Reversible Polymerization of Adamantane‐type [P₄N₁₀]¹⁰⁻ Anions to Honeycomb‐type [P₂N₅]⁵⁻ Layers under High‐Pressure

Abstract

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.