Comprehensive Investigation of Anion Species in Crystalline Li + ion Conductor Li 27-x [P 4 O 7+x N 9-x ]O 3 (x≈1.9(3)).

The Li + ion conductor Li 27-x [P 4 O 7+x N 9-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 I 4 - ${\mathrel{\mathop{{\rm { 4}}}\limits^{{\rm -}}}}$ 3d with a=12.0106(14) Å and Z=4. It contains both non-condensed [PO 2 N 2 ] 5- and [PO 3 N] 4- tetrahedra as well as O 2- 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 -8  S cm -1 with an activation energy of 0.46±0.02 eV and a bulk ionic conductivity of 4×10 -6  S cm -1 at 25 °C, which is close to the ionic conductivity of amorphous lithium nitridophosphate. This makes Li 27-x [P 4 O 7+x N 9-x ]O 3 an interesting candidate for investigation of structural factors affecting ionic conductivity in lithium oxonitridophosphates.