Molecular Engineering as an Approach To Design a New Beryllium-Free Fluoride Carbonate as a Deep-Ultraviolet Nonlinear Optical Material

It is a great challenge to explore deep-ultraviolet (deep-UV) nonlinear optical (NLO) materials that can achieve a subtle balance among large nonlinear coefficients, moderate birefringence, and deep-ultraviolet (UV) transparency. A new beryllium-free fluoride carbonate Ca2Na3(CO3)3F was successfully synthesized through molecular engineering design, and large single crystals were grown by spontaneous crystallization with molten fluxes. The substitution of NLO-active [BO3] groups for [CO3] groups resulted in an optimal balance among the SHG coefficient, birefringence, and UV transparency. Via comparison of these two iso-structural compounds, the second-harmonic generation coefficients and birefringence of Ca2Na3(CO3)3F have been greatly improved. Remarkably, Ca2Na3(CO3)3F exhibited a wide transparent region with a deep-UV absorption edge at 190 nm. These results demonstrated Ca2Na3(CO3)3F is a promising NLO material in the UV or deep-UV region.探索能够在大非线性系数、适中双折射与深紫外（deep-UV）透光性能之间实现精妙平衡的深紫外（deep-UV）非线性光学（NLO）材料，是一项极具挑战性的研究课题。本研究通过分子工程设计，成功合成了一种新型无铍氟碳酸盐Ca₂Na₃(CO₃)₃F，并采用熔剂自发结晶法制备出大尺寸单晶。将具有非线性光学（NLO）活性的[BO₃]基团替代[CO₃]基团，可实现二次谐波产生（SHG）系数、双折射与紫外透光性之间的最优平衡。通过对这两种同构化合物的对比分析，Ca₂Na₃(CO₃)₃F的二次谐波产生系数与双折射性能均得到了显著提升。值得注意的是，Ca₂Na₃(CO₃)₃F展现出宽透光区间，其深紫外吸收边低至190 nm。上述研究结果表明，Ca₂Na₃(CO₃)₃F是一种极具应用前景的紫外或深紫外波段非线性光学（NLO）材料。