Li18 P6 N16 was synthesized by reaction of LiPN2 and Li7 PN4 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 Li3 N flux enabled a complete structure determination using single-crystal X-ray diffraction. The hitherto unknown tricyclic [P6 N16 ]18- anion is composed of six vertex-sharing PN4 tetrahedra forming one vierer- and two additional dreier-rings. To confirm the structure, Rietveld refinement, 7 Li and 31 P 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.
Keywords: conductivity; high-pressure chemistry; nitrides; phosphates; solid-state structure.
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