Design and Synthesis of a Deep-Ultraviolet Nonlinear Optical Crystal KRb₃B₆Ge₃O₁₇ Composed of [B₃O₈] Units and [GeO₄] 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 KRb₃B₆Ge₃O₁₇ (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 <i>Cc</i>. The structure consists of two-dimensional (2D) infinite layers formed by the interconnection and coplanar arrangement of the [B₃O₈] groups and [GeO₄] tetrahedra. The layers are alternately connected by isolated [GeO₄] tetrahedra bridging and [GeO₄] tetrahedra sharing O atom connections, effectively eliminating the dangling bonds of [B₃O₈] units and shifting the UV cutoff edge to the DUV region. Experimental and theoretical calculations show that KRBG has a DUV cutoff edge (λ_cutoff < 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.