Source: https://pubs.rsc.org/en/content/articlehtml/2018/qm/c8qm00439k
Timestamp: 2019-04-21 02:55:16+00:00

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Paralleling the extensively studied organic ligands, the light inorganic ligands are also promising candidates for constructing Gd-based magnetic cryocoolants bearing large MCE in gravimetric units (J kg−1 K−1) and volumetric units (mJ cm−3 K−1) thanks to the weak exchange interactions and dense frame in the resulting compounds.8,9 The shielded 4f orbitals of GdIII make the GdIII–GdIII magnetic coupling usually weak, even if the ligand is a bridging-oxygen atom, which is well corroborated in Gd(OH)3 (−ΔSmaxm = 62.0 kg−1 K−1 or 346 mJ cm−3 K−1 with ΔH = 7 T) and Gd2O(OH)4(H2O)2 (−ΔSmaxm = 59.1 J kg−1 K−1 or 217 mJ cm−3 K−1 with ΔH = 7 T).8a Furthermore, the inorganic Gd-based magnetic coolants usually display good chemical and thermal stability.8b,c The effectiveness and efficiency of inorganic ligands to fabricate ideal Gd-based cryogenic magnetic coolers has been well testified in publications.8,9 Representative examples include the early investigated Gd2(SO4)3·8H2O,9a,b recently reported GdF3 (−ΔSmaxm = 71.6 J kg−1 K−1 or 506 mJ cm−3 K−1 with ΔH = 7 T),9c GdPO4 (−ΔSmaxm = 62.0 J kg−1 K−1 or 376 mJ cm−3 K−1 with ΔH = 7 T),9d and K3Li3Gd7(BO3)9 (−ΔSmaxm = 56.6 J kg−1 K−1 or 278 mJ cm−3 K−1 with ΔH = 7 T).9e Different from the Gd-based molecular magnetic coolers supported by organic ligands, their counterparts like Gd2(SO4)3·8H2O, Gd(OH)3, GdF3 and GdPO4 have usually been known for many years, which gives us a hint that utilising known results is also an avenue to explore brilliant molecular cryogenic magnetic coolers under the guidance of molecular design strategies.
Herein, we report the structure, magnetic susceptibility and isothermal magnetization of an inorganic frame material with the formula of Gd(OH)SO4. This material features weak magnetic exchange, low molecular weight (270.32 g mol−1) and dense frame structure (5.163 g cm−3), which is helpful for boosting the maximum entropy change (−ΔSmaxm). The −ΔSmaxm derived from isothermal magnetization is up to 53.5 J kg−1 K−1 or 276 mJ cm−3 K−1, comparable to the performance of commercial gadolinium gallium garnet Gd3Ga5O12 (GGG, −ΔSmaxm = 38.4 J kg−1 K−1 or 272 mJ cm−3 K−1 with ΔH = 7 T) and its derivative Gd3(Ga1−xFex)5O12 (GGIG).10 It is notable that the −ΔSmaxm of 1 still reaches 27.5 J kg−1 K−1 (T = 2 K and ΔH = 2 T) and 38.3 J kg−1 K−1 (T = 2 K and ΔH = 3 T) even in low fields, which already surpasses GGG (about 24 J kg−1 K−1 with ΔH = 3 T). All these magnetic characteristics make 1 an outstanding cryogenic magnetic cooler.
All chemicals were analytical grade and used directly. IR data were collected on a MAGNA-560 (Nicolet) FT-IR spectrometer with KBr as pellets. Experimental powder X-ray diffraction (PXRD) data were collected on a Bruker D8 FOCUS diffractometer with a Cu-target tube and a graphite monochromator. Simulated PXRD data were obtained from the single crystal X-ray diffraction (SCXRD) data via the Mercury program. Thermogravimetric (TG) data were collected on a Rigaku Thermo plus EVO2 TG-DTA8121 analyzer. The magnetic data were collected using a MPMS XL-5 SQUID magnetometer. The diamagnetic corrections were finished with Pascal's constants.
Synthesis of Gd(OH)SO4 (1). 1 could be prepared according to the literature procedure.111 could also be synthesized by the following method. A mixture of Gd2(SO4)3·8H2O (0.075 g, 0.10 mmol), H2C2O4·2H2O (0.189 g, 1.50 mmol), urea (0.015 g, 0.25 mmol) and H2O (10 mL) was sealed in a Teflon-lined autoclave (20 mL) and heated to 180 °C for 7 days and then slowly cooled to ambient temperature. Yield: ca. 15% based on H2C2O4·2H2O. IR (cm−1): 3480(s), 3425(s), 2970(m), 2917(w), 1621(s), 1382(w), 1172(s), 1087(s), 990(s), 867(s), 773(s), and 600(s).
The SCXRD data were collected on a XtaLAB-mini diffractometer at 293(2) K with Mo-Kα radiation (λ = 0.71073 Å) by ω scan mode. SHELX-2016 software was used to solve the frame of 1.12 Detailed crystallographic data are presented in Table 1 and the selected bond lengths and angles are given in Table S1 (ESI†). Further details on the structural investigation is available from the Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: (+49) 7247-808-666; e-mail: crysdata@fiz-karlsruhe.de), on quoting the depository number CSD-261284 (Gd(OH)SO4).
