English

• 1. INTRODUCTION

  Selenite compounds have attracted much attention due to their rich structural chemistry and interesting magnetic properties. Usually, selenite groups can exist in two forms of SeO₃^2- and Se₂O₅^2-, in which the lone-pair electrons of Se⁴⁺ shift the coordinated oxygen atoms towards one side, leading to "chemical scissors" for the linkages of transition-metal polyhedral units that give rise to a variety of frameworks^{[1, 2]}. In M–Se–O systems, there are a series of MSeO₃ and MSe₂O₅ (M = Cu²⁺, Ni²⁺, Co²⁺, Mn²⁺) compounds^[3-8]. Among these compounds, Cu₂OSeO₃^[9] is one of the most representative transition-metal based selenite compounds which is composed of Cu²⁺ ions and SeO₃^2- groups with a cubic space-group P2₁3, showing an unusual ferrimagnetic ordering and magnetoelectric effect at low temperature^[10]. Furthermore, this compound has also been found to exhibit magnetoelectric skyrmions that can magnetically induce electric polarization^[11].

  Recently, we have successfully synthesized two new mercury selenite compounds of HgM(SeO₃)₂(H₂O)₂ (M = Co, Ni)^[12], in which both two compounds crystallize in monoclinic structure with space group C2/c, exhibiting quite interesting magnetic properties. HgCo(SeO₃)₂(H₂O)₂ undergoes a canted antiferromagnetic order at T_N = 7.6 K, showing two successive magnetic transitions under the application of a magnetic field, while HgNi(SeO₃)₂(H₂O)₂ exhibits a collinear antiferromagnetic order below T_N = 9.0 K with only one field-induced magnetic transition at low temperature. Here, we report on a new selenite compound CdNi(SeO₃)₂(H₂O)₂, which is isostructural to HgM(SeO₃)₂(H₂O)₂ (M = Co, Ni) series.

  2. EXPERIMENTAL

  2.1 Synthesis of CdNi(SeO₃)₂(H₂O)₂

  Single crystal of CdNi(SeO₃)₂(H₂O)₂ was obtained by a conventional hydrothermal method. The mixture of 0.6 mmol NiO (98%, 0.05 g), 0.6 mmol CdO (99%, 0.08 g), 2.0 mmol of SeO₂ (99%, 0.22 g), and 6 mL of deionized water was sealed in autoclaves equipped with a Teflon liner (28 mL) and the solution was adjusted by sulfuric acid with the pH value of 2. The autoclaves were gradually heated to 463 K and held for 4 days, and then cooled to room temperature at a rate of 4 K·h^-1 for 4 days. Finally, the green rhombic-like crystals were selected by manual under a microscope and washed several times using deionized water and ethanol (99%), and further dried at 353 K for 2 hours. The energy dispersive spectrometer (EDS) analysis confirmed the molar ratio of Ni/Cd/Se with 1.0:0.9:1.9, which is in good agreement with the single-crystal X-ray structure analysis. The powdered samples for physical measurements were prepared by crushing small single crystals and the phase purity was confirmed by powder X-ray diffraction analysis (Fig. 1).

  Figure 1

  

  Figure 1.  Simulated (black line) and experimental (red line) powder X-ray (Cu-Kα) diffraction patterns from 5~85° for CdNi(SeO₃)₂(H₂O)₂

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  2.2 X-ray crystallographic studies

  The small crystals of CdNi(SeO₃)₂(H₂O)₂ (0.1mm × 0.1mm × 0.05mm) were selected and mounted on glassy fibers for single-crystal X-ray diffraction (XRD) measurements. Data collection was performed on a Rigaku Mercury CCD diffrac-tometer equipped with a graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) at 293 K. The data sets were corrected for Lorentz and polarization factors as well as for the absorption by Multi-scan method^[13]. The structure was solved by direct methods and refined by full-matrix least-squares fitting on F² by SHELX-97^[14]. 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 final refined structural parameters were checked by the PLATON program^[15]. The final refined atomic positions and structural parameters are seen in the Supporting Information (Tables S1 and S2).

  2.3 Magnetic measurements

  Magnetic measurements were performed using a commercial Quantum Design Physical Property Measurement System (PPMS). Magnetic susceptibility was measured at 0.1 T from 300 to 2 K and magnetization was measured at 2 K in applied field from 0 to 8 T.

  2.4 Thermal analysis

  Thermogravimetric analyses (TGA) were performed in a NETZSCH STA 449C instrument in nitrogen atmosphere at a heating rate of 10 K/min. The samples were placed in Al₂O₃ crucibles and heated from room temperature to 823 K.

