ZFC data above 100 K were fitted by the Curie− Weiss law; the effective magnetic moment (μeff) and Weiss constant (θ) were determined to be 8.03(1) μB and −72.3(8) K, respectively. The μeff per Ni ion (μeff/√6 = 3.28 μB) is somewhat larger than the spin-only value for S = 1 (2.83 μB), but it is within values often observed for the Ni2+ ion in an octahedral crystal field (2.9−3.9 μB). In such cases the g value deviates from the spin-only value by the spin−orbit coupling: g

Table 2. Selected Bond Lengths (Å) and Bond Valence Sums for PbM2Ni6Te3O18 (M = Mn and Cd)a Pb−O1 × 6 Pb−O4 × 6 average Pb−O BVS M−O2 × 3 M−O3 × 3 average M−O BVS Ni−O4 × 1 Ni−O1 × 1 Ni−O2 × 1 Ni−O2′ × 1 Ni−O1′ × 1 Ni−O3 × 1 average Ni−O BVS Te−O4 × 1 Te−O2 × 2 Te−O1 × 2 Te−O3 × 1 average Te−O BVS

PbMn2Ni6Te3O18 ND, 150 K

PbMn2Ni6Te3O18 ND, 18 K

PbMn2Ni6Te3O18 XRD, RT

PbCd2Ni6Te3O18 XRD, RT

2.632(2) 3.232(2) 2.932 1.76 2.141(3) 2.318(3) 2.229 1.88 2.016(2) 2.036(2) 2.064(2) 2.083(2) 2.083(2) 2.114(2) 2.066 1.98 1.899(4) 1.934(2) 1.950(2) 1.982(4) 1.942 5.64

2.631(1) 3.232(2) 2.930 1.77 2.142(2) 2.309(3) 2.225 1.90 2.019(1) 2.041(2) 2.068(1) 2.082(2) 2.083(1) 2.107(1) 2.067 1.97 1.890(4) 1.933(2) 1.946(2) 1.989(4) 1.940 5.66

2.630(10) 3.250(11) 2.941 1.75 2.148(10) 2.350(9) 2.249 1.80 2.062(11) 2.045(10) 2.035(11) 2.069(10) 2.134(10) 2.107(11) 2.075 1.93 1.832(15) 1.925(10) 1.937(10) 1.936(16) 1.915 6.06

2.626(10) 3.253(9) 2.939 1.77 2.201(8) 2.361(8) 2.281 2.05 2.063(9) 2.067(9) 2.087(9) 2.090(10) 2.124(8) 2.102(10) 2.089 1.86 1.862(14) 1.957(9) 1.946(8) 2.014(14) 1.947 5.57

The BVS values were calculated by ∑iexp((l0 − li)/B) where li is bond length, l0 = 2.112 (Pb2+), 1.790 (Mn2+), 1.875(Cd2+), 1.654 (Ni2+), and 1.917 Å (Te6+), and B = 0.37. a

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Inorganic Chemistry = 2(1−4λ/10Dq).17 The negative Weiss constant indicates that the dominant magnetic interaction between Ni2+ ions is antiferromagnetic. The magnetic susceptibility of PbCd2Ni6Te3O18 shows a maximum at 45 K. This anomaly is broad in shape and similar to the feature found in the low-dimensional antiferromagnet rather than the typical antiferromagnetic transition. In the crystal structure of PbCd2Ni6Te3O18, the NiO6 and TeO6 octahedra form the double chain. If the diamagnetic Te6+ ions are ignored, it can be regarded as a zigzag chain consisting of magnetic Ni2+ ions (Figure 5). This chain has two kinds of

magnetization (Figure 6) does not show the feature of a Haldane phase, i.e., the singlet ground state with a spin gap, as

Figure 6. Field dependence of the magnetization for PbCd2Ni6Te3O18.

