English

• Low-dimensional magnetic materials have been a hot topic in materials chemistry and condensed matter physics due to their reduced dimensionality, which often gives rise to magnetic behaviors that are absent in conventional three-dimensional systems^[1-2]. In 1D spin chains, strong intrachain interactions combined with much weaker interchain coupling enhance quantum fluctuations and suppress the establishment of long-range magnetic order (LRO)^[3-5]. Such characteristics make 1D magnets ideal platforms for investigating unusual magnetic ground states and quantum phenomena^[6]. For example, spinon excitation in spin-1/2 Heisenberg antiferromagnetic chain is observed in classic compounds such as KCuF₃^[7], the Haldane gap in integer-spin antiferromagnetic chains appears in typical S=1 systems such as CsNiCl₃^[8] and Ni(C₂H₈N₂)₂NO₂(ClO₄)^[9], and spin-Peierls transition occurs in a 1D S= 1/2 chain CuGeO₃^[10], while field-induced magnetic phase transitions^[11-13] and pronounced short-range magnetic ordering^[14-15] are also commonly found in many 1D magnetic systems. Compared with transition-metal-based 1D magnets, the rare-earth-based magnetic systems exhibit significantly richer magnetic behaviors owing to the strong spin-orbit coupling, large unquenched orbital angular momentum, and pronounced crystal-field splitting of rare-earth ions^[16-17]. These intrinsic characteristics often result in strong single-ion anisotropy^[18-19], anisotropic exchange interactions^[20-21], and enhanced magnetocaloric effects^[22-23].

  To date, extensive efforts have been devoted to 1D magnetic chains constructed from 3d transition-metal ions, but 1D magnetic chains based on rare-earth ions remain comparatively underexplored^[24]. Nevertheless, rare-earth 1D magnetic systems mainly include oxyborates^[25], hydroxyl-bridged chains^[26], carboxylate-based coordination polymers^[27], halide chains^[28], and so on^[29]. Depending on the bridging mode and chain geometry, these systems exhibit diverse magnetic properties. Among various rare-earth magnetic systems, rare-earth borate compounds have attracted considerable attention due to their rich structural diversity^[30-31]. Recently, the Zou and Chen groups have reported the interesting luminescence properties of Cd₄YbO(BO₃)₃ (1) and Cd₄DyO(BO₃)₃ (2)^[32-33], which have a 1D chain structure. However, their magnetic properties have not yet been investigated. Herein, to further study the magnetic properties of this system, we synthesize 1 and 2 by the high-temperature solid-state method. Magnetic susceptibility results indicate dominant antiferromagnetic exchange interactions and the absence of LRO down to 2 K for 1 and 2.

  1.   Experimental

  1.1   Materials and methods

  Ytterbium oxide (Yb₂O₃, 99.99%, Adamas, Titan Scientific Co., Ltd.), dysprosium oxide (Dy₂O₃, 99.99%, Anhui Senrise Technologies Co., Ltd.), cadmium oxide (CdO, 99.99%, Shanghai Aladdin Biochemical Technology Co., Ltd.), boric acid (H₃BO₃, AR, Energy Chemical, Titan Scientific Co., Ltd.), sodium fluoride (NaF, 99%, Adamas, Titan Scientific Co., Ltd.), sodium tetraborate decahydrate (Na₂B₄O₇·10H₂O, 99%, Bide Pharmatech Co., Ltd.) and anhydrous ethanol (C₂H₅OH, AR, Sinopharm Chemical Reagent Co., Ltd.) were used as received. Powder X-ray diffraction (PXRD) patterns were recorded on a Rigaku SmartLab SE powder X-ray diffractometer using Cu Kα radiation (λ=0.154 059 3 nm) at room temperature in the 2θ range of 10°-70°, in which the X-ray tube was operated at 40 kV and 45 mA. The FTIR spectra were collected on a Bruker INVENIO-R FT-IR Spectrometer in the range of 400-4 000 cm^-1 with the use of a KBr pellet. Thermogravimetric analysis (TGA) was carried out on a Diamond TG-DTA thermal analyzer instrument with a heating rate of 10 ℃·min^-1 in an atmosphere of flowing N₂ in the temperature range from ambient temperature to 1 000 ℃. The UV-Vis-NIR optical diffuse reflectance spectra were measured at room temperature with a UV-2600 Shimadzu spectrometer. A BaSO₄ plate was used as a standard (100% reflectance). The magnetization data were recorded using a superconducting quantum interference device (SQUID). Powder samples 1 and 2 were individually wrapped in a preservative film and placed in a sample holder. Magnetic susceptibilities were measured under an applied field of 1 000 Oe between 2 and 300 K. Magnetization was measured at 2 K in an applied field from -7 to 7 T. The diamagnetic contribution was subtracted by using Pascal′s constants^[34].

