Abstract A crystalline form of lithium phosphorus oxynitride, Li 2.88 PO 3.73 N 0.14 , has been prepared by solid state reaction between Li 3 N and LiPO 3 under a flowing N 2 atmosphere. The structure of the oxynitride was refined by Rietveld analysis of neutron powder diffraction data using the orthorhombic unit cell of γ-Li 3 PO 4 with space group Pmnb . High-performance liquid chromatography measurement indicates that most nitrogen substitutes for oxygen to form PO 3 N structural units with small concentrations of P-N[]P due to some Li ↔ P disorder in the structure. The ionic conductivity of Li 2.88 PO 3.73 N 0.14 (σ25°C ∼ 10 -13 S · cm-1) is several orders of magnitude higher than that of γ-Li 3 PO 4 (σ 25°C ∼ 10 -18 S · cm -1 ).