Electronic and structural in6uences on ionic conductivity in the crystalline lithium phosphorus oxynitrides -Li 3 PO 4 and Li 2.88 PO 3.73 N 0.14 are analyzed, using approximate molecular orbital calculations. Starting from -Li 3 PO 4 , we construct a model compound for the new nonstoichiometric oxynitride, Li 11 P 4 O 14 N, in which an oxygen in a bridge position (O II ) in the parent -Li 3 PO 4 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.