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

Herein, a new fluorooxoborate α-KBOF is synthesized successfully in a closed system. It crystallizes in the 2/ space group and features the rare one-dimensional (1D) zigzag [BOF] chains built by fundamental building blocks (FBBs) [BOF]. To the best of our knowledge, KBOF 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.