Pb8M(BO3)6 (M = Mg, Ca): Two new borates with large birefringence activated by the ∞[Pb8B6O18]2- double layers

Two new isostructural lead borates , Pb 8 Mg(BO 3 ) 6 and Pb 8 Ca(BO 3 ) 6 , have been synthesized via high temperature solution method. The single crystal structures of title compounds contain ∞ [Pb 8 B 6 O 18 ] 2- double layers composed of planar BO 3 triangles, Pb(2)O 9 and distorted Pb(1)O 5 polyhedra, these layers condense in c -axis, and the MgO 6 (or CaO 6 ) polyhedra fill into the layers, forming a three-dimensional framework. The stereochemical activity of lone pair (SCALP) electrons were found in the Pb(1)O 5 polyhedra, which combine with the BO 3 units, generating strong optical anisotropy . The calculated birefringences of Pb 8 Mg(BO 3 ) 6 and Pb 8 Ca(BO 3 ) 6 using first-principles method are 0.15@1064 nm and 0.12@1064 nm, respectively, which are comparable to the commercial α -BaB 2 O 4 (α-BBO) crystal and imply that they probably can be used as birefringent materials. The electronic structures, density of states of title compounds were calculated by density functional theory (DFT). Meanwhile, the TG-DSC curves, infrared spectral and UV–Vis–NIR diffuse reflectance spectral were measured.