New Second-Order NLO Materials Based on Polymeric Borate Clusters and GeO₄ Tetrahedra: A Combined Experimental and Theoretical Study

Three novel rubidium borogermanates with three types of noncentrosymmetric structures, namely, RbGeB₃O₇, Rb₂GeB₄O₉, and Rb₄Ge₃B₆O₁₇, have been synthesized by high-temperature solid-state reactions in platinum crucibles. The structure of RbGeB₃O₇ features a three-dimensional (3D) anionic framework composed of cyclic B₃O₇ groups corner-sharing GeO₄ tetrahedra. The structure of Rb₂GeB₄O₉ shows a 3D anionic framework based on B₄O₉ clusters connected by GeO₄ tetrahedra via corner sharing. The structure of Rb₄Ge₃B₆O₁₇ is a novel 3D anionic framework composed of cyclic B₃O₈ groups, Ge₂O₇ dimers, and GeO₄ tetrahedra that are interconnected via corner sharing. Second harmonic generation (SHG) measurements indicate that RbGeB₃O₇, Rb₂GeB₄O₉, and Rb₄Ge₃B₆O₁₇ display moderate SHG responses that are approximately 1.3, 2.0, and 1.3 × KH₂PO₄ (KDP), respectively, which are slightly smaller than those from theoretical calculations (about 3.7, 2.8, and 2.4 × KDP, respectively).