Influence of the Cation Size on Symmetry Breaking and Optical Performances in Ae2Mg3(BO3)2F4 (Ae = Sr, Ba)

The rational design of noncentrosymmetric (NCS) borates remains challenging because subtle variations in the arrangement of microscopic functional units can profoundly influence macroscopic symmetry and nonlinear optical (NLO) behavior. Herein, we report two new alkaline-earth borate fluorides, Sr2Mg3(BO3)2F4 and Ba2Mg3(BO3)2F4, featuring identical [BO3] triangles and Mg-O/F polyhedra, but crystallizing in distinct space groups, namely P1̅ for Sr2Mg3(BO3)2F4 and Fdd2 for Ba2Mg3(BO3)2F4. Structural analysis indicates that replacing Sr2+ with the larger Ba2+ cation modifies the local coordination environment around the alkaline-earth cation and the distortion of the Mg-O/F framework, which results in the reorganization of [BO3] units from an antiparallel arrangement to a cooperative polar alignment. Property measurements indicate that Ba2Mg3(BO3)2F4 exhibits a significant second-harmonic generation response, suitable birefringence, and a short UV cutoff edge down to 190 nm. A structure-property relationship study confirms that the [BO3] units are the primary source of the optical response. This work not only provides a promising deep-ultraviolet NLO crystal but also reveals a cation-mediated symmetry engineering strategy to design NCS borates.