Magnetocaloric Effect of Two Gd-Based Frameworks

Abstract

Magnetic refrigeration material is the key to adiabatic demagnetization refrigeration technology. In this work, two magnetic refrigerants, Gd₅(C₄O₄)(HCOO)₃(CO₃)₂(OH)₆·2.5H₂O (1) and Gd₂(OH)₄SO₄ (2), were prepared through hydrothermal reaction. Magnetic study reveals that their magnetic entropy changes are 24.8 J kg⁻¹ K⁻¹ for 1 and 15.1 J kg⁻¹ K⁻¹ for 2 at 2 K and 2 T, respectively. The magnetic entropy changes of 1 and 2 at T = 2 K and ∆H = 2 T exceed most gadolinium hydroxyl compounds, indicating that magnetic refrigerants with large magnetic entropy changes at low magnetic fields can be obtained by introducing more weak magnetic exchange ligands to replace hydroxyl groups in gadolinium hydroxyl compounds.

1. Introduction

Since the magnetocaloric effect (MCE) of Fe and Ni was observed by Warburg and Weiss in 1881 [1] and 1917 [2], respectively, the adiabatic demagnetization refrigeration (ADR), which is based on the MCE, has gained extensive attention over the century, not only because it has the advantages of environmental friendliness and energy efficiency [3,4,5] but it is also a promising method to reach the subkelvin temperature region (below 1 K) without the use of rare ³He gas [6,7]. Owing to the magnetic entropy changes (−∆S_m) of magnetic refrigerants being the driving force of ADR, a great many efforts have been made in the preparation of large MCE magnetic refrigerants in the past decades. Although some large MCE magnetic refrigerants, such as Gd(HCOO)₃ [8], Gd(OH)₃ [9], Gd(OH)CO₃ [10], GdPO₄ [11] and GdF₃ [12], have been successfully obtained so far, on one hand, these compounds are already known and thus it is necessary to find new magnetic refrigerants with large MCE. On the other hand, although various Gd-based materials, such as inorganic salts [11,12,13], molecule-based clusters [14,15,16,17,18,19,20], inorganic metallic oxides [21,22,23] and coordination polymers [24,25,26], were selected as magnetic refrigerants in the past decades, few of them contain four different inorganic ligands and thus how the collaboration among these ligands affects their MCE remains unclear. Here we report syntheses and MCE of two magnetic refrigeration materials, namely, Gd₅(C₄O₄)(HCOO)₃(CO₃)₂(OH)₆·2.5H₂O (1) and Gd₂(OH)₄SO₄ (2), of which compound 1 represents the very rare example of inorganic Gd-based compound formed by four different inorganic anions.

2. Results and Discussion

Compound 1 was synthesized through the hydrothermal reaction of gadolinium chloride hexahydrate with squaric acid in the mixed solvent of N,N-dimethylformamide (DMF) and water. Compound 2 was synthesized by the hydrothermal reaction of gadolinium nitrate hexahydrate with ammonium carbonate and sulfuric acid in aqueous solution. Powder X-ray diffraction (PXRD) patterns of the bulk sample 1 and 2 reveal that the experimental diffraction patterns are consistent with those simulated by the single crystal data (Figure S1, see the Supplementary Materials), demonstrating the purity of the bulk sample 1 and 2. Thermogravimetric analysis (TGA) under nitrogen atmosphere indicates the weight loss at 265 °C is 3.89% for 1, attributed to the loss of two and a half H₂O. The total weight loss of 29.78% corresponds well to the theoretical value of 30.31% when the residue is Gd₂O₃. Sample 2 has good thermal stability and can remain stable up to 350 °C. The weight loss of 7.29% matches well with that of 7.52% calculated by two H₂O at the temperature of 430 °C (Figure S2). Both the experimental PXRD patterns and TGA further confirm the purity of the bulk sample 1 and 2. The O atoms in the main structure are assigned to hydroxyl groups based on bond valence sum (BVS) calculations [27] and charge balance requirements (Tables S1 and S2).