  1.2   Synthesis

  White powder samples of 1 and 2 were prepared by the solid-state method. Stoichiometric amounts of CdO (0.256 8 g, 2.00 mmol), Yb₂O₃ (0.098 5 g, 0.25 mmol) or Dy₂O₃ (0.093 2 g, 0.25 mmol), and H₃BO₃ (0.092 7 g, 1.50 mmol) were weighed and ground thoroughly with 3-4 mL of anhydrous ethanol. After the anhydrous ethanol was removed by evaporation in an oven at 60 ℃, the powder sample was transferred to the alumina crucible. Then, the mixtures were heated to 950 ℃ (for 1) or 900 ℃ (for 2), held for 120 h (for 1) or 72 h (for 2), and cooled to ambient temperature at a rate of 1.5 ℃·min^-1.

  For the growth of single crystal samples of 1, CdO (0.719 1 g, 5.60 mmol), Yb₂O₃ (0.275 9 g, 0.70 mmol), H₃BO₃ (0.346 3 g, 5.60 mmol), Na₂B₄O₇·10H₂O (1.067 8 g, 2.80 mmol), and NaF (0.117 6 g, 2.80 mmol) were ground and then loaded into a platinum crucible. The sample was heated to 600 ℃ and kept at this temperature for one day, followed by heating at 950 ℃ for 6 h, then cooled slowly to 700 ℃ at 1.5 ℃·h^-1, to 400 ℃ at 5 ℃·h^-1, and finally rapidly to room temperature at 20 ℃·h^-1. Excess flux was washed away with hot water, and then the transparent prism-like crystals of 1 were obtained.

  1.3   X-ray crystallographic analysis

  Crystallographic data of 1 were collected on a Rigaku XtaLAB mini Ⅱ with Mo Kα radiation (λ=0.071 073 nm) at 293.15 K. The cell refinement, data collection, and reduction were run by the CrysAlisPro software package. The structure was solved by intrinsic phasing using the program SHELXT^[35] and subsequently refined by the full-matrix least squares method on F ² with anisotropic thermal parameters for all non-hydrogen atoms using the program SHELXL^[36] with the Olex2 package^[37]. No higher symmetry was found by using the PLATON program^[38]. The crystallographic data of 1 are shown in Table 1.

  Table 1

  

  Table 1.  Crystal data and structure refinement parameters of compound 1

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  Parameter	1		Parameter	1
  Formula weight	815.07		Dc / (g·cm-3)	6.409
  Temperature / K	293.15		μ / mm-1	20.914
  Crystal system	Monoclinic		F(000)	714
  Space group	Cm		2θ range/(°)	5.166-59.988
  a / nm	0.792 22(4)		Reflection collected	2 656
  b / nm	1.576 85(6)		Goodness-of-fit on F 2	1.095
  c / nm	0.343 46(2)		R1, wR2 [I≥2σ(I)]*	0.033 7, 0.092 4
  β / (°)	100.144(4)		R1, wR2 (all data)	0.033 8, 0.092 5
  V / nm3	0.422 35(4)		Rint	0.038 1
  Z	2		Flack parameter	0.50(3)
  * R1=∑||Fo|-|Fc||/∑|Fo|, wR2={∑[w(Fo2-Fc2)2]/∑w(Fo2)2}1/2.

