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.