is observed in the typical Haldane chain compound Ni(C2H8N2)2NO2ClO4 (NENP)6 and PbNi2V2O8.8 The variation of the magnetization increases linearly, and its magnitude is large for the nonmagnetic state. These results indicate that this compound has an antiferromagnetically ordered ground state, and it is expected to be brought about the interchain magnetic interaction (J3). In order to obtain further information about the magnetic ordering of this compound, the temperature dependence of the specific heat (Cp) was measured (Figure 7). A broad anomaly is

Figure 5. Structure of the double chain and the magnetic interaction pathways between Ni ions.

superexchange pathways (Ni−O−Ni) with different Ni−Ni distances and bond angles: 2.886(4) Å and 86.7(3)−88.8(3)° (J1) and 3.151(4) Å and 97.9(3)° (J2). In addition, there exists another interaction pathway between neighboring chains with 3.664(5) Å and 121.9(3)° (J3). Among them, the intrachain interactions (J1 and J2) are expected to be much stronger than the interchain ones (J3), because the former have shorter Ni− Ni distances and bond angles near 90°, which is ideal for the superexchange interaction. Therefore, we have fitted the observed magnetic susceptibility using an alternating 1D chain model by Borrás-Almenar et al.:18 χr = [ATr2 + BTr + C ]/[Tr3 + DTr2 + ETr + F ]

(1)

χM|J1|/[2NAg2μ2B/3],

where χr = Tr = kBT/|J1|, and A−F are second degree polynomials of α (= J2/J1). For the definition of J in this model, the style of the Hamiltonian is H = −J Si Sj, and the values of the coefficients in the polynomials are listed in Table 1 of ref 18. The single-ion anisotropy parameter D cannot be determined by the fitting; thus the equation without the D parameter was used. The calculation curve is shown as a red solid line in Figure 4, and it is in good agreement with the experimental data. The three fitting parameters, J1, J2 (assumed as |J1| > |J2|), and g are determined to be −38(2) K, − 29(3) K, and 2.21(1), respectively. The magnetic interaction between adjacent Ni ions in the chain is antiferromagnetic. These features found in the Ni zigzag chain of PbCd2Ni6Te3O18 are similar to the Haldane chain, an antiferromagnetic 1D chain consisting of integer spins, having a spin gap at low temperatures.1,2 In the case of an alternating chain (0 < α < 1), the ground state of the chain is the Haldane phase (0.6 < α < 1) or the dimer phase (0 < α < 0.6).19 The determined value α = 0.76(12) may mean that this compound belongs to the former. However, the field dependence of the

Figure 7. Temperature dependence of the specific heat (Cp), specific heat divided by temperature (Cp/T), and magnetic entropy (Smag) for PbCd2Ni6Te3O18. Red lines are the lattice specific heat calculated by eq 2. The inset shows the Cp/T vs T curve below 35 K.

observed at lower temperatures, indicating the magnetic transition. This anomaly is well identified in the Cp/T vs T plot in Figure 7 and contains two small peaks at 25.7 and 22.9 K. The magnetic specific heat (Cmag) was estimated by extracting the lattice specific heat (Clat) from Cp. The Clat was calculated from the sum of the Debye and Einstein models:20 D

DOI: 10.1021/acs.inorgchem.5b01619 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry ⎛ T ⎞3 C lat = 9R ⎜ ⎟ ⎝ θD ⎠ 3

+ R∑ i=1

∫0

θD/ T

(θD/T )4 e θD/ T (e θD/ T − 1)2

d(θD/T )

(θEi /T )2 e θ Ei / T (e θ Ei / T − 1)2

(2)

where θD and θEi are the Debye and Einstein temperatures, respectively. The calculated Clat curve is shown as a solid line in Figure 7. The Cmag and magnetic entropy (Smag) were calculated by Smag = ∫ T0 (Cmag/T) dT. The Smag at 100 K is 53.6 J mol−1 K−1, which is close to 6R ln(2S + 1) = 54.8 J mol−1 K−1 expected from the magnetic entropy of the Ni2+ (S = 1) ion. This result indicates that the magnetic moments of Ni2+ ions are fully ordered at sufficiently low temperatures. For two Cp peaks observed at 25.7 and 22.9 K, the former is considered to be the Néel temperature; it corresponds to an inflection point of χM vs T curve (Figure 4). On the other hand, the origin of the latter is unclear at present. We consider that it may be a spin rearrangement due to the competition between two magnetic ordered states in terms of the sign of interchain interaction (J3) and structural features of this compound. If this interaction is antiferromagnetic (J3 < 0), the spin arrangement is the same for all the chains (Figure 8a). Conversely, if it is