  2.   Results and discussion

  2.1   Description of crystal structure

  The obtained structural parameters are consistent with those reported previously^[32]. Because 1 and 2 are isostructural, only the crystal structure of 1 is described below. 1 crystallizes in the monoclinic space group Cm, and in its asymmetric unit (Fig. 1b), there are two Cd²⁺ ions, one Yb³⁺ ion, six O atoms, and two B atoms. As shown in Fig. 1a, 1 consists of isolated 1D Yb³⁺-based chains along the c-axis separated by nonmagnetic Cd²⁺ ions and BO₃^3- groups, which are nearly parallel to the ab plane. The B—O distances within the BO₃^3- groups are in the range of 0.133-0.143 nm, and the Cd²⁺ centers are coordinated by oxygen atoms with Cd—O bond lengths of 0.223-0.240 nm. Fig. 1c clearly illustrates that the adjacent Yb³⁺ centers are linked through two bridging oxygen atoms from BO₃^3- and [CdO₆] groups to form an infinite linear chain. The Yb³⁺ ions form a uniform magnetic chain along the c-axis, and the nearest-neighbor Yb…Yb distance within the chain is 0.343 nm (Fig. 1c). In the ab plane, the nearest interchain Yb…Yb separation is 0.792 nm, which is significantly longer than the intrachain Yb…Yb distance, suggesting that the magnetic exchange is predominantly confined along the chain direction. Moreover, as shown in Fig. 1d, each Yb³⁺ ion is coordinated by oxygen atoms in a distorted polyhedral environment, with Yb—O bond lengths ranging from 0.223 to 0.233 nm. The Yb—O—Yb bond angles are 96.2° and 100.4°.

  Figure 1

  

  Figure 1.  (a) Crystal structure of compound 1 along the c-axis; (b) Asymmetric unit of 1 with 80% probability displacement ellipsoids; (c) Yb³⁺-based chain; (d) Bond lengths of Yb—O and bond angles of Yb—O—Yb

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  2.2   PXRD analysis

  The phase purities of 1 and 2 were confirmed by the PXRD measurement (Fig. 2). The peak positions of 1 and 2 were consistent with the simulated patterns of 1, indicating that they are pure phases.

  Figure 2

  

  Figure 2.  PXRD patterns of compounds 1 and 2

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  2.3   Thermal stability analysis

  To investigate the thermal stabilities of 1 and 2, thermogravimetric measurements were performed. It can be found that the TGA curves of 1 and 2 were similar. The TGA results show that 1 and 2 had thermal stability up to 930 ℃ (for 1) and 900 ℃ (for 2) (Fig. 3). These decomposition temperatures were higher than those of other borates^[39-40], indicating that the compounds have good thermal stability and a wide range of applicable temperatures.

  Figure 3

  

  Figure 3.  TGA curves of compounds 1 and 2

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  2.4   FTIR spectra

  The FTIR spectra of 1 and 2 are shown in Fig. 4. The strong absorption bands located at ca. 1 245 cm^-1 and ca. 1 182 cm^-1 are assigned to the asymmetric stretching vibrations of the trigonal planar BO₃^3- units^[41]. The band at ca. 719 cm^-1 can be attributed to the in-plane bending vibration of the B—O bonds within the BO₃ groups^[41]. Additional peaks at ca. 597 cm^-1 and ca. 493 cm^-1 are associated with out-of-plane bending modes of the borate units^[41]. A broad band centered at around 3 434-3 435 cm^-1 was observed in both samples, which is attributed to the O—H stretching vibration from water molecules absorbed on the sample surface from air^[42].

  Figure 4

  

  Figure 4.  FTIR spectra of compounds 1 and 2

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  2.5   Optical band gap analysis

  The optical band gap analysis based on the UV-Vis-NIR diffuse reflectance data (Fig. 5) reveals that 1 and 2 are wide-band-gap semiconductors with optical band gaps of 3.74 and 3.57 eV, respectively. These values were close to those of Cd₄YO(BO₃)₃ (3.78 eV)^[43], Cd₄GdO(BO₃)₃ (3.80 eV)^[43], and Cd₄LuO(BO₃)₃ (3.70 eV)^[43].

