(C3N2H5)B3O3F2(OH)2: Realizing Large Birefringence via a Synergistic Effect between Anion F/OH-Ratio Optimization and Cation Activation

Borates have emerged as a significant resource for exploring birefringent materials in the UV frequency region. The π-conjugated planar anions, such as the [BO₃], [B₂O₅], and [B₃O₆] units, exhibiting strong polarizability anisotropy, are traditionally good structural models. Both fluorination and hydroxylation have been proven effective strategies to modulate the optical properties of the planar anions. Controlling the ratio between F and OH is the key to realizing a balanced performance between birefringence and bandgap. Based on a fluorination mechanism proposed recently, a novel hydroxyfluorooxoborate (C₃N₂H₅)B₃O₃F₂(OH)₂ with an optimal F/OH ratio of 1/1 is synthesized. During the synthesis, another strategy known as “cation activation” is also employed, which further increases birefringence by introducing active cation units. With its superior performance, this compound exhibits a short UV cutoff edge at 214 nm and a substantial birefringence of 0.205 at 546 nm, surpassing the commercial birefringent crystal α-BBO (0.122@532 nm). The synergy between F/OH-ratio optimization and cation activation offers a practical methodology for developing UV birefringent materials.