K3Sr3Li2Ga4B6O20F: A Sr2Be2B2O7-Type Ultraviolet Nonlinear Optical Crystal

A novel gallium-based nonlinear optical (NLO) crystal without the toxic Be element, K3Sr3Li2Ga4B6O20F, was successfully realized using a chemical substitution strategy from Sr2Be2B2O7 (SBBO), in which Sr2+ and Be2+ cations are cosubstituted by K+ and Sr2+ and Li+ and Ga3+, respectively. In the structure, BO3, GaO4, (Ga/Li)O4, GaO3F, and LiO3F units are connected by shared oxygen atoms to form two distinct types of single layers, [LiGa2B3O10F] and [(Ga/Li)3B3O11]. Two adjacent [LiGa2B3O10F] single layers are coupled through bridging oxygen atoms and fluorine atoms to generate the [Li2Ga4B6O20F] double layer, and two identical [(Ga/Li)3B3O11] single layers are interconnected by sharing apical oxygen atoms to produce the [(Ga/Li)6B6O21] double layer. These two distinct types of double layers are alternately stacked perpendicular to the ab plane, forming a 3D crystal network. The powder second-harmonic-generation efficiency of the title crystal at a fundamental wavelength of 1064 nm is 1/4 of that of KH2PO4. It exhibits a broad band gap of 5.9 eV and moderate birefringence (0.073 @ 532 nm). Simulation results reveal that the shortest phase-matching threshold is 232 nm, which can be utilized for Nd-based laser fourth-harmonic-generation output.