Reversible Polymerization of Adamantane-type [P4 N10 ]10- Anions to Honeycomb-type [P2 N5 ]5- Layers under High-Pressure.

The high-pressure polymorph Li5 P2 N5 of Li10 P4 N10 (="2 Li5 P2 N5 ") was synthesized by high-pressure/high-temperature reaction of LiPN2 and Li7 PN4 or β-Li10 P4 N10 at 9 GPa, using the Li3 N self-flux method in a Walker-type multianvil assembly. Li5 P2 N5 is the first lithium nitridophosphate with a layered structure and is made up of corner sharing PN4 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, 6 Li, 7 Li and 31 P solid-state NMR spectroscopy were conducted. To corroborate Li5 P2 N5 as the corresponding high-pressure polymorph of β-Li10 P4 N10 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.