Gd(OH)F2: A Promising Cryogenic Magnetic Refrigerant

Achieving Magnetic Refrigerants with Large Magnetic Entropy Changes and Low Magnetic Ordering Temperatures

Adiabatic demagnetization refrigeration (ADR) is a promising cooling technology with high efficiency and exceptional stability in achieving ultralow temperatures, playing an indispensable role at the forefront of fundamental and applied science. However, a significant challenge for ADR is that existing magnetic refrigerants struggle to concurrently achieve low magnetic ordering temperatures (T0) and substantial magnetic entropy changes (-ΔSm) at ultralow temperatures. In this work, we propose the combination of Gd3+ and Yb3+ to effectively regulate both -ΔSm and T0 in ultralow temperatures. Notably, the -ΔSm values for Gd0.1Yb0.9F3 (1) and Gd0.3Yb0.7F3 (2) in the 0.4-1.0 K range exceed those of all previously reported magnetic refrigerants within this temperature interval, positioning them as the most efficient magnetic refrigerants for the third stage to date. Although the -ΔSm values for Gd0.5Yb0.5F3 (3) in 1-4 K are less than those of the leading magnetic refrigerant Gd(OH)F2, the -ΔSm values for Gd0.7Yb0.3F3 (4) in 1-4 K at 2 T surpass those of all magnetic refrigerants previously documented within the same temperature range, making it the superior magnetic refrigerant for the fourth stage identified thus far.

Synthesis, Structure, and Magnetic Properties of Cyanurates RE5(C3N3O3)(OH)12 (RE = Gd-Lu): Cryogenic Magnetocaloric Candidate Gd5(C3N3O3)(OH)12

Rare-earth (RE)-based frustrated magnets are fertile playgrounds for discovering exotic quantum phenomena and exploring adiabatic demagnetization refrigeration applications. Here, we report the synthesis, structure, and magnetic properties of a family of rare-earth cyanurates RE5(C3N3O3)(OH)12 (RE = Gd-Lu) with an acentric space group P6̅2m. Magnetic susceptibility χ(T) and isothermal magnetization M(H) measurements manifest that RE5(C3N3O3)(OH)12 (RE = Gd, Dy-Yb) compounds exhibit no magnetic ordering down to 2 K, while Tb5(C3N3O3)(OH)12 shows long-range magnetic ordering around 3.6 K. Among them, magnetically frustrated spin-7/2 Gd5(C3N3O3)(OH)12 shows long-range magnetic ordering around 1.25 K and a large magnetocaloric effect with a maximum magnetic entropy change ΔSm of up to 58.1 J kg-1 K-1 at ΔH = 7 T at liquid-helium temperature regimes.

Accurate Prediction of the Magnetic Ordering Temperature of Ultralow-Temperature Magnetic Refrigerants

Adiabatic demagnetization refrigeration is known to be the only cryogenic refrigeration technology that can achieve ultralow temperatures (≪1 K) at gravity-free conditions. The key indexes to evaluate the performance of magnetic refrigerants are their magnetic entropy changes (-ΔSm) and magnetic ordering temperature (T0). Although, based on the factors affecting the -ΔSm of magnetic refrigerants, one has been able to judge if a magnetic refrigerant has a large -ΔSm, how to accurately predict their T0 remains a huge challenge due to the fact that the T0 of magnetic refrigerants is related to not only magnetic exchange but also single-ion anisotropy and magnetic dipole interaction. Here, we, taking GdCO3F (1), Gd(HCOO)F2, Gd2(SO4)3·8H2O, GdF3, Gd(HCOO)3 and Gd(OH)3 as examples, demonstrate that the T0 of magnetic refrigerants with very weak magnetic interactions and small anisotropy can be accurately predicted by integrating mean-field approximation with quantum Monte Carlo simulations, providing an effective method for predicting the T0 of ultralow-temperature magnetic refrigerants. Thus, the present work lays a solid foundation for the rational design and preparation of ultralow-temperature magnetic refrigerants in the future.

