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 −ΔS_m,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.