  3. RESULTS AND DISCUSSION

  3.1 Crystal structure

  CdNi(SeO₃)₂(H₂O)₂ crystallizes in monoclinic C2/c space group with a = 12.376(6), b = 7.662(3), c = 9.282(4) Å and β = 123.679(6)º. There is one crystallographic Cd, one Ni and one Se site in an asymmetrical unit. As shown in Fig. 2a~2c, all of Ni atoms are equivalent, which are surrounded by six oxygen atoms, forming a slightly distorted NiO₆ octahedron with bond lengths ranging from 2.0530(1) to 2.0699(1) Å. Se atoms are in a trigonal pyramidal geometry with Se–O bond lengths from 1.6825(1) to 1.7093(1) Å (Table 1), while Cd atoms are coordinated with six oxygen atoms with Cd–O distances ranging from 2.239 to 2.602 Å. BVS calculations gave the total bond valences of 1.99, 1.91 and 4.21 for Ni, Cd, and Se, indicating the oxidation states of Ni, Cd, and Se atoms with +2, +2, and +4, respectively. CdNi(SeO₃)₂(H₂O)₂ shows a layer structure along the a-axis (Fig. 2d), in which the layers are constructed by NiO₆ octahedra and SeO₃ trigonal pyramids, and Cd²⁺ cations are located inside between the layers. The shortest distance between the neighboring layers is 5.289 Å. To check the linkage of polyhedra in the layers (Fig. 2e), it is noted that each NiO₆ octahedron is surrounded by four SeO₃ via corner-sharing O (O(1) or O(2)), thus forming eight-ring holes with the Ni–Se–Ni–Se–Ni–Se–Ni–Se manners.

  Figure 2

  

  Figure 2.  View of the oxygen-coordination environments for (a) Cd, (b) Ni, (c) Se and crystal structure of CdNi(SeO₃)₂(H₂O)_2. (d) 3D framework viewed along the b direction. (e) 2D layer projected in the bc plane

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  Table 1

  

  Table 1.  Selected Bond Lengths (Å) and Bond Angles (°) of CdNi(SeO₃)₂(H₂O)₂

  DownLoad: CSV

  Bond	Dist.		Bond	Dist.
  Se(1)–O(2)	1.6825(2)		Se(1)–O(3)	1.6980(2)
  Se(1)–O(1)	1.7093(2)		Ni(1)–O(1)#4	2.0530(2)
  Ni(1)–O(1)	2.0530(2)		Ni(1)–O(2)#5	2.0643(2)
  Ni(1)–O(2)#6	2.0643(2)		Ni(1)–O(4)#4	2.0699(2)
  Ni(1)–O(4)	2.0699(2)
  Angle	(°)		Angle	(°)
  O(2)–Se(1)–O(1)	100.25(9)		O(3)–Se(1)–O(1)	101.53(9)
  O(2)–Se(1)–O(3)	97.59(9)		O(1)#4–Ni(1)–O(2)#5	93.42(8)
  O(1)–Ni(1)–O(2)#5	86.58(8)		O(1)#4–Ni(1)–O(2)#6	86.58(8)
  O(1)–Ni(1)–O(2)#6	93.42(8)		O(2)#5–Ni(1)–O(2)#6	180.00
  O(1)#4–Ni(1)–O(4)#4	91.39(7)		O(1)–Ni(1)–O(4)#4	88.61(7)
  O(2)#5–Ni(1)–O(4)#4	83.04(7)		O(2)#6–Ni(1)–O(4)#4	96.96(7)
  O(1)#4–Ni(1)–O(4)	88.61(7)		O(1)–Ni(1)–O(4)	91.39(7)
  O(2)#5–Ni(1)–O(4)	96.96(7)		O(2)#6–Ni(1)–O(4)	83.04(7)
  O(4)#4–Ni(1)–O(4)	180.00		O(1)#4–Ni(1)–O(1)	180.0(8)
  Symmetry transformations used to generate the equivalent atoms: #1: –x, –y, –z+1; #2: x, y+1, z; #3: –x+1, y+1/2, –z+3/2; #4: –x, y+1/2, –z+3/2; #5: –x, y–1/2, –z+3/2; #6: x–1, y, z

  Compared with isomorphic compound HgCo(SeO₃)₂(H₂O)₂, Se−O bond lengths and O–Se–O angles are similar in CdNi(SeO₃)₂(H₂O)₂, while the Hg–O bond length (ranging from 2.263 to 2.626 Å) is longer than the Cd–O bond length, which leads to larger distance between the neighboring layers (5.34 Å). Besides, in terms of the intraplane exchange couplings through Co−O−Se−O−Co and Ni−O−Se−O−Ni pathways, the Co−O−Se angles (115.0 and 132.3°) are larger than Ni−O−Se (114.8 and 130.1°), which also brings about larger Co–Co (6.05 Å) distance between the nearest neighboring than Ni–Ni (6.01 Å) in the bc plane.

  3.2 Magnetic properties

  As shown in Fig. 3, the magnetic susceptibility increases with decreasing the temperature, and a peak is observed at 15 K, indicating the onset of antiferromagnetic (AFM) ordering. The inverse susceptibility is fitted well by Curie-Weiss law χ = C/(T – θ) above 50 K, giving the Curie constant C = 1.215 emu·mol^-1·K and the Weiss temperature θ = –16.4 K. The effective magnetic moment of Ni²⁺ ions is calculated to be 3.118(1) μ_B, which is larger than the theoretical spin-only value of 2.828 μ_B for Ni²⁺ (S = 1) ions, indicating a large orbital moment contribution of Ni²⁺ ions in such an oxygen octahedral environment. The negative θ suggests that the dominant interactions between magnetic Ni²⁺ ions are AFM. Fig. 4 shows the magnetization (M) as a function of applied field (H) at 2 K. The magnetization increases linearly with the increasing field and does not saturate at 8 T. Furthermore, no hysteresis and remanent magnetization are observed. These features also support an AFM ordering at low temperature.