Because the structure of 1 has been reported,11 the structural delineations are introduced briefly. SCXRD analysis indicates that compound 1 crystallizes in the monoclinic space group of P21/n. The asymmetric unit of 1 has one crystallographically independent Gd atom, one sulfate (SO42−), and one hydroxy anion (OH−). As displayed in Fig. 1a, all Gd atoms are in the [GdO9] capped square antiprism geometry. The SO42− groups have the Harris notation of [6.2211] to link six Gd atoms,13 while the OH− moieties exhibit μ3 mode to link three Gd atoms. The presence of sulfate and OH− moieties in the frame has also been confirmed by their corresponding characteristic IR peaks (3480, 867, 773 cm−1 for OH− and 1172 cm−1 for sulfate) (Fig. S1, ESI†). The μ3-OH− together with the η2-O-atom from sulfate connect symmetry-related Gd-ions to form the 1D Gd-oxygen chain (Fig. 1b), which is further connected to the other η2-O-atom of sulfate to generate the 2D layer (Fig. 1c). The adjacent 2D layers connect with each other via the η1-O-atom of sulfate to give rise to the resultant 3D dense framework (Fig. 1d).
Fig. 1 (a) The coordination environments of sulfate and Gd atoms; (b) the 1D Gd-oxygen chain; (c) the 1D Gd-oxygen chain connected by sulfate to form the 2D layer; (d) the 2D layer linked by an additional O-atom of sulfate to generate the 3D structure.
TG analysis for 1 was measured under air atmosphere between 25 and 1000 °C (10 °C min−1). As illustrated in Fig. S2 (ESI†), no apparent weight loss was observable from ambient temperature to 600 °C, implying good thermal stability of 1. An apparent weight loss is present upon further heating of the sample above 610 °C, indicating the collapse of the framework. To confirm the phase purities before property measurements, PXRD was conducted. Fig. S3 (ESI†) illustrates that the positions of the experimental peak values are in accordance with the fitted ones derived from SCXRD, suggesting the purity of the samples. The distinct intensities between the peaks may be attributable to the variation in the preferred orientation of the experimental sample.
The mole magnetic susceptibility (χM) of 1 was investigated in the solid state in the temperature range of 2–300 K at 1 kOe field. All magnetic measurements were obtained with crushed crystalline samples. The observed χMT product of 8.04 cm3 K mol−1 at 300 K basically matches with the value for one isolated GdIII (7.88 cm3 K mol−1, 8S7/2, g = 2) in 1 (Fig. 2a). During the process of cooling, the χMT products of 1 decline very slowly from 300 K to 10 K. Below 10 K, an apparent fall was present with the minimum value of 7.51 cm3 K mol−1 at 2 K. The declining trend of the χMT vs. T plot for 1 manifests that adjacent GdIII ions feature antiferromagnetic (AF) coupling, which is further corroborated by the negative Weiss constant θ = −0.22 K from Curie–Weiss simulation (Fig. 2a). The accordance between zero field cooled (ZFC) and field cooled (FC) magnetizations excluded the possibilities of long-range magnetic ordering (Fig. 2b).
Fig. 2 (a) The plot of χMT vs. T for 1 (red part for the Curie–Weiss simulation). (b) FC and ZFC magnetization of 1 in the dc field of 50 Oe. (c) Variable-field magnetization plots at specific temperatures for 1. (d) −ΔSm derived from experimental magnetization data of 1 at distinct fields and temperatures.
−ΔSmaxm [mJ cm−3 K−1] = −ΔSmaxm [J kg−1 K−1]*ρcald [g cm−3] D: dimensionality.
The structure, magnetic susceptibility and isothermal magnetizations for the inorganic Gd-based coordination polymer (CP) Gd(OH)SO4 (1) have been investigated. The title CP exhibits the 3D dense architecture with a Gd-oxygen chain as supramolecular building units. Magnetic characterisations indicated that adjacent GdIII ions show weak magnetic exchange in 1. Because of the integration of the large isotropic spin of GdIII, the dense structure and weak magnetic couplings, the observed maximum entropy change (−ΔSmaxm) for 1 was 53.5 J kg−1 K−1 for ΔH = 7 T and T = 2 K and 38.3 J kg−1 K−1 for T = 2 K and ΔH = 3 T, which already surpasses the commercial gadolinium gallium garnet Gd3Ga5O12 (GGG) (38.4 J kg−1 K−1 with ΔH = 7 T and about 24 J kg−1 K−1 with ΔH = 3 T), indicating it to be an excellent cryogenic magnetic cooler. This work highlights the role of inorganic ligands in the generation of excellent molecular cryogenic magnetic coolers. Further work will focus on utilising other known results to explore brilliant molecular cryogenic magnetic coolers under the guidance of molecular design strategies.
This work was supported by the Natural Science Foundation of China (21571111, 21601099 and 21601100), the China Postdoctoral Science Foundation (2016M592137), the Young Scientist Foundation of Shandong Province of China (ZR2016BB25), and the project of applied and fundamental research of Qingdao City of China (16-5-1-87-jch).
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