Exceptional Magnetocaloric Responses in a Gadolinium Silicate with Strongly Correlated Spin Disorder for Sub-Kelvin Magnetic Cooling

The development of magnetocaloric materials with a significantly enhanced volumetric cooling capability is highly desirable for the application of adiabatic demagnetization refrigerators in confined spatial environments. Here, the thermodynamic characteristics of a magnetically frustrated spin-7/2 Gd9.33[SiO4]6O2 is presented, which exhibits strongly correlated spin disorder below ≈1.5 K. A quantitative model is proposed to describe the magnetization results by incorporating nearest-neighbor Heisenberg antiferromagnetic and dipolar interactions. Remarkably, the recorded magnetocaloric responses are unprecedentedly large and applicable below 1.0 K. It is proposed that the S = 7/2 spin liquids serve as versatile platforms for investigating high-performance magnetocaloric materials in the sub-kelvin regime, particularly those exhibiting a superior cooling power per unit volume.

Giant Low-Field Cryogenic Magnetocaloric Effect in a Polycrystalline EuB4O7 Compound

To deal with the shortage and high price of helium-3 resources, adiabatic demagnetization refrigeration technology as an alternative to helium-3-based refrigeration technology has received much attention. The magnetism and ultralow-temperature magnetocaloric effect (MCE) of the EuB4O7 compound have been investigated. The results of magnetic and quasi-adiabatic demagnetization measurements suggest the absence of a magnetic order above 0.4 K for EuB4O7. The dipolar interaction between the nearest-neighbor Eu atoms has a characteristic energy of about 800 mK, which may induce a large MCE. The maximum magnetic entropy change reaches 22.8, 36.2, and 47.6 J·kg-1 K-1 at μ0H = 0-10 kOe, 0-20 kOe, and 0-50 kOe, respectively. Measurements by a quasi-adiabatic demagnetization device show that the lowest temperature achievable (289 mK) for polycrystalline EuB4O7 is lower than that (362 mK) for the commercial refrigerant Gd3Ga5O12 (GGG) single crystals. The hold time is more than 70 min below 700 mK, with an environment temperature of 2 K, proving that EuB4O7 exhibits superior cooling performance.

{GdIII7} and {GdIII14} Cluster Formation Based on a Rhodamine 6G Ligand with a Magnetocaloric Effect

Heptanuclear {GdIII7} (complex 1) and tetradecanuclear {GdIII14} (complex 2) were synthesized using the rhodamine 6G ligand HL (rhodamine 6G salicylaldehyde hydrazone) and characterized. Complex 1 has a rare disc-shaped structure, where the central Gd ion is connected to the six peripheral GdIII ions via CH3O-/μ3-OH- bridges. Complex 2 has an unexpected three-layer double sandwich structure with a rare μ6-O2- ion in the center of the cluster. Magnetic studies revealed that complex 1 exhibits a magnetic entropy change of 17.4 J kg-1 K-1 at 3 K and 5 T. On the other hand, complex 2 shows a higher magnetic entropy change of 22.3 J kg-1 K-1 at 2 K and 5 T.

Improvement on Magnetocaloric Effect through Structural Evolution in Gadolinium Borate Halides Ba2Gd(BO3)2X (X = F, Cl)

A new Gd3+-containing borate Ba2Gd(BO3)2F has been successfully grown via the high-temperature solution method using BaF2-NaF-B2O3 flux. Ba2Gd(BO3)2F crystallizing in the orthorhombic space group Pnma is with lattice parameters a = 7.571(4) Å, b = 10.424(5) Å, c = 8.581(4) Å, α = β = γ = 90°, and Z = 2. Its three-dimensional framework was constructed from interesting pinwheel-like [Gd(BO3)F]∞ layers bridged by sharing [BO3]3-, which is different from the [Gd(BO3)]∞ layer in the model structure Ba2Gd(BO3)2Cl. The magnetic measurements indicated that Ba2Gd(BO3)2F has a larger magnetocaloric effect with -ΔSm,max = 27.82 J·kg-1·K-1at 2 K and 9 T than that of Ba2Gd(BO3)2Cl under the same conditions. Moreover, thermal stability, infrared spectrum (IR), and ultraviolet-visible-near-infrared diffuse reflectance spectrum were carried out to characterize the title compounds. The first-principles computations also looked into the electronic band structures, densities of states, and refractive indices.

