Large Magnetocaloric Effect in Li₃K₉Gd₃(BO₃)₇ Crystal Featuring Sandwich-Like Three-Dimensional Framework

Abstract

A new Gd-based borate crystal, Li3K9Gd3(BO3)7, has been successfully obtained via the high-temperature solution method using Li2O–K2O–B2O3 self-flux. It crystallizes in monoclinic space group P2/n (no. 10) with lattice parameters a = 11.3454(6) Å, b = 9.9881(4) Å, c = 11.4467(7) Å, α = γ = 90 o, β = 114.782(7) o, and Z = 2. Li3K9Gd3(BO3)7 exhibits an intriguing sandwich-like three-dimensional (3D) framework constructed from [Gd–B–O]∞ layers, KO n (n = 6 and 8) polyhedra, and LiO4 tetrahedra, in which [Gd–B–O]∞ layers are built from two types of GdO8 polyhedra and triangular BO3 units. Magnetic measurements showed that Li3K9Gd3(BO3)7 exhibits a large magnetocaloric effect with −ΔS m = 39.3 J kg–1 K–1 at 2.0 K for ΔH = 7 T, which is slightly higher than that of the commercial gadolinium gallium garnet under the same conditions. The powder X-ray diffraction, infrared spectrum, and UV–vis–NIR diffuse reflectance spectrum were also performed to characterize Li3K9Gd3(BO3)7. The electronic band structures, partial density of states, and refractive indices of Li3K9Gd3(BO3)7 were investigated via the first-principle calculations.

Anion-Centered Polyhedron Strategy for Strengthening Photon Emission Induced by Electron–Phonon Coupling

“…Herein, we synthesized eight Gd-containing borates: Gd:ScBO 3 , Gd:YAl 3 (BO 3 ) 4 , Ba 3 Gd­(B 3 O 6 ) 3 , Li 3 K 9 Gd 3 (BO 3 ) 7 , Rb 7 SrGd 2 (B 5 O 10 ) 3 , RbBaGd­(BO 3 ) 2 , GdCa 4 O­(BO 3 ) 3 (GdCOB), and Gd:Na 3 La 9 O 3 (BO 3 ) 8 (Gd:NLBO). − Their crystal structure information and fundamental properties are listed in Table S1. The former six compounds are type-I without the “free-O” motif, while the latter two compounds are type-II containing the “free-O” motif.…”

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

“…The diffuse reflectance spectrum in UV–vis–IR of Ba 2 Gd­(BO 3 ) 2 F is depicted in Figure S4. The 6 I 17/2 transition of Gd 3+ is responsible for the absorption peak located at around 274 nm, which has also been discovered in other Gd-based materials. ,,− Above 275 nm, there is no absorption peak. At 250 nm, the reflectance reaches 70.45%, the cutoff edge of Ba 2 Gd­(BO 3 ) 2 F is lower than 250 nm.…”

“…Rare earth ion Gd 3+ has a large spin ground state, low spin excited state, and negligible magnetic anisotropy, meanwhile, the magnetic coupling between ions is very weak, allowing for a substantial MCE. As a result, Gd 3+ -based materials are favorable to for the fabrication of low-temperature magnetic refrigerants. − Several Gd 3+ -based compounds, such as cluster molecules, − coordination polymers, − metallic oxides, − and inorganic salts, ,− have been used as magnetic refrigerants in the sub-Kelvin temperature range in the last few decades. Inorganic Gd 3+ -based borates are considered to be promising candidates for magnetocaloric materials because of outstanding physicochemical stability, high thermal conductivity, and diverse molecular structures. ,,− By investigating the ICSD database and earlier published papers, it is noted that Gd 3+ -based borate chloride Ba 2 Gd­(BO 3 ) 2 C1 − has high Gd content, light molecular mass, and small-size ligands, which are beneficial to MCE. − Moreover, the Cl atoms in the structure coordinate to metal ions rather than B atoms, which provides the stable frame construction, making it easy for processing and applications.…”

A bidentate-anionic-group strategy for enhancing electron–phonon coupling and vibronic fluorescence in rare-earth crystals

“…In order to further confirm the positive contribution of the bidentate-BO 3 group to electron–phonon coupling, another RE-borate, Li 3 K 9 Gd 3 (BO 3 ) 7 , 37 has been synthesized. Its high-purity powder XRD pattern is plotted in Fig.…”