Single crystal X-ray diffraction analysis shows that 1 and 2 crystallize in the monoclinic system with space groups P2₁/c and C2/m, respectively (Figures S3 and S4). Crystal data and structural refinement details are shown in Table S3. The asymmetric unit of 1 consists of five Gd³⁺ ions, six OH⁻ anions, two CO₃²⁻ anions, three HCOO⁻ anions, one C₄O₄²⁻ anion and two and a half H₂O. Gd1 to Gd4 are all coordinated with four OH⁻, one HCOO⁻, two CO₃²⁻ and one C₄O₄²⁻ in monodentate mode. Gd5 is coordinated with two OH⁻, two monodentate HCOO⁻ and two CO₃²⁻ in chelated mode. Gd1 and Gd2 are in triangular dodecahedron geometry calculated by continuous shape measurements [28] (CShM). Gd3 and Gd4 are in square antiprism, while Gd5 is in biaugmented trigonal prism (Table S4). Figure 1a shows that four Gd³⁺ ions are linked by four OH⁻, generating a [Gd₄(OH)₄]_n⁸ⁿ⁺ cubane. Four [Gd₄(OH)₄]_n⁸ⁿ⁺ cubanes centered by a Gd³⁺ ion through the connection of HCOO⁻, CO₃²⁻ and OH⁻ form a butterfly-shaped unit of [Gd₁₇(OH)₂₀(HCOO)₃(CO₃)₂]_n²⁴ⁿ⁺, as shown in Figure 1b. It is worth noting that the two cubanes on the left side of the butterfly-shaped unit are connected to the central Gd³⁺ through two HCOO⁻ and one CO₃²⁻, while these on the right side of the butterfly-shaped unit are connected to the central Gd³⁺ through one HCOO⁻ and one CO₃²⁻. Based on previous work, the ligands of CO₃²⁻ and HCOO⁻ were generated from the decomposition of squaric acid in situ under hydrothermal conditions [29,30]. Adjacent butterfly-shaped units expand into a 2D layer of [Gd₅(OH)₆(HCOO)₃(CO₃)₂]_n²ⁿ⁺ through sharing edges, as shown in Figure 1c. The adjacent layers are pillared by squarate in a μ₄−η¹:η¹:η¹:η¹ coordination mode, forming into a 3D framework of 1 (Figure 1d).

Compound 2 is a known compound [31,32], but its single crystal structure has not yet been obtained. The asymmetric unit of 2 contains half of one Gd³⁺ ion, one OH⁻ anion and a quarter of one SO₄²⁻ anion. The central Gd³⁺ is coordinated by six OH⁻ anions and two SO₄²⁻ anions in chelated mode in metabidiminished icosahedron geometry (Table S5). Figure 2a reveals that the Gd³⁺ ions are bridged by μ₃−OH⁻, forming a classic 2D layer of [Gd₂(OH)₄]_n²ⁿ⁺ as observed in Gd(OH)₂Cl [33]. The 3D framework can be viewed as the adjacent layers connected by sulfates in a μ₄−η¹:η¹:η²:η² coordination mode, as shown in Figure 2b. It was mentioned that although the 2D layer structure in 1 is very similar to that in Gd(OH)₂Cl, the latter is in fact a 3D supramolecular network viewed as a connection of adjacent 2D layer structures through electrostatic interactions between Cl⁻ anions and [Gd(OH)₂]_nⁿ⁺ layers.

The lengths of Gd–O bond in 1 and 2 range from 2.298(6) to 2.516(5) Å and 2.375(8) to 2.518(10) Å, and the Gd–O–Gd angles vary from 104.37(14) ~ 116.37(16)° and 92.9(5) ~ 110.8(4)°, respectively. The distances between Gd···Gd are 3.815(6) ~ 8.561(4) Å and 3.650(6) ~ 6.762(15) Å for 1 and 2 (Tables S6 and S7). The bond lengths and angles are comparable to those reported in Gd(OH)₂Cl [33].