  Figure 5

  

  Figure 5.  Tauc plots for direct optical band gap estimation of compounds 1 and 2 based on the UV-Vis-NIR diffuse reflectance data

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  2.6   Magnetic properties

  Fig. 6a and 7a show the temperature-dependent magnetic susceptibilities of compounds 1 and 2, measured at 1 000 Oe between 2 and 300 K. Both magnetic susceptibilities increased monotonically, with no divergence between zero-field cooling (ZFC) and field-cooling (FC) curves, ruling out spin-canting behavior as the temperature decreased. No anomaly for LRO was observed down to 2 K. Considering that crystal-field effects can influence the temperature dependence of magnetic susceptibility in Ln³⁺ compounds, Curie-Weiss fittings were performed in different temperature ranges to evaluate the magnetic behavior.

  Figure 6

  

  Figure 6.  (a) Temperature-dependent magnetic susceptibilities of compound 1 measured at 0.1 T; (b) χ_m^-1-T plot (blue) and χ_mT-T plot (green) of 1; (c) Magnetic field-dependent magnetization of 1 measured at 2 K

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  Figure 7

  

  Figure 7.  (a) Temperature-dependent magnetic susceptibilities of compound 2 measured at 0.1 T; (b) χ_m^-1-T plot (blue) and χ_mT-T plot (green) of 2; (c) Magnetic field-dependent magnetization of 2 measured at 2 K

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  As shown in Fig. 6b, a high-temperature Curie-Weiss fit (150-300 K) applied to inverse susceptibility χ_m^-1 resulted in values of C=2.13 cm³·mol^-1·K and μ_eff(HT)=4.13μ_B (per mole of Yb³⁺, the same hereinafter), which are in good agreement with the expected value of 4.53μ_B (g=8/7, J=7/2). The Weiss temperature θ_HT was found to be -37.80 K, suggesting that the dominant exchange interaction in 1 is antiferromagnetic. In contrast, a low-temperature Curie-Weiss fit (2-15 K) resulted in values of C=1.45 cm³·mol^-1·K, μ_eff(LT)=3.41μ_B, and θ_LT=-2.34 K. The difference between the Curie-Weiss parameters may be related to the crystal-field splitting of Yb³⁺ ions and the associated thermal depopulation of excited Stark levels upon cooling. The χ_mT value at room temperature was 1.9 cm³·mol^-1·K and gradually decreased upon cooling. This behavior is commonly observed in anisotropic Yb³⁺ compounds and may be related to crystal-field effects and the progressive depopulation of excited Stark levels. Compared with related six-coordinate mononuclear Yb³⁺ compounds^[44-45], the lower χ_mT value of 1 supports the presence of weak antiferromagnetic correlations between Yb³⁺ ions. Fig. 6c shows the isothermal magnetization curve of 1 at 2 K. Below 1 T, the magnetization increased linearly with the increase of the magnetic field, and when the field reached 7 T, the magnetization of 1 was 1.8μ_B without reaching full saturation, compared to the theoretical saturation magnetization of 4.0μ_B per mole of Yb³⁺. The non-saturating magnetisation behaviour may be attributed to anisotropic effective moments and crystal-field effects of the Yb³⁺ ions. At the same time, there was no hysteresis, indicating the absence of significant ferromagnetic order or magnetic hysteresis behavior in 1.

