Two reported tavorite-type compositions, LiAlPO4F and LiMgSO4F, are evaluated by ab initio calculation to explore their viability as solid electrolyte materials for the development of solid state Li ion batteries. The two materials are determined to be intrinsically capable of 1D Li ion diffusion along channels formed by a series of zigzag paths passing in between the nearest neighbor trans-position Al (Mg) octahedral pairs. The calculated Li migration energies (ME) (0.55 and 0.20 eV, respectively) are comparable with fast Li ionic conductors currently employed in conventional Li batteries. The covalency of the polyanion group is the dominant factor that directly minimizes the interaction of mobile Li with the lattice, thus leading to ME lowering. (C) 2013 Elsevier B.V. All rights reserved.