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

The first lithium flurooxoborate 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 [Pna2(1), Z = 4, a = 7.6555(1) angstrom, b = 8.5318(1) angstrom, c = 10.7894(2) angstrom, R-1 = 0.0184, wR(all) = 0.0487, 853 independent reflections], the fluorooxoborate anion forms a two-dimensional infinite network with boroxine rings, connected by a 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.