Theoretical approach to ionic conductivity in phosphorus oxynitride compounds

B3YLP density functional calculations have been performed to study the ioni c conductivity in gamma -Li3PO4 and gamma -Li2.88PO3.73N0.14. Starting from the crystal structure of gamma -Li2.88PO3.73N0.14, we construct a model cl uster without defects, Li15PO10, as well as another new oxynitride, Li14PO8 N, in which lithium and oxygen defects are introduced as one oxygen is subs tituted by nitrogen. To model the ionic conductivity in these materials, di fferent pathways of lithium motion are considered. The first one involves a Li+ motion between two crystallographic sites through faces of adjacent Li O4 tetrahedron via an unoccupied octahedral site. The second one involves a direct Li+ motion through faces of adjacent LiO4 tetrahedra. Both mechanis ms are unlikely for the parent model cluster because of the high computed e nergy 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 t o be about 1.26 eV for Li+ mobility through tetrahedra] faces for the nitri de structure, compared to 4.8 eV in the parent cluster. Considering paramet ers such as Li-N covalency, ionic radius, and tetrahedral distortion, the n itridation 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. (C) 200 1 Academic Press.