LiB6O9F, the First Lithium Fluorooxoborate – Crystal Structure and Ionic Conductivity

The first lithium fluorooxoborate LiB6O9F was synthesized from LiF and B2O3 in an all‐solid state reaction at 673 K. Single crystals have been grown by subsequent long‐term annealing at 473 K. According to X‐ray analysis of the crystal structure [Pna21, Z = 4, a = 7.6555(1) Å, b = 8.5318(1) Å, c = 10.7894(2) Å, R1 = 0.0184, wR(all) = 0.0487, 853 independent reflections], the fluorooxoborate anion forms a two‐dimensional infinite network with boroxine rings, connected by bridging oxygen atoms constituting the basic building units. To one boron atom in every second ring, a fluorine atom is added. Pentacoordinate lithium cations interconnect neighboring fluorooxoborate layers. LiB6O9F is thermally stable up to 743 K and decomposition is accompanied by evolution of BF3. Ionic conductivities were investigated with impedance spectroscopy, and the new compound can be classified as a solid ionic conductor.