CsB3GeO7and K2B2Ge3O10: Explorations of New Second-Order Nonlinear Optical Materials in the Borogermanate Systems

Two novel alkali(I) borogermanates with noncentrosymmetric structures, namely, CsB 3GeO 7 and K 2B 2Ge 3O 10, have been synthesized by high-temperature solid-state reactions in a platinum crucible. The structure of CsB 3GeO 7 features a novel three-dimensional (3D) framework composed of cyclic B 3O 7 (5-) groups that are interconnected by Ge(IV) cations, whereas the structure of K 2B 2Ge 3O 10 is a new 3D network based on cap-shaped [Ge 3B 2O 14] (10-) clusters that are interconnected via Ge-O-B bridges. CsB 3GeO 7 exhibits a second-harmonic generation (SHG) response that is about 1.5 times that of KDP (KH 2PO 4), whereas the SHG signal of K 2B 2Ge 3O 10 is very weak. Both compounds are insulators and transparent in the range of 300-5000 nm. The electronic structure calculations for both compounds also have been performed.