Synthesis and characterisation of the new oxyfluoride Li+ ion conductor, Li5SiO4F

A new polymorphic phase, Li5SiO4F, with Li+ ion dynamics that are structurally distinct from Li4SiO4, is described. The γ polymorph forms at 750–775 °C, melts incongruently at 800 °C and crystallizes in a monoclinic unit cell: a = 8.67(1) Å b = 16.51(1) Å c = 11.23(2) Å β = 96.61(4) ° and V = 1602.3(3) Å3. The α polymorph forms at 650 °C and appears to be metastable. The γ polymorph has a Li+ ion conductivity of 1.2 × 10−7 S cm−1 at 40 °C, that is several orders of magnitude higher than both the α polymorph and Li4SiO4, with an associated activation energy of 0.51(1) eV. Variable temperature 19F and 7Li static NMR measurements show the absence of F− mobility but the existence of Li+ mobility and confirm that Li+ is the main charge carrier in both polymorphs of Li5SiO4F. 7Li line narrowing NMR experiments suggest that the barrier to local Li hopping is fairly small (0.2–0.3 eV) with faster Li+ ion dynamics in γ-Li5SiO4F compared to α-Li5SiO4F. γ-Li5SiO4F has negligible electronic conductivity, is stable in contact with Li metal and is a new type of stoichiometric, mixed anion, Li+ ion conductor.