Unlocking Advanced UV NLO Crystals in Rare‐Earth Metal Borate Fluorides via [B ₃ O ₆ ]‐Mediated Structural Modulation

Abstract Developing new short‐wavelength nonlinear optical (NLO) crystals has always been a significant and challenging area of research. Herein, guided by the cooperative optimization strategy, three new rare‐earth metal borate fluorides, K 2 GdB 3 O 6 F 2 , Rb 2 LuB 3 O 6 F 2 , and Cs 2 LuB 3 O 6 F 2 , are rationally designed and fabricated by synergically assembling advantageous functional groups. Among them, a structural evolution from centrosymmetric K 2 GdB 3 O 6 F 2 to non‐centrosymmetric Rb 2 LuB 3 O 6 F 2 and Cs 2 LuB 3 O 6 F 2 reveals that the [B 3 O 6 ] group contributes to the control of structural symmetry, owing to its sensitivity to the coordination of rare earth metal polyhedra. Notably, all the three title compounds exhibit short cutoff edges less than 200 nm, with Cs 2 LuB 3 O 6 F 2 displaying a large experimental frequency doubling effect of 1.5 × KH 2 PO 4 . The type‐I shortest phase‐matching wavelengths for Rb 2 LuB 3 O 6 F 2 and Cs 2 LuB 3 O 6 F 2 are evaluated to be 210 and 202 nm, respectively, indicating their potential for direct output of 213 coherent lights through a fifth harmonic generation process of Nd: YAG laser. This study provides new insights into the rational design and development of short‐wavelength NLO materials by exploring the sensitivity of the [B 3 O 6 ] groups to the surrounding coordination environment, thereby fostering innovation in the field of NLO materials.