Sr14.06Gd14.63(BO3)24: A Gadolinium-Rich Borate with Magnetic Refrigeration Performance

A single crystal of Sr_14.06Gd_14.63(BO₃)₂₄ has been successfully grown through a high-temperature solution technique with K₂O-KF-B₂O₃ as the flux. It crystallizes in the Pnma space group with parameters a = 22.3153(5) Å, b = 15.9087(4) Å, c = 8.7507(2) Å, and Z = 2. Sr_14.06Gd_14.63(BO₃)₂₄ has a three-dimensional (3D) framework built from [GdO] chains, in which the isolate [BO₃]^3- groups and Sr²⁺ ions fill in the space of the 3D framework. The magnetic measurements revealed that the title compound exhibits a large magnetocaloric effect with the magnetic entropy change of -ΔS_m = 42.2 J kg^-1 K^-1 at 2 K for 7 T, which is higher than that of the commercial material, Gd₃Ga₅O₁₂ (GGG), with -ΔS_m of 38.4 J kg^-1 K^-1 under the same conditions. Moreover, the infrared spectrum (IR), UV-vis-NIR diffuse reflectance spectrum, and thermal stability were investigated.

Magnetic and Magnetocaloric Properties of the A2LnSbO6 Lanthanide Oxides on the Frustrated fcc Lattice

Frustrated lanthanide oxides are promising candidates for cryogen-free magnetic refrigeration due to their suppressed ordering temperatures and high magnetic moments. While much attention has been paid to the garnet and pyrochlore lattices, the magnetocaloric effect in frustrated face-centered cubic (fcc) lattices remains relatively unexplored. We previously showed that the frustrated fcc double perovskite Ba₂GdSbO₆ is a top-performing magnetocaloric material (per mol Gd) because of its small nearest-neighbor interaction between spins. Here we investigate different tuning parameters to maximize the magnetocaloric effect in the family of fcc lanthanide oxides, A₂LnSbO₆ (A = {Ba²⁺, Sr²⁺} and Ln = {Nd³⁺, Tb³⁺, Gd³⁺, Ho³⁺, Dy³⁺, Er³⁺}), including chemical pressure via the A site cation and the magnetic ground state via the lanthanide ion. Bulk magnetic measurements indicate a possible trend between magnetic short-range fluctuations and the field-temperature phase space of the magnetocaloric effect, determined by whether an ion is a Kramers or a non-Kramers ion. We report for the first time on the synthesis and magnetic characterization of the Ca₂LnSbO₆ series with tunable site disorder that can be used to control the deviations from Curie-Weiss behavior. Taken together, these results suggest fcc lanthanide oxides as tunable systems for magnetocaloric design.

Strong Magnetocaloric Coupling in Oxyorthosilicate with Dense Gd3+ Spins

Searching for working refrigerant materials is the key element in the design of magnetic cooling devices. Herein, we report on the thermodynamic and magnetocaloric parameters of an X₁ phase oxyorthosilicate, Gd₂SiO₅, by field-dependent static magnetization and specific heat measurements. An overall correlation strength of |J|S² ≈ 3.4 K is derived via the mean-field estimate, with antiferromagnetic correlations between the ferromagnetically coupled Gd-Gd layers. The magnetic entropy change -ΔS_m is quite impressive, reaches 0.40 J K^-1 cm^-3 (58.5 J K^-1 kg^-1) at T = 2.7 K, with the largest adiabatic temperature change T_ad = 23.2 K for a field change of 8.9 T. At T = 20 K, the lattice entropy S_L is small enough compared to the magnetic entropy S_m, S_m/S_L = 21.3, which warrants its potential in 2 -20 K cryocoolers with both the Stirling and Carnot cycles. Though with relatively large exchange interactions, the layered A-type spin arrangement ultimately enhances the magnetocaloric coupling, raising the possibilities of designing magnetic refrigerants with a high ratio of cooling capacity to volume.

An unusual F-bridged dual-trinuclear Mg–organic framework as a luminescent thermometer for highly efficient low-temperature detection

A novel Mg-MOF with unusual μ3-F dual-trinuclear cluster was successfully afforded by utilizing a solvent system of DMA/DMPU/HFP. Interestingly, as a luminescent thermometer, this MOF exhibits excellent low-temperature sensing capabilities.