The Li + ion conductor Li 27 À x [P 4 O 7 + x N 9 À x ]O 3 (x � 1.9) has been synthesized from P 3 N 5 , Li 3 N and Li 2 O in a Ta ampoule at 800 °C under Ar atmosphere. The cubic compound crystallizes in space group I43d with a = 12.0106(14) Å and Z = 4. It contains both non-condensed [PO 2 N 2 ] 5À and [PO 3 N] 4À tetrahedra as well as O 2À ions, surrounded by Li + ions. Charge neutrality is achieved by partial occupancy of Li positions, which was refined with neutron powder diffraction data. Measurements of the partial ionic and electronic conductivity show a total ionic conductivity of 6.6 × 10 À 8 S cm À 1 with an activation energy of 0.46 � 0.02 eV and a bulk ionic conductivity of 4 × 10 À 6 S cm À 1 at 25 °C, which is close to the ionic conductivity of amorphous lithium nitridophosphate. This makes Li 27 À x [P 4 O 7 + x N 9 À x ]O 3 an interesting candidate for investigation of structural factors affecting ionic conductivity in lithium oxonitridophosphates.x N 1 + x with pairs of corner-sharing tetrahedra [P 2 O 6 N] 5À with a bridging N atom has been reported. [21] Crystalline lithium [a] S.