Li(0.8)Mg(2.1)B2O5F: the first borate fluoride with magnesium-oxygen-fluorine octahedral chains.

The first alkali metal and magnesium mixed borate fluoride, Li(0.8)Mg(2.1)B2O5F, was prepared by the high-temperature solution method. It crystallizes in the monoclinic space group P2(1)/n (no. 14) with lattice constants a = 3.0881(4) Å, b = 13.6020(2) Å, c = 9.6563(1) Å, β = 91.744(9)°, Z = 4. In the structure, a special magnesium-oxygen-fluorine octahedral chain is found in borates for the first time. The octahedral chains are linked into a three-dimensional framework containing tunnels with (1)(∞)[LiO4](7-) chains and isolated B2O5 groups filling in. The differences of octahedral chains in Li(0.8)Mg(2.1)B2O5F and magnesium inosilicate minerals are discussed. The TG-DSC curves, IR spectrum and UV-Vis-NIR diffuse reflectance spectrum were measured.