  Figure 3

  

  Figure 3.  Temperature dependencies of the magnetic susceptibility (χ) and corresponding reciprocal value (χ⁻¹) for CdNi(SeO₃)₂(H₂O)₂. The red solid line is a fit to the Curie-Weiss law

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  Figure 4

  

  Figure 4.  Field dependence of magnetization measured at 2 K

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  3.3 Thermal analysis

  To investigate the thermal stability of CdNi(SeO₃)₂(H₂O)₂, the sample was heated up to 823 K under a nitrogen atmosphere in Fig. 5. It is noted that the compound is stable with increasing the temperature up to 523 K, while the onset of weight loss is observed. The clear loss in the weight of sample starts at 623 K with a rapid drop, while a plateau is seen in the temperature range of 673~773 K. This step may be attributed to the loss of H₂O molecules, since the observed weight loss of 8.0% for the plateau is close to the calculated of two H₂O in CdNi(SeO₃)₂(H₂O)₂, 7.8%.

  Figure 5

  

  Figure 5.  Thermogravimetric curves for CdNi(SeO₃)₂(H₂O)₂

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  4. CONCLUSION

  A new selenite compound CdNi(SeO₃)₂(H₂O)₂ has been obtained by a hydrothermal method. This compound was found to crystallize in the monoclinic system of space group C2/c, which exhibits a layer structure running along the a-axis. The layers show an eight-rings hole network. The compound is stable at room temperature while the loss of H₂O molecules happens with heating above 523 K. Magnetic measurements conformed that CdNi(SeO₃)₂(H₂O)₂ is an antiferromagnet and possesses an antiferromagnetic ordering at T_N = 15 K.

  
  
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• 

  Figure 1  Simulated (black line) and experimental (red line) powder X-ray (Cu-Kα) diffraction patterns from 5~85° for CdNi(SeO₃)₂(H₂O)₂

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  Figure 2  View of the oxygen-coordination environments for (a) Cd, (b) Ni, (c) Se and crystal structure of CdNi(SeO₃)₂(H₂O)_2. (d) 3D framework viewed along the b direction. (e) 2D layer projected in the bc plane

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  Figure 3  Temperature dependencies of the magnetic susceptibility (χ) and corresponding reciprocal value (χ⁻¹) for CdNi(SeO₃)₂(H₂O)₂. The red solid line is a fit to the Curie-Weiss law

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  Figure 4  Field dependence of magnetization measured at 2 K

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  Figure 5  Thermogravimetric curves for CdNi(SeO₃)₂(H₂O)₂

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  Table 1.  Selected Bond Lengths (Å) and Bond Angles (°) of CdNi(SeO₃)₂(H₂O)₂

  Bond	Dist.		Bond	Dist.
  Se(1)–O(2)	1.6825(2)		Se(1)–O(3)	1.6980(2)
  Se(1)–O(1)	1.7093(2)		Ni(1)–O(1)#4	2.0530(2)
  Ni(1)–O(1)	2.0530(2)		Ni(1)–O(2)#5	2.0643(2)
  Ni(1)–O(2)#6	2.0643(2)		Ni(1)–O(4)#4	2.0699(2)
  Ni(1)–O(4)	2.0699(2)
  Angle	(°)		Angle	(°)
  O(2)–Se(1)–O(1)	100.25(9)		O(3)–Se(1)–O(1)	101.53(9)
  O(2)–Se(1)–O(3)	97.59(9)		O(1)#4–Ni(1)–O(2)#5	93.42(8)
  O(1)–Ni(1)–O(2)#5	86.58(8)		O(1)#4–Ni(1)–O(2)#6	86.58(8)
  O(1)–Ni(1)–O(2)#6	93.42(8)		O(2)#5–Ni(1)–O(2)#6	180.00
  O(1)#4–Ni(1)–O(4)#4	91.39(7)		O(1)–Ni(1)–O(4)#4	88.61(7)
  O(2)#5–Ni(1)–O(4)#4	83.04(7)		O(2)#6–Ni(1)–O(4)#4	96.96(7)
  O(1)#4–Ni(1)–O(4)	88.61(7)		O(1)–Ni(1)–O(4)	91.39(7)
  O(2)#5–Ni(1)–O(4)	96.96(7)		O(2)#6–Ni(1)–O(4)	83.04(7)
  O(4)#4–Ni(1)–O(4)	180.00		O(1)#4–Ni(1)–O(1)	180.0(8)
  Symmetry transformations used to generate the equivalent atoms: #1: –x, –y, –z+1; #2: x, y+1, z; #3: –x+1, y+1/2, –z+3/2; #4: –x, y+1/2, –z+3/2; #5: –x, y–1/2, –z+3/2; #6: x–1, y, z

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