From β‑KB<sub>3</sub>O<sub>4</sub>F<sub>2</sub> to α‑KB<sub>3</sub>O<sub>4</sub>F<sub>2</sub>: Phase Transition-Directed Great\nChanges in Structure and Optical\nAnisotropy

Herein, a new fluorooxoborate\nα-KB<sub>3</sub>O<sub>4</sub>F<sub>2</sub> is synthesized successfully\nin a closed system. It\ncrystallizes in the <i>P</i>2<sub>1</sub>/<i>n</i> space group and features the rare one-dimensional (1D) zigzag [B<sub>6</sub>O<sub>8</sub>F<sub>4</sub>]<sub>∞</sub> chains built\nby fundamental building blocks (FBBs) [B<sub>6</sub>O<sub>9</sub>F<sub>4</sub>]. To the best of our knowledge, KB<sub>3</sub>O<sub>4</sub>F<sub>2</sub> is a unique example of inorganic anhydrous borate\nwhose two polymorphs show 1D B–O/F chains constructed by different\nFBBs but the same symmetry operation. Different from <i>β-</i>KB<sub>3</sub>O<sub>4</sub>F<sub>2</sub>, the inconsistent arrangement\nof π-conjugated [B<sub>2</sub>O<sub>5</sub>] units in <i>α-</i>KB<sub>3</sub>O<sub>4</sub>F<sub>2</sub> makes it\nexhibit a small birefringence (0.011@546 nm). Further, <i>α-</i>KB<sub>3</sub>O<sub>4</sub>F<sub>2</sub> owns a deep-ultraviolet\n(DUV) cutoff edge (<200 nm), suggesting that it has a potential\napplication as the zero-order waveplate material in the DUV region.