From β-KB3O4F2 to α-KB3O4F2: Phase Transition-Directed Great Changes in Structure and Optical Anisotropy.

Herein, a new fluorooxoborate α-KB₃O₄F₂ is synthesized successfully in a closed system. It crystallizes in the P2₁/n space group and features the rare one-dimensional (1D) zigzag [B₆O₈F₄]_∞ chains built by fundamental building blocks (FBBs) [B₆O₉F₄]. To the best of our knowledge, KB₃O₄F₂ is a unique example of inorganic anhydrous borate whose two polymorphs show 1D B-O/F chains constructed by different FBBs but the same symmetry operation. Different from β-KB₃O₄F₂, the inconsistent arrangement of π-conjugated [B₂O₅] units in α-KB₃O₄F₂ makes it exhibit a small birefringence (0.011@546 nm). Further, α-KB₃O₄F₂ owns a deep-ultraviolet (DUV) cutoff edge (<200 nm), suggesting that it has a potential application as the zero-order waveplate material in the DUV region.