Li18P6N16-A Lithium Nitridophosphate with Unprecedented Tricyclic [P6N16]18- Ions

Li18P6N16 was synthesized by reaction of LiPN2 and Li7PN4 at 5.5 GPa and 1273 K employing the multi-anvil technique. It is the first lithium nitridophosphate obtained by high-pressure synthesis. Moreover, it is the first example received by reaction of two ternary lithium nitrides. The combination of high-pressure conditions with a Li3N flux enabled a complete structure determination using single-crystal X-ray diffraction. The hitherto unknown tricyclic [P6N16](18-) anion is composed of six vertex-sharing PN4 tetrahedra forming one vierer- and two additional dreier-rings. To confirm the structure, Rietveld refinement, Li-7 and P-31 solid-state NMR spectroscopy, FTIR spectroscopy and EDX measurements were carried out. To validate the ionic properties, the migration pathways of the Li+ ions were evaluated, and the conductivity and its temperature dependence were determined by impedance spectroscopy measurements. In order to obtain a clearer picture of the formation mechanism of this compound class, different synthetic approaches were compared, enabling targeted syntheses of unprecedented P/N-anion topologies with intriguing properties.