Figure 3a illustrates the temperature-dependent magnetic susceptibility of 1 and 2 measured in a temperature range of 2 to 300 K under a direct current (dc) magnetic field of 1000 Oe. At room temperature, the values of χ_MT of 1 and 2 are 39.21 cm³ K mol⁻¹ and 15.48 cm³ K mol⁻¹, respectively, corresponding to the calculated theoretical values of 39.40 cm³ K mol⁻¹ for five spin−only Gd³⁺ ions (⁸S_7/2, S = 7/2, g = 2) and 15.48 cm³ K mol⁻¹ for two Gd³⁺ ions. From 300 K to 100 K, the χ_MT values of the two compounds remain unchanged. Then, the χ_MT values decrease slowly till temperature continues to cool down to 25 K. Further decreasing the temperature, the χ_MT values start to decrease rapidly and reach 20.78 cm³ K mol⁻¹ for 1 and 5.49 cm³ K mol⁻¹ for 2 at 2 K. The rapid decreasing χ_MT vs. T curves indicate the existence of antiferromagnetic (AFM) interactions between adjacent Gd³⁺ ions in both 1 and 2. Consistently, fitting the χ_M⁻¹ vs. T data of 1 and 2 with the Curie–Weiss law gives C = 39.57 cm³ K mol⁻¹ and θ = −2.25 K for 1 and C = 15.71 cm³ K mol⁻¹, θ = −2.78 K for 2. The negative Weiss constants further manifest the presence of antiferromagnetic coupling between Gd³⁺ ions of 1 and 2. Figure S5 shows the isothermal magnetization data in applied magnetic field from 0 to 7 T in the temperature range from 2 to 10 K for 1 and 2. The magnetization increases with the decrease of temperature at a given applied magnetic field and decreases with the decrease of magnetic field at a given temperature. At 2 K and 7 T, the magnetizations of the two samples approach the saturation values and reach 34.78 Nμ_B for 1 and 13.93 Nμ_B for 2, which are consistent with the theoretical values of 35 Nμ_B and 14 Nμ_B calculated through five Gd³⁺ ions (J = 7/2, L = 0, S = 7/2) and two Gd³⁺ ions.

Since 1 and 2 both have large mass ratio of metal to ligand, the experimental MCE of 1 and 2 was investigated by using multiple temperature data of magnetization to fit the Maxwell equation [34] below:

$${\mathsf{\Delta }S}_{\mathrm{m}}{\left(T\right)}_{\mathsf{\Delta }H}={{\displaystyle \int }}^{ }{\left[\partial M\left(T,H\right)/\partial T\right]}_H \mathrm{d}H$$

(1)

As shown in Figure 3b,c, the magnetic entropy changes (−ΔS_m) of 1 and 2 increase monotonically with the decrease of temperature and the increase of magnetic field, reaching the maximum value of 59.8 J kg⁻¹ K⁻¹ (221 mJ cm⁻³ K⁻¹) and 57.6 J kg⁻¹ K⁻¹ (293 mJ cm⁻³ K⁻¹), respectively, at 2 K and 7 T.

For comparison, the theoretical −ΔS_m for 1 and 2 was also calculated by the equation −ΔS_m = nRln(2S + 1)/M_w [35,36], which shows that the −ΔS_m is 66.5 J kg⁻¹ K⁻¹ for 1 and 72.2 J kg⁻¹ K⁻¹ for 2. Both the experimental MCE of 1 and 2 were obviously smaller than theoretical MCE of 1 and 2; this is attributed to the existence of relatively strong antiferromagnetic interactions in 1 and 2, which degrade the spin degeneracy causing the inability to obtain large magnetic entropy changes under limited magnetic field [37].

Table 1. Weiss constants and magnetic entropy changes for selected Gd-based magnetocaloric materials (−∆S_m).

