Polymorphism is a subtle but important phenomenon in solid-state chemistry and material sciences. Generally, promoting the formation of a specific polymorph enables the modulation of symmetries and thus the optical properties, but the discovery of such polymorphs with tunable structures is extremely challenging. Herein, by focusing on the recently emerging fluorooxoborate system with a flexible skeleton, a series of new fluorooxoborate phases with the formula of BaBOF3 were obtained by a combined computational–experimental method. Among them, noncentrosymmetric α- and Cc-BaBOF3 phases show nonlinear optical effects and short deep-UV absorption edges, indicating that both compounds can be used as nonlinear optical materials. The small torsion and deformation in their anionic units, that is, [BO2F2] units, cause a huge difference in symmetry and thus in the nonlinear optical property. We demonstrated that the BaBOF3 polymorph with tunable structures and symmetries is a simple but perfect case to understand the influence of B–O/F interactions on creating new phases, which also enriches crystal chemistry of the species of fluorooxoborates. The first-principle calculations on the title compounds were performed to elucidate the structure–property relationship.

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The polarization modulation of deep‐UV light is an important process that incorporates functionality to selectively respond to light‐mater interaction. Typically, optical anisotropy is foremost to the use efficiency of deep‐UV birefringent crystals. Herein, a new congruently melting polyborate with extremely large birefringence (Δn(001)=0.14@589.3 nm) and band gap (6.89 eV) is discovered as a high performance birefringent crystal, which breaks the current deadlock of deep‐UV polyborates that usually show small birefringence. The rigid tetrahedra, including [ZnO4] and edge‐sharing [BO4] tetrahedra, make all the planar [BO3] triangles in the lattice adopt preferential arrangement and thereby lead to an extraordinary large birefringence that is larger than all the deep‐UV borates with experimentally measured values. Structural analyses with the additional theoretical calculations were used to study the origin of strong optical anisotropy in BaZnB4O8.