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.