New second-order NLO materials based on polymeric borate clusters and GeO_4 tetrahedra: A combined experimental and theoretical study

摘要

Three novel rubidium borogermanates with three types of noncentrosymmetric structures, namely, RbGeB_3O_7, Rb_2GeB _4O_9, and Rb_4Ge_3B_6O _(17), have been synthesized by high-temperature solid-state reactions in platinum crucibles. The structure of RbGeB_3O_7 features a three-dimensional (3D) anionic framework composed of cyclic B _3O_7 groups corner-sharing GeO_4 tetrahedra. The structure of Rb_2GeB_4O_9 shows a 3D anionic framework based on B_4O_9 clusters connected by GeO _4 tetrahedra via corner sharing. The structure of Rb _4Ge_3B_6O_(17) is a novel 3D anionic framework composed of cyclic B_3O_8 groups, Ge _2O_7 dimers, and GeO_4 tetrahedra that are interconnected via corner sharing. Second harmonic generation (SHG) measurements indicate that RbGeB_3O_7, Rb_2GeB _4O_9, and Rb_4Ge_3B_6O _(17) display moderate SHG responses that are approximately 1.3, 2.0, and 1.3 × KH_2PO_4 (KDP), respectively, which are slightly smaller than those from theoretical calculations (about 3.7, 2.8, and 2.4 × KDP, respectively).