Synthesis, Structure and Properties of a New Molybdate-tellurite Compound with 2D Triangular Lattice①

XIE Y-Xin ZHANG Wn-Wn ZHAO Zhi-Ying HE Zhng-Zhen②

a (State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China) b (University of Chinese Academy of Sciences, Beijing 100049, China)

ABSTRACT A new 3d transition-metal molybdate-tellurite (NH4)2Co(TeMo2O9)2 was obtained through a conventional hydrothermal method. This compound was confirmed to crystallize in the monoclinic system of space group P21/c with a = 10.5133(5), b = 10.6456(5), c = 7.6078(5) Å, β = 108.879(6)°, V = 805.66(8) Å3, Z = 2, Mr = 1021.97, Dc = 4.213 g/cm3, μ = 7.685 mm-1, F(000) = 930, the final R = 0.0399 and wR = 0.1025 for 1580 observed reflections with I ＞ 2σ(I), showing a tunnel structure with spin triangle-lattice composed by CoO6 octahedra. Magnetic and heat capacity measurements confirmed a paramagnetic behavior down to 2 K with a negative Weiss temperature of -20 K, suggesting remarkable spin frustration in the system.

Keyword: molybdate-tellurite, triangular spin-lattice, crystal structure, magnetic properties;

1 INTRODUCTION

Geometrically frustrated magnets have attracted a great deal of interest because of their fascinating magnetic properties. The two-dimensional (2D) triangular lattice antiferromagnet (TLA) is one of the most frustrated spin systems, since the antiferromagnetic (AFM) interactions between the neighboring spins in a triangle cannot be satisfied completely, usually preventing a long-range magnetic order at low temperature and exhibiting various fascinating magnetic properties such as quantum spin liquid[1]and valence-bond condensation[2]. Over the past two decades, some significant TLAs have been synthesized and investigated such as κ-(BEDT-TTF)2Cu2(CN)3[3], EtMe3Sb[Pd(dmit)2]2[4], Cs2CuCl4[5]and YbMgGaO4[6]. However, ideal TLAs with a perfect triangular spin-lattice are still rare and therefore the exploration of new 2D TLAs is a great challenge.

To synthesize new 2D TLAs, one strategy is to employ the anionic groups with stereochemically active lone electron pairs (SeO32-, TeO32-, etc.) that are usually regarded as “chemical scissors” and adopted in the design of low-dimen- sional spin systems[7,8]. Many 2D TLAs with lone-pair anionic groups including HgM(SeO3)2(H2O)2(M = Co, Ni)[9]and Ni5(TeO3)4Br2[10,11]have been studied. On the other hand, the d0metal ion Mo6+usually forms distorted MoO4tetrahedra and MoO6octahedra, where MoO4tetrahedra are usually adopted as linkage groups to weaken the magnetic exchange interactions between spin-layers[12], while MoO6octahedra can polymerize into a variety of Mo-O groups, such as butterfly-like groups[13], hexanuclear-ring groups[14], and so on. In our present work, we try to explore new compounds with a 2D triangular spin lattice by considering the combination of Te-O and Mo-O groups. It is noted that the transition-metal molybdate-tellurite compounds are rather rare, including M(TeMoO6) (M = Mn, Co)[15,16], (M(H2O)6)3(TeMo6O24) (M = Co, Ni)[14], Ni3(MoO4)(TeO3)2[12], Ni3(Mo2O8)(TeO3)[13], and Co3(Mo2O8)(TeO3)[17]. In this paper, we report a new cobalt molybdate-tellurite compound (NH4)2Co(TeMo2O9)2with a 2D spin triangle-lattice and its magnetic properties.

2 EXPERIMENTAL

2. 1 Synthesis of (NH4)2Co(TeMo2O9)2

Single crystals of (NH4)2Co(TeMo2O9)2were obtained by a conventional hydrothermal method. A mixture of 1.5 mmol CoO (AR, 0.1124 g), 0.3 mmol Yb2O3(4 N, 0.1182 g), 0.5 mmol (NH4)6Mo7O24·4H2O (AR, 0.6179 g), 0.5 mmol TeO2(4 N, 0.0798 g), 5 drops NH4·H2O (AR, 25%) and 8 mL deionized water was sealed in an autoclave equipped with a Teflon liner (28 mL). The autoclaves were gradually heated to 230 °C and held for 4 days and then cooled to room temperature at a rate of 4 °C/h. The product contains the desired brown noodle-like crystals with a yield of 50%. The powdered samples for physical measurements were prepared by crushing small single crystals and the purity was confirmed by powder X-ray diffraction analysis (Fig. 1). Furthermore, the Energy Dispersive Spectrometer (EDS) analysis confirmed the molar ratio of Co/Mo/Te with 1.0/4.1/2.0, which is in good agreement with the single-crystal X-ray structure analysis. Moreover, the reagent of Yb2O3plays the role of mineralizer since the quality of crystals is unsatisfactory without it.