Figure 9. Temperature dependence of the magnetic susceptibility for PbMn2Ni6Te3O18. The green line is the fitting curve by the Curie− Weiss model.

predominant magnetic interaction is antiferromagnetic. In fact, the magnetic susceptibility of this compound shows a broad peak at 95 K, which is similar to the result in PbCd2Ni6Te3O18. In the case of PbMn2Ni6Te3O18, the magnetic ions reside not only in the Ni zigzag chain but also in the Mn2O9 unit. There exist additional superexchange Mn−O−Mn and Ni−O−Mn pathways (Figure S3 and Table S3); thus, the alternating chain model cannot be applied. Figure 10 shows the temperature dependence of specific heat. A sharp λ-type anomaly is observed at 86.0 K, which

Figure 8. Spin arrangement models between Ni antiferromagnetic chains in PbCd2Ni6Te3O18 for the interchain interaction (a) J3 < 0 and (b) J3 > 0.

ferromagnetic (J3 > 0), the spin arrangement is opposite between neighboring chains (Figure 8b); in this case, there exists a magnetic frustration (geometric frustration) because of the triangle-based (Kagomé-like) array of the chains (see Figure S2). The existence of magnetic frustration can explain the Cp peak at 22.9 K; that is, it may correspond to the spin rearrangement between the frustrated and nonfrustrated magnetic structures or between different frustrated ones (e.g., partially disordered, modulated, and 120° arrangements often found in the frustrated system of a triangular lattice). Magnetic Properties of PbMn2Ni6Te3O18. The temperature dependence of the magnetic susceptibility of PbMn2Ni6Te3O18 is shown in Figure 9. In this case, both Mn2+ and Ni2+ ions are magnetic. The effective magnetic moment and Weiss constant are 11.56(1) μB and −93.4(10) K, respectively. The former is in good agreement with 11.60 μB calculated from (2 μ2Mn2+ + 6 μ2Ni2+)1/2 in which μMn2+ = 5.92 μB (spin-only value for S = 5/2) and μNi2+ = 3.28 μB (observed value in PbCd2Ni6Te3O18), and the latter indicates that the

Figure 10. Temperature dependence of the specific heat (Cp), specific heat divided by temperature (Cp/T), and magnetic entropy (Smag) for PbMn2Ni6Te3O18. Red lines are the lattice specific heat calculated by eq 2.

indicates that the long-range antiferromagnetic ordering occurs. The magnetic specific heat was estimated in the same way as for PbCd2Ni6Te3O18, and the magnetic entropy due to the observed magnetic transition is calculated to be 82.4 J mol−1 K−1. This value is close to 2R ln(2SMn + 1) + 6R ln(2SNi + 1) = 84.6 J mol−1 K−1; therefore, the magnetic moments of both Mn2+ and Ni2+ ions order antiferromagnetically. The linear M vs H curve (Figure S4) observed below TN also supports this result. In order to clarify the magnetic structure of PbMn2Ni6Te3O18, powder neutron diffraction measurements E

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Figure 11. Magnetic structure of PbMn2Ni6Te3O18: (a) spin arrangements of all magnetic ions; (b) positions of magnetic ions viewed from a different direction.

