[B3O3F4(OH)]2–: A Versatile Hydroxyfluorooxoborate Heteroanion Producing Compounds with Highly Tunable Optical Anisotropy

Structural alteration based on existing molecules is an effective way to study new substances. However, a limited control range results, especially when the band gaps are close together, from the difficulty of achieving high optical anisotropic enhancement in compounds with the same chemical formula as comparable behaviors of cations and anions that have the same organic and inorganic characteristics. This article presents a methodical investigation of the highly versatile hydroxyfluorooxoborate family formulated A2[B3O3F4(OH)] (A = monovalent cations), which coordinates various inorganic and organic cations with the identical heteroanionic anion of [B3O3F4(OH)]. We found more than 10 times the optical anisotropy enhancement in this family and validated the anion–anion and tethered–heteroanion interaction for this highly tunable optical anisotropy. Furthermore, it is noted that the recently discovered new member [CN4H7]2[B3O3F4(OH)] achieved a well-balanced optical property with a short deep-ultraviolet cutoff edge and strong birefringence. Our results confirm that it is feasible to achieve highly tunable optical anisotropy in compounds with identical molecular formulas.