Theoretical Approach to Ionic Conductivity in PhosphorusOxynitride Compounds

B3YLP density functional calculations have been performed to study the ionic conductivity in γ-Li3PO4 and γ-Li2.88PO3.73N0.14. Starting from the crystal structure of γ-Li2.88PO3.73N0.14, we construct a model cluster without defects, Li15PO10, as well as another new oxynitride, Li14PO8N, in which lithium and oxygen defects are introduced as one oxygen is substituted by nitrogen. To model the ionic conductivity in these materials, different pathways of lithium motion are considered. The first one involves a Li+ motion between two crystallographic sites through faces of adjacent LiO4 tetrahedron via an unoccupied octahedral site. The second one involves a direct Li+ motion through faces of adjacent LiO4 tetrahedra. Both mechanisms are unlikely for the parent model cluster because of the high computed energy barrier associated with Li+ mobility in the cluster. In contrast, we obtain a reasonable energy barrier in the nitride cluster which has Li+ and O2− defects creation and incorporates nitrogen. The barrier was computed to be about 1.26 eV for Li+ mobility through tetrahedral faces for the nitride structure, compared to 4.8 eV in the parent cluster. Considering parameters such as Li–N covalency, ionic radius, and tetrahedral distortion, the nitridation could be expected to enhance the ionic conductivity. We connect the magnitude of the ionic conductivity to the height of the energy barrier computed for Li+ jumping between different crystallographic sites.