Improvement on Magnetocaloric Effect through Structural Evolution in Gadolinium Borate Halides Ba 2 Gd(BO 3 ) 2 X (X = F, Cl).

A new Gd 3+ -containing borate Ba 2 Gd(BO 3 ) 2 F has been successfully grown via the high-temperature solution method using BaF 2 -NaF-B 2 O 3 flux. Ba 2 Gd(BO 3 ) 2 F 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(BO 3 )F] ∞ layers bridged by sharing [BO 3 ] 3- , which is different from the [Gd(BO 3 )] ∞ layer in the model structure Ba 2 Gd(BO 3 ) 2 Cl. The magnetic measurements indicated that Ba 2 Gd(BO 3 ) 2 F has a larger magnetocaloric effect with -Δ S m,max = 27.82 J·kg -1 ·K -1 at 2 K and 9 T than that of Ba 2 Gd(BO 3 ) 2 Cl 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.