2. 2 X-ray crystallography

Single-crystal X-ray diffraction (XRD) measurement for (NH4)2Co(TeMo2O9)2was performed on Rigaku Mercury CCD diffractometer equipped with a graphite-monochromated MoKα radiation (λ = 0.71073 Å) at 293 K. The crystal structure was solved using intrinsic phasing method and refined by Olex-2 software[18]. The final refined structure parameters were checked by the PLATON program[19]. All non-hydrogen atoms were refined with anisotropic thermal parameters. The hydrogen atoms were located at calculated positions and refined with isotropic thermal parameters. The selected bond distances and bond angles of the compound are summarized in Table 1. The hydrogen bond lengths and bond angles are listed in Table 2.

Table 1. Selected Bond Lengths (Å) and Bond Angles (°)

Table 2. Hydrogen Bond Lengths (Å) and Bond Angles (°)

Fig. 1. Simulated (red line) and experimental (black line) powder X-ray (CuKα) diffraction patterns for (NH4)2Co(TeMo2O9)2

2. 3 Magnetic measurements

Powdered sample of (NH4)2Co(TeMo2O9)2was placed in a gel capsule sample holder suspending in a brass tube. Magnetic susceptibilities and heat capacity were measured by PPMS (Quantum Design). Magnetic susceptibilities were measured under an applied field of 0.1 T from 300 to 2 K. Heat capacity was measured at zero field by a relaxation method with the sample of 2.0 × 2.0 × 1.0 mm3(9.8 mg).

2. 4 Thermal analysis

Thermogravimetric analysis (TGA) was measured in a N2atmosphere with a heating rate of 10 °C/min by NETZSCH STA 449C instruments. The sample was placed in Al2O3crucible and heated from room temperature to 1200 °C.

3 RESULTS AND DISCUSSION

3. 1 Crystal structure of (NH4)2Co(TeMo2O9)2

(NH4)2Co(TeMo2O9)2crystallizes in the monoclinic system of space group P21/c. There is one Co, one Te, two Mo (Mo(1), Mo(2)) and one N site in an asymmetrical unit. As shown in Fig. 2, all of the Co and Mo atoms form distorted CoO6and MoO6octahedra. For CoO6octahedra, the Co-O bond lengths in the range of 2.054(3)～2.075(4) Å and all trans. O-Co-O angles are close to the ideal 180°, but the cis angles are in the range of 84.99(2)°～95.01(2)°, leading to a slightly distorted CoO6octahedron. For the MoO6octahedra, the Mo-O bond distances fall in the range of 1.702(4)～2.281(4) Å, indicating the MoO6octahedron has a large distortion, which is very common for molybdate. Te atoms form a trigonal-pyramidal TeO3with Te-O bond lengths ranging from 1.840(4) to 1.940(3) Å. CoO6octahedra are surrounded by four Mo and two Te atoms, while TeO3groups are surrounded by one Co and three Mo atoms. Mo1 atoms connect two Te atoms, while Mo(2) atoms connect one Te and two Co atoms. It is noted that Mo1 atoms have one dangling bond O(8) with a possible O···H-N route, and N atoms are surrounded by four H atoms, forming NH4+groups. The BVS calculations give the rational values of 2.181, 3.793, 6.104 and 6.205 for Co, Te, Mo1 and Mo2, respectively, indicating their oxidation states of +2, +4 and +6.

As shown in Fig. 3, a tunnel structural feature can be seen in the three-dimensional framework of (NH4)2Co(TeMo2O9)2, where NH4+ions are located inside the tunnels. There are [Mo2O10] dimers parallel to the a-axis and the [Mo2O10] dimers connect with each other through TeO3trigonal- pyramids and CoO6octahedra, forming unique motif seen from the ab plane. It is worth noting that CoO6octahedra connect MoO6octahedra via corner-sharing O atoms with Co-O-Mo-O-Co exchange routes, forming a triangular spin-lattice on the bc plane. One of the most remarkable structural features is that [Mo2O10] dimers form a two-leg ladder-chain running along the c-axis (Fig. 3c), in which Mo1O6octahedra connect to each other via corner-sharing O1 atoms and Mo2O6octahedra connect to each other via corner-sharing O(2) atoms, forming so-called legs, while Mo1O6octahedra connect to Mo2O6octahedra via edge- sharing O atoms (O(3)···O(5)), forming so-called rungs. It must be noted that the distances between the nearest and next-nearest neighbored Co atoms in a triangular spin-lattice are 6.542(3) and 7.607(8) Å, confirming it is an isosceles spin triangle.