Compound	θ	−∆Sm/ J kg−1 K−1 (mJ cm−3 K−1)		Ref.
		7 T	2 T
GdF3	+0.7	71.6 (506) *	45.5 (322)	[12]
Gd(HCOO)3	−0.3	55.9 (216) *	43.7 (169) *	[8]
Gd(OH)CO3	−1.05	66.4 (355) *	33.7 (180)	[10]
Gd(OH)3	−1.69	62.0 (346)	26.9 (150)	[9]
Gd(OH)2Cl	−1.99	61.8 (319)	17.6 (91)	[33]
Gd3(OH)8Cl	−2.78	59.8 (310)	14.2 (74)	[33]
[Gd3(CO3)(OH)6]OH	−4.37	61.5 (262)	10.1 (43)	[42]
Gd2(OH)5Cl·1.5H2O	−3.1	51.9 (/) *	/	[43]
Gd4(OH)4(SO4)4(H2O)4	−1.57	51.3 (199)	25.5 (99)	[44]
[Gd60]	−3.71	48.0 (133)	12.7 (35)	[15]
1	−2.25	59.8 (221)	24.8 (92)	This
2	−2.78	57.6 (293)	15.1 (77)	Work

*: Maximum value in current magnetic field.

lists the magnetic interactions and the magnetic entropy changes of some excellent magnetic refrigerants reported to date under the applied magnetic field of 2 T and 7 T. For comparison, the magnetic interactions and the magnetic entropy changes of some magnetic refrigerants containing OH⁻ group are also listed. Although the MCE of 1 and 2 at 7 T is significantly smaller than that of GdF₃ and Gd(OH)CO₃, it is comparable to that of [Gd₃(CO₃)(OH)₆]OH, Gd(OH)₃ and Gd(OH)₂Cl. It was mentioned that although the MCE of Gd(HCOO)₃ at 7 T is obviously smaller than that of 1 and 2, its MCE at 2 T is significantly larger than that of 1 and 2. This result distinctly indicates that weak magnetic interaction favors to obtain large MCE magnetic refrigerants at low applied magnetic field. Because of the limitation of weight and high magnetic field interference in space missions, the magnetic field above 4 T cannot be generated properly [38], and commercial magnets such as NbTi [39] and NdFeB [40,41], can easily provide a magnetic field of 2 T. Except for Gd(OH)CO₃, Gd(OH)₃ and Gd₄(OH)₄(SO₄)₄(H₂O)₄, the magnetic entropy changes of other gadolinium hydroxyl compounds listed in Table 1 are all lower than that of 1 in low magnetic field. In combination with the composition of these compounds, this result indicates that magnetic refrigerants with large MCE at low fields can be obtained by introducing more weak magnetic exchange ligands to replace hydroxyl groups in gadolinium hydroxyl compounds, consistent with the fact that the MCE of Gd(OH)CO₃ is significantly larger than that of Gd(OH)₃. Based on the Table 1, the MCE of Gd(OH)₃ larger than that of 1 is obviously related to the fact that its magnetic density is higher than that of 1. Therefore, in order to obtain gadolinium hydroxyl compounds with large magnetic entropy changes, it is necessary to maintain high magnetic density of the gadolinium hydroxyl compounds, in addition to introducing more weak magnetic exchange ligands to replace hydroxyl groups in gadolinium hydroxyl compounds. It was noted that although the mass magnetic entropy change of 1 (24.8 J kg⁻¹ K⁻¹) at 2 T is obviously larger than that of Gd(OH)₂Cl (17.6 J kg⁻¹ K⁻¹) at 2 T, the volumetric magnetic entropy change of 1 (92 mJ cm⁻³ K⁻¹) at 2 T is close to that of Gd(OH)₂Cl (91 mJ cm⁻³ K⁻¹) at 2 T. No obvious difference in the volumetric magnetic entropy change between them is attributed to that the crystal density of 1 is significantly smaller than that of Gd(OH)₂Cl. In this sense, the introduction of heavy atoms into gadolinium hydroxyl compounds is beneficial to improving their volumetric magnetic entropy change when their magnetic density and magnetic interaction are close.

