In the search for solid electrolytes for application in all-solid-state batteries, synthesis of nitrogen-doped, crystalline, Li3PO4-based materials by partial substitution of oxygen by nitrogen in order to create interstitial lithium ions and enhance Li-ion conductivity was investigated. Amorphous lithium phosphorus oxynitrides have been previously studied and excellent ionic conductivity and chemical stability was discovered in thin film LiPON; however, the detailed structure and the effect of nitrogen on its properties are not known due to the glassy state. In two previous studies on crystalline forms of this material, lithium vacancy formation in the structure of N-doped Li3PO4 and N-doping of LiPO3 was examined. Previously, computational studies on Li-ion conduction of Li3PO4 predicted that an interstitial defect mechanism would provide the most efficient Li-ion transport. In this work we report the first study to investigate nitrogen doping of crystalline, Li3PO4-based materials, to create interstitial Li+ ions. Samples were prepared by solid-state reaction with Li3N, as nitrogen-containing reactant, by a new modified synthesis technique. Li3+X(P,V)O4-XNX and Li2+Y(Zn,Mg)SiO4-YNY solid solutions have been successfully synthesised. N-doping of all compositions yielded a γ-type phase, except in Li3VO4 that resulted in a β phase. Structure of Li3+X(P,V)O4-XNX is similar to their undoped polymorphs, the unit cell variation is small upon adding nitrogen and the solid solution limit is X≤0.2. The unit cell variation is more significant in Li2+Y(Zn,Mg)SiO4-YNY. Compositions with low concentration of nitrogen have a γ0 structure, similar to undoped polymorphs. Nitrogen doping encourages monoclinicity by distorting the γ0 phase to lithia-γ0. Compositions with higher concentrations of nitrogen match with Li-rich compositions in Li2+2X(Zn,Mg)1-XSiO4. Electrical properties were studied by impedance spectroscopy using high-density pellets prepared by one-step, reaction-sintering synthesis. N-doping improved the conductivity significantly by about two orders of magnitude in N-doped Li3(P,V)O4 and by 4-6 orders of magnitude in N-doped Li2(Zn,Mg)SiO4. The activation energy decreased by about 0.3 eV in N-doped Li3(P,V)O4, between 0.05-0.15 eV in N-doped Li2ZnSiO4 with a low concentration of nitrogen and between 0.6-0.7 eV in N-doped Li2ZnSiO4 with a high concentration of nitrogen and N-doped Li2MgSiO4. The impedance data indicated that the level of electronic conductivity in N-doped compositions is small or negligible and the principal current carriers are lithium ions. Higher ionic conductivity and lower activation energy in N-doped samples was attributed to the high number of interstitial lithium ions created as the result of the substitution of nitrogen for oxygen. Nitrogen content was determined using combustion analysis, which confirmed nitrogen incorporation into the N-doped samples. It was revealed that a great deal of nitrogen loss had occurred. Nitrogen loss that was also reported in previous studies of crystalline LiPON, was suggested to be due to the high temperature required for the solid-state reaction, early decomposition of Li3N and/or by decomposition of N-doped products before N-analysis. Electrochemical testing was performed on Li3.4VO3.6N0.4 and showed improved electrochemical properties compared to Li3VO4, attributed to its enhanced ionic conductivity. As a result of its low working voltage (0.8-1.2V), high capacity and reasonable cycling performance it was suggested as a potential candidate for anode material.