Electronic and structural effects of nitrogen doping on the ionic conductivity of γ-Li3PO4

Electronic and structural influences on ionic conductivity in the crystalline lithium phosphorus oxynitrides gamma-Li3PO4 and Li2.88PO3.73N0.14 are analyzed, using approximate molecular orbital calculations. Starting from gamma-Li3PO4, we construct a model compound for the new nonstoichiometric oxynitride, Li11P4O14N, in which an oxygen in a bridge position (O-II) in the parent gamma-Li3PO4 structure is replaced by a nitrogen; in addition, oxygen and lithium defects are introduced in a systematic way. We examine the distortion of the lattice in response to substitution and defect formation. To study the P-N-P units observed in chromatographic studies of the oxynitride, density functional calculations are also carried out on small cluster models [(HO)(3)PNP(HO)(3))](1+), [(HO)(3)POP(HO)(3)](2+), [O(P-3(OH)(3))](4+), and [N(P-3(OH)(3))](3+). To produce a high mobility of lithium species in the lattice, across tetrahedral faces rather than edges, our calculations suggest that a high concentration of defects is needed. (C) 1999 Academic Press.