Electronic and Structural Effects of Nitrogen Doping on the Ionic Conductivity of -Li3PO4

Electronic and structural influences on ionic conductivity in the crystalline lithium phosphorus oxynitrides -Li3PO4 and Li2.88PO3.73N0.14 are analyzed, using approximate molecular orbital calculations. Starting from -Li3PO4, we construct a model compound for the new nonstoichiometric oxynitride, Li11P4O14N, in which an oxygen in a bridge position (OII) in the parent 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) 3PNP(HO)3)]1+, [(HO)3POP(HO)3]2+, [O(P3(OH)3)]4+, and [N(P3(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.