Li2B3O4F3, a new lithium-rich fluorooxoborate

The new lithium fluorooxoborate, Li2B3O4F3, is obtained by a solid state reaction from LiBO2 and LiBF4 at 553 K and crystallizes in the acentric orthorhombic space group P212121 (no. 19) with the cell parameters a=4.8915(9), b=8.734(2), and c=12.301(2) Å. Chains of fluorinated boroxine rings along the b axis consists of BO3 triangles and BO2F2 as well as BO3F tetrahedra. Mobile lithium ions are compensating the negative charge of the anionic chain, in which the fourfold coordinated boron atoms bear a negative formal charge. Annealing Li2B3O4F3 at temperatures above 573 K leads to conversion into Li2B6O9F2. The title compound is an ionic conductor with the highest ion conductivity among the hitherto know lithium fluorooxoborates, with conductivities of 1.6×10−9 and 1.8×10−8 S cm−1 at 473 and 523 K, respectively.