Comprehensive Investigation of Anion Species in Crystalline Li+ ion Conductor Li27-x [P4 O7+x N9-x ]O3 (x≈1.9(3))

Abstract

The Li⁺ ion conductor Li_27-x [P₄ O_7+x N_9-x ]O₃ (x≈1.9) has been synthesized from P₃ N₅ , Li₃ N and Li₂ O in a Ta ampoule at 800 °C under Ar atmosphere. The cubic compound crystallizes in space group $I\bar{4}3d$ with a=12.0106(14) Å and Z=4. It contains both non-condensed [PO₂ N₂ ]^5- and [PO₃ 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 [P₄ O_7+x N_9-x ]O₃ an interesting candidate for investigation of structural factors affecting ionic conductivity in lithium oxonitridophosphates.

Keywords: NMR spectroscopy; conducting materials; lithium; neutron diffraction; solid-state structures.