Reversible Polymerization of Adamantane-type [P N ] Anions to Honeycomb-type [P N ] Layers under High-Pressure.

The high-pressure polymorph Li P N of Li P N (="2 Li P N ") was synthesized by high-pressure/high-temperature reaction of LiPN and Li PN or β-Li P N at 9 GPa, using the Li N self-flux method in a Walker-type multianvil assembly. Li P N is the first lithium nitridophosphate with a layered structure and is made up of corner sharing PN tetrahedra forming a corrugated honeycomb-type layer of linked sechser-rings in chair conformation. The arrangement of the P atoms is analogous to that of black phosphorus. The structure was elucidated from single-crystal X-ray data. To confirm the structure Rietveld refinement, Li, Li and P solid-state NMR spectroscopy were conducted. To corroborate Li P N as the corresponding high-pressure polymorph of β-Li P N DFT calculations and temperature dependent X-ray powder diffraction were carried out. DFT calculations estimated the transition pressure to 6.5 GPa (without accounting for temperature), which is in line with the synthesis pressure.