Heterologous Isomorphic Substitution Induces Optical Property Enhancement for Deep-UV Crystals: a Case in Rb[B 3 O 3 F 2 (OH) 2 ].

Optical anisotropy is pivotal for optical crystals, and it can be characterized by the maximum algebraic difference in refractive indices. Improving the optical anisotropy, especially for deep-ultraviolet (UV) crystals, is still a challenge and of interest. Herein, a new hydroxyfluorooxoborate, Rb[B 3 O 3 F 2 (OH) 2 ], was obtained by the heterologous isomorphic substitution strategy. Dual enhancement for the band gap and birefringence compared with the parent A[B 3 O 3 F 2 (OH) 2 ] (A = [Ph 4 P]/[Ph 3 MeP]) compounds was achieved in Rb[B 3 O 3 F 2 (OH) 2 ]. This considerable enhancement originates from the removal of organic components and the retention of a birefringence-active anionic framework. This enhancement pushes the application region from UV to deep-UV. This discovery not only expands the structural chemistry of borates but also demonstrates the viability of heterologous isomorphic substitution to design deep-UV crystals with enhanced optical property.