were performed below and above the magnetic transition temperature. As shown in Figure 2, the magnetic reflection peaks are observed in the ND profile measured at 18 K. The integer indices of magnetic reflections mean that the magnetic unit cell is the same as the crystal unit cell with the propagation vector k = (0, 0, 0). To determine the spin arrangement, the representation analysis was carried out by using the program SARAh.21 The symmetry and directions of ordered magnetic moments compatible with the crystal symmetry (P63/m) were obtained; for the Ni2+ ions (12i site) and Mn2+ ions (4f site), the decomposition of magnetic representation is ΓNi = (1) (1) and ΓMn = ∑12 3∑12 i=1Γi i=1Γi , respectively. The magnetic structure models from these representations are summarized in Table S4. Among them, the best fit was achieved using a model including only the representation Γ2 (the basis vector ψ6 for Ni2+ and ψ2 for Mn2+). The directions of magnetic moments for both ions are collinear along the c axis, which is consistent with the absence of 00l magnetic peaks. The ordered magnetic moments, μMn = 4.71(3) μB and μNi = 1.96(2) μB, indicate that both magnetic moments are fully ordered at 18 K. The magnetic structure of PbMn2Ni6Te3O18 is drawn in Figure 11. The magnetic moments of Ni ions order antiferromagnetically in the zigzag chain, which is in common with the result of PbCd2Ni6Te3O18. Three chains forming a trigonal tunnel (Figure 3) have the same spin arrangement, indicating the magnetically nonfrustrated ground state. The Mn moments in the Mn2O9 unit are coupled antiferromagnetically, and their directions are collinear together with the Ni moments. The spin arrangement between the nearest Mn and Ni ions is ferromagnetic. As compared to the PbCd2Ni6Te3O18, an apparent antiferromagnetic transition without an additional transition and much higher Néel temperature are due to the magnetic interaction between Mn and Ni ions. This interaction fixes the arrangement of magnetic moments of Ni chains and accordingly weakens the magnetic frustration. To elucidate more detailed magnetic behaviors of PbM2Ni6Te3O18-type compounds, further investigations of structural and magnetic properties for new related compounds are needed.

interaction of adjacent Ni ions in this chain is antiferromagnetic, and the characteristic temperature dependence of magnetic susceptibility observed in PbCd2Ni6Te3O18 is well explained by the 1D alternating chain model with J1 = −39 K and J2 = −28 K. The magnetization and specific heat measurements show that the ground state of this compound is not the nonmagnetic Haldane state but a three-dimensional antiferromagnetic ordered state (TN = 25.7 K) due to the interchain magnetic interaction. Another Cp peak was observed at 22.9 K, which may be due to the spin rearrangement caused by the geometric magnetic frustration. On the other hand, PbMn2Ni6Te3O18 shows an antiferromagnetic transition at 86.0 K. From the analysis of ND data, the magnetic structure was determined. It is a collinear arrangement consisting of both Mn2+ and Ni2+ magnetic moments, and these moments are antiferromagnetically ordered in the Mn2O9 dimeric unit and Ni zigzag chain, respectively.

CONCLUSION The pentanary oxides PbM2Ni6Te3O18 (M = Mn and Cd) were synthesized, and their crystal structures were refined by powder XRD and ND measurements. These compounds have a hexagonal structure with space group P63/m, in which the Ni2+ ions form a zigzag chain along the c axis. The magnetic

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b01619. Tables S1−S4 and Figures S1−S5 (PDF) Crystal data (CIF) Crystal data (CIF) Crystal data (CIF) Crystal data (CIF)

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AUTHOR INFORMATION

Corresponding Author

*Tel (Y. Doi): +81-11-706-2715. Fax: +81-11-706-4931. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was partially supported by a Grant-in-Aid for Scientific Research (No. 23750052) from the Ministry of Education, Culture, Sports, Science and Technology of Japan.

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REFERENCES

(1) Haldane, F. D. M. Phys. Lett. A 1983, 93, 464−468. (2) Haldane, F. D. M. Phys. Rev. Lett. 1983, 50, 1153−1156. (3) Yamashita, M.; Ishii, T.; Matsuzaka, H. Coord. Chem. Rev. 2000, 198, 347−366.

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DOI: 10.1021/acs.inorgchem.5b01619 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.5b01619 Inorg. Chem. XXXX, XXX, XXX−XXX