Evaluation of migration energy of lithium ions in chalcogenides and halides by first principles calculation

Migration energies of Li ion in Li3N, Li2X (X=O, S, Se, and Te) and LiX (X=F, Cl, Br, I) via vacancy mechanism have been calculated by first principles pseudopotential method using plane-wave basis. The energy was obtained as the difference in total energies of supercells by two separate calculations; one with a Li+ ion at the normal point and the other with a Li+ ion at the saddle point. Positions of atoms within the second nearest neighbor of the jumping ion were fully relaxed. Two kinds of diffusion paths, i.e., direct and indirect jumps, were considered. Results show rough agreement with available experimental data. The migration energies for the indirect jump in both halides and chalcogenides show a tendency to decrease with the increase in the periodic number in the Periodic table. This is consistent with the widely accepted view. However, the migration energies for the direct jump of chalcogenides do not obey the rule. Comparison of two polymorphs of LiF implies that not only the anionic species but also the crystal structure plays an important role in determining migration energy.