  As shown in Fig. 7a, a high-temperature Curie-Weiss fit (150-300 K) applied to inverse susceptibility χ_m^-1 resulted in values of C=14.91 cm³·mol^-1·K and μ_eff(HT)=10.92μ_B (per mole of Dy³⁺, the same hereinafter), which is in good agreement with the expected value of 10.65μ_B (g=4/3, J=15/2) for free Dy³⁺. The Weiss temperature θ_HT was found to be -11.46 K, suggesting that the dominant exchange interaction in 2 is antiferromagnetic. In contrast, a low-temperature Curie-Weiss fit (2-15 K) resulted in values of C=12.70 cm³·mol^-1·K, μ_eff=10.08μ_B, and θ_LT=-2.93 K, and the pronounced reduction of the effective moment upon cooling may be related to the progressive depopulation of excited Stark levels arising from crystal-field splitting. The χ_mT value at room temperature was 14.35 cm³·mol^-1·K and then decreased upon cooling. This behavior may be associated with the strong crystal-field effects of Dy³⁺ ions. Compared with related six-coordinate mononuclear Dy³⁺ compounds^[46-47], the reduced low-temperature χ_mT value of 2 supports the presence of weak antiferromagnetic correlations between Dy³⁺ ions. Fig. 7c shows the field-dependent magnetization of 2 at 2 K. The M-B plot initially increased linearly with the increase in field strength in the range of 0-2 T and then gradually approached saturation upon further increasing the magnetic field. At 7 T, the magnetization reached 6.04μ_B. The theoretical saturation magnetization per mole of Dy³⁺ is 10μ_B, and the measured magnetization is approximately half of the theoretical saturation magnetization, which can be attributed to the strong magnetic anisotropy of Dy³⁺ ions and the powder-averaging effect. This phenomenon also occurs in Dy(BaBO₃)₃^[48], KBaDy(BO₃)₂^[49], Dy₂Be₂GeO₇^[50], and so on^[51-52]. At the same time, similar to 1, no hysteresis was observed on the curve, indicating the absence of significant ferromagnetic order or magnetic hysteresis behavior in 2.

  3.   Conclusions

  In summary, two isostructural 1D chain rare-earth borate compounds Cd₄LnO(BO₃)₃ [Ln=Yb (1), Dy (2)] were successfully synthesized via the high-temperature solid-state method. Magnetic data analysis reveals that antiferromagnetic interactions dominate in both compounds, and no long-range magnetic order occurs down to 2 K. These Ln-based magnetic compounds with a 1D structure provide a platform for exploring more physical phenomena in rare-earth-based low-dimensional materials.

  
  Acknowledgments: This work is supported by the National Natural Science Foundation of China (Grants No.22271107, 22422111) and the National Key Research and Development Program (Grant No.2023YFA1406500).
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  Figure 1  (a) Crystal structure of compound 1 along the c-axis; (b) Asymmetric unit of 1 with 80% probability displacement ellipsoids; (c) Yb³⁺-based chain; (d) Bond lengths of Yb—O and bond angles of Yb—O—Yb

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  Figure 2  PXRD patterns of compounds 1 and 2

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  Figure 3  TGA curves of compounds 1 and 2

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  Figure 4  FTIR spectra of compounds 1 and 2

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  Figure 5  Tauc plots for direct optical band gap estimation of compounds 1 and 2 based on the UV-Vis-NIR diffuse reflectance data

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  Figure 6  (a) Temperature-dependent magnetic susceptibilities of compound 1 measured at 0.1 T; (b) χ_m^-1-T plot (blue) and χ_mT-T plot (green) of 1; (c) Magnetic field-dependent magnetization of 1 measured at 2 K

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  Figure 7  (a) Temperature-dependent magnetic susceptibilities of compound 2 measured at 0.1 T; (b) χ_m^-1-T plot (blue) and χ_mT-T plot (green) of 2; (c) Magnetic field-dependent magnetization of 2 measured at 2 K

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  Table 1.  Crystal data and structure refinement parameters of compound 1

  Parameter	1		Parameter	1
  Formula weight	815.07		Dc / (g·cm-3)	6.409
  Temperature / K	293.15		μ / mm-1	20.914
  Crystal system	Monoclinic		F(000)	714
  Space group	Cm		2θ range/(°)	5.166-59.988
  a / nm	0.792 22(4)		Reflection collected	2 656
  b / nm	1.576 85(6)		Goodness-of-fit on F 2	1.095
  c / nm	0.343 46(2)		R1, wR2 [I≥2σ(I)]*	0.033 7, 0.092 4
  β / (°)	100.144(4)		R1, wR2 (all data)	0.033 8, 0.092 5
  V / nm3	0.422 35(4)		Rint	0.038 1
  Z	2		Flack parameter	0.50(3)
  * R1=∑||Fo|-|Fc||/∑|Fo|, wR2={∑[w(Fo2-Fc2)2]/∑w(Fo2)2}1/2.

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