Abstract
The synthesis of polynitrogen compounds is of fundamental importance due to their potential as environmentally-friendly high energy density materials. Attesting to the intrinsic difficulties related to their formation, only three polynitrogen ions, bulk stabilized as salts, are known. Here, magnesium and molecular nitrogen are compressed to about 50 GPa and laser-heated, producing two chemically simple salts of polynitrogen anions, MgN4 and Mg2N4. Single-crystal X-ray diffraction reveals infinite anionic polythiazyl-like 1D N-N chains in the crystal structure of MgN4 and cis-tetranitrogen N44− units in the two isosymmetric polymorphs of Mg2N4. The cis-tetranitrogen units are found to be recoverable at atmospheric pressure. Our results respond to the quest for polynitrogen entities stable at ambient conditions, reveal the potential of employing high pressures in their synthesis and enrich the nitrogen chemistry through the discovery of other nitrogen species, which provides further possibilities to design improved polynitrogen arrangements.
Highlights
The synthesis of polynitrogen compounds is of fundamental importance due to their potential as environmentally-friendly high energy density materials
Upon the full pressure release, the β-Mg2N4 compound undergoes an isosymmetric phase transition into the α-Mg2N4 salt, comprised of cistetranitrogen N44− units. These results demonstrate the recoverability to ambient conditions of the pressure-produced N44−entity, emphasizing the potential and importance of the high pressure approach for the discovery and synthesis of improved polynitrogen species
With the goal of synthesizing nitrogen-rich Mg-N solids, we compressed micrometer-size pure magnesium pieces surrounded by a large volume of molecular nitrogen—used as both a pressure transmitting medium and a reagent—up to about 60 GPa in four diamond anvil cells (DACs)
Summary
The synthesis of polynitrogen compounds is of fundamental importance due to their potential as environmentally-friendly high energy density materials. Laser-heating at pressures below 52.2 GPa resulted in the formation of a previously known compound, Mg3N2 (space group C2/m),[20] identified by both X-ray diffraction and Raman spectroscopy measurements (see Supplementary Figures 1 and 2 as well as Supplementary Table 2).
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