Fig. 2. Oxygen-coordination environments for (a) Co, (b) Te, (c) Mo1 and (d) Mo2 atoms in (NH4)2Co(TeMo2O9)2. The symmetry transformations are shown under Table 1

Fig. 3. Structure framework of (NH4)2Co(TeMo2O9)2 viewed on the ab (a) and (b) bc planes. (c) MoO6 ladder chain running along the c-axis. (d) Topological arrangement of magnetic Co2+ ions

3. 2 Magnetic properties of (NH4)2Co(TeMo2O9)2

As shown in Fig. 4, the magnetic susceptibility increases with decreasing temperature and not any anomalies are observed down to 2 K. The inverse susceptibility is fitted well by Curie-Weiss law χ = χ0+ C/(T - θ) above 30 K, giving χ0= 1.96(1) × 10-5emu/mol, the Curie constant C = 3.84(4) emu·mol-1·K and the Weiss temperature θ = -20.38(2) K. The effective magnetic moment of Co2+ions is calculated to be 5.54(5) μB, indicating a large orbital moment contribution. The negative Weiss temperature suggests a dominative antiferromagnetic interaction between the neighboring Co2+ions. No bifurcation is found between zero-field-cooled (ZFC) and field-cooled (FC) susceptibilities in the field of 0.1 T (Fig. 5) and no λ-like anomalies can be observed from the heat capacity data down to 2 K with zero applied field (Fig. 6), indicating the absence of long-range magnetic order at low temperature.

Fig. 4. Temperature dependence of the magnetic susceptibility and corresponding reciprocal susceptibility of (NH4)2Co(TeMo2O9)2

Fig. 5. ZFC-FC magnetic susceptibility at a field of 0.1 T of (NH4)2Co(TeMo2O9)2

Fig. 6. Heat capacity data of (NH4)2Co(TeMo2O9)2

It is well-known that magnetic properties are strongly related to the topological spin structures of solid magnets. Removing nonmagnetic MoO66-, TeO32-and NH4+groups from the structure of (NH4)2Co(TeMo2O9)2, the topological arrangement of magnetic Co2+ions is a typical 2D triangular spin-lattice (Fig. 3d). We note the value of frustration factor f =|θ|/TN＞ 10.19 with Weiss temperature θ = -20.38 K and Neel temperature TN＜ 2 K, indicating a remarkable spin frustration in the system[20]. There are two different magnetic exchange interactions (J1and J2) in the isosceles spin-triangle of (NH4)2Co(TeMo2O9)2(Fig. 3d). It must be noted that J1with hydrogen bonding formed by NH4+groups may enhance such exchange interactions, and further the competition between AFM J1and J2leads to a remarkable spin frustration in (NH4)2Co(TeMo2O9)2. The spin frustration with super-super exchange interactions in (NH4)2Co(TeMo2O9)2can also be observed in other 2D triangular spin systems such as BaAg2Cu(VO4)2[21]and Ca3CoNb2O9[22].

3. 3 Thermogravimetric analysis

TGA analysis under N2atmosphere is shown in Fig. 7, indicating that (NH4)2Co(TeMo2O9)2is stable below 360 °C. A rapid weight loss can be observed above 360 °C with a plateau in the temperature range from 460 to 600 °C. The loss of weight from 360 to 460 °C is about 3.91%, which is close to the calculated value of two NH3(3.33%). Furthermore, a remarkable loss about 32.19% of weight can be seen from 600 to 1200 °C, which is close to the calculated value of two TeO2(31.23%). The total weight loss is about 36.2% up to 1200 °C. Since the final residual of (NH4)2Co(TeMo2O9)2has been melted with Al2O3crucible under 1200 °C, the residual is not characterized.

Fig. 7. TGA curve of (NH4)2Co(TeMo2O9)2

4 CONCLUSION

We have successfully obtained a new 3d transition-metal molybdate-tellurite compound (NH4)2Co(TeMo2O9)2through a conventional hydrothermal method. This compound shows a tunnel structure with triangular spin-lattice with two exchange interactions J1and J2. Our experimental results combined from magnetic and heat capacity measurements have confirmed that (NH4)2Co(TeMo2O9)2does not show any magnetic orderings down to 2 K with a negative Weiss temperature of -20.241 K, indicating a remarkable spin frustration in the system.

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