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.