Cd₈(BO₃)₄SiO₄: Metal Cation Inducing the Formation of Isolated [BO₃] and [SiO₄] Units in Borate Silicate

The first compound of cadmium-borate silicate Cd₈(BO₃)₄SiO₄, crystallizing in space group P4₂/n (no. 86), has been successfully synthesized by the conventional high-temperature solution method and melts congruently. The zero-dimensional anionic groups of Cd₈(BO₃)₄SiO₄ are isolated [BO₃] triangles and isolated [SiO₄] tetrahedra which are filled in the framework formed by [CdO₆] polyhedra. It has a moderate birefringence (Δn = 0.053 at 546 nm), which is measured by experiment and evaluated by first-principles calculations; meanwhile, the source of birefringence is revealed through the response electronic distribution anisotropy method. The UV-vis-NIR diffuse reflectance spectrum indicates that Cd₈(BO₃)₄SiO₄ possesses a wide optical transparency range, with a UV cutoff edge at about 254 nm. This work enriches the structure chemistry of borate silicates, and we discussed the possible methods for the exploration and synthesis of novel optical crystals possessing zero-dimensional anionic groups in the borate silicate system.