Direct Reaction of Nitrogen and Lithium up to 75 GPa: Synthesis of the Li₃N, LiN, LiN₂, and LiN₅ Compounds

A wide variety of Li-N compounds are predicted as stable under pressure and associated with various nitrogen anionic moieties. Accordingly, the LiN₅ compound was recently synthesized at 45 GPa by the direct reaction of nitrogen and lithium. In this study, we present an experimental investigation of the Li-N binary phase diagram from ambient pressure up to 73.6 GPa. The samples loaded in the diamond anvil cells were constituted of pure lithium pieces embedded in a much greater quantity of molecular nitrogen and, at incremental pressure steps, were laser-heated to produce the thermodynamically favored solid. The following compounds are observed: Li₃N, LiN₂, LiN as well as LiN₅, and their pressure stability domain is disclosed. Two are synthesized for the first time, namely Cmcm LiN and P6₃/ mmc LiN₂. Both are structurally resolved and characterized by X-ray diffraction and Raman spectroscopy measurements. Their high bulk modulus is characteristic of charged N₂ dimers.