Design and Synthesis of a Deep-Ultraviolet Nonlinear Optical Crystal KRbBGeO Composed of [BO] Units and [GeO] Tetrahedra.

Alkali metal borogermanates have emerged as promising candidates for ultraviolet (UV) and deep-ultraviolet (DUV) nonlinear optical (NLO) crystals due to their excellent comprehensive performance of a short absorption edge and moderate second harmonic response in the UV regions. A new DUV borogermanate KRbBGeO (KRBG) has been successfully synthesized by a high-temperature solid-state method using a boron-rich B/Ge molar ratio of 2:1. KBRG has a noncentrosymmetric structure and crystallizes in the polar space group . The structure consists of two-dimensional (2D) infinite layers formed by the interconnection and coplanar arrangement of the [BO] groups and [GeO] tetrahedra. The layers are alternately connected by isolated [GeO] tetrahedra bridging and [GeO] tetrahedra sharing O atom connections, effectively eliminating the dangling bonds of [BO] units and shifting the UV cutoff edge to the DUV region. Experimental and theoretical calculations show that KRBG has a DUV cutoff edge (λ < 200 nm), a large bandgap (6.11 eV), and a moderate second harmonic generation (SHG) effect (1.1 × KDP). The above findings reveal that KRBG is a highly potential NLO crystal in UV and even DUV regions, enriching the structural variety of borogermanate.