3. Conclusions

In summary, we reported on the crystal structures and MCE of the two magnetic refrigeration reagents 1 and 2. Magnetic study reveals that 1 and 2, respectively, exhibit magnetic entropy changes of 59.8 J kg⁻¹ K⁻¹ and 57.6 J kg⁻¹ K⁻¹ at T = 2 K and ∆H = 7 T. Significantly, the magnetic entropy changes of 24.8 J kg⁻¹ K⁻¹ for 1 and 15.1 J kg⁻¹ K⁻¹ for 2 at T = 2 K and ∆H = 2 T exceed most gadolinium hydroxyl compounds, indicating that magnetic refrigerants with large MCE at low fields can be obtained by introducing more weak magnetic exchange ligands to replace hydroxyl groups in gadolinium hydroxyl compounds when the magnetic density remains unchanged.

4. Materials and Methods

4.1. General Information

All materials and reagents were commercially available and used without further purification.

Powder X-ray diffraction data (PXRD) was collected on a Rigaku Ultima IV powder X-ray diffractometer (Cu Kα, λ = 1.54184 Å) in the 2θ range of 5–60° at room temperature. Thermogravimetric analysis (TGA) curve was conducted on an SDT_Q600 thermal analyzer at a rate of 10 °C per minute up to 800 °C under a constant nitrogen gas. Elemental analyses were carried out using a CE Instruments EA 1110 elemental analyzer. Magnetic measurement was carried out using a Quantum Design MPMS−XL5 superconducting quantum interference device (SQUID).

4.2. Synthesis of Gd₅(C₄O₄)(HCOO)₃(CO₃)₂(OH)₆·2.5H₂O (1)

Compound 1 was prepared by a mixture of GdCl₃·6H₂O (0.186 g, 0.5 mmol) and squaric acid (0.057 g, 0.5 mmol) dissolved in the mixed solvent of 5 mL N,N-dimethylformamide (DMF) and 5 mL deionized water. The resulting solution was stirred for 30 min before transferred into a Teflon-lined autoclave at 170 °C for 4 days and cooled down to room temperature at a rate of 3 °C h⁻¹. Colorless crystals were obtained in 54% yield based on Gd³⁺. Elem. anal. Calculated (found): C, 8.31% (8.85%); H, 0.93% (0.61%).

4.3. Synthesis of Gd₂(OH)₄SO₄ (2)

Compound 2 was synthesized by a mixture of Gd(NO₃)₃·6H₂O (0.226 g, 0.5 mmol) and ammonium carbonate (0.144 g, 1.5 mmol) dissolved in 5 mL deionized water. An amount of 10 μL concentrated sulfuric acid was introduced into the solution while stirring. The resulting solution was stirred for another 30 min before being transferred into a Teflon-lined autoclave at 250 °C for 3 days and cooled down to room temperature at a rate of 3 °C h⁻¹. Colorless crystals were obtained in 72% yield based on Gd³⁺. Elem. anal. Calculated (found): H, 0.84% (0.59%).

4.4. X-ray Crystallographic Analysis of 1 and 2

Single crystal data of 1 and 2 were collected by a Rigaku XtaLAB Synergy diffractometer with monochromatic Cu Kα radiation (λ = 1.54184 Å). Data reduction and absorption correction were applied by using the multi-scan program. The structures were determined and refined using full matrix least squares based on F² with SHELXS and SHELXL [45] within Olex2 [46]. All non-hydrogen atoms were refined anisotropically. CCDC numbers 2164597 and 2164596 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html. (Accessed on 24 June 2022)

4.5. The Bond Valence Sum (BVS) Analysis of 1 and 2

The bond valence sum (BVS) analysis was used to determine the oxidation states of oxygen atoms in compound 1 and 2. The calculation formula is S_ij = exp[(R₀ − R_ij)/b], in which S_ij is the valence of the individual bond, R_ij is the observed bond length, R₀ is a constant depended upon the bonded elements, and b is a constant of 0.37. As shown in Tables S1 and S2, the total BVS values of O atoms are very close to the state of +1, for which we identify the states of all O atoms assigned to hydroxyl groups.