A new polymorphic phase, Li5SiO4F, with Li+ ion dynamics that are structurally distinct from Li4SiO4, is d( scribed. The y polymorph forms at 750-775 degrees C, melts incongruently at 800 C and crystallizes in a monoclini unit cell: a --= 8.67(1) angstrom, b = 16.51(1) angstrom, c = 11.23(2) angstrom, 13 = 96.61(4) and V = 1602.3(3) A3. The a poi} morph forms at 650 degrees C and appears to be metastable. The y polymorph has a Li+ ion conductivity 1.2 x 10(-7) S cm(-1) at 40 degrees C, that is several orders of magnitude higher than both the a polymorph and Li4SiO4 with an associated activation energy of 0.51(1) eV. Variable temperature 19F and Li-7 static NMR measurement show the absence of F-mobility but the existence of Li+ mobility and confirm that Li+ is the main charg carrier in both polymorphs of Li5SiO4F. Li-7 line narrowing NMR experiments suggest that the barrier to local I hopping is fairly small (0.2-0.3 eV) with faster Li ion dynamics in y-Li5SiO4F compared to a-Li5SiO4F. Li5SiO4F has negligible electronic conductivity, is stable in contact with Li metal and is a new type of sto chiometric, mixed anion, Li+ ion conductor.