Synthesis of magnesium-nitrogen salts of polynitrogen anions

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 N₄^4- 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.

Fig. 1

The crystal structure of the MgN₄ and β-Mg₂N₄ salts at 58.5 GPa. a The unit cell of MgN₄ (the light blue, dark blue and orange spheres represent the N1, N2 and Mg atoms, respectively); b a projection of the MgN₄ structure along the c-axis, emphasising 1D chains of nitrogen atoms aligned along the a-axis; c a repeating N₄²⁻ subunit of a chain with the N-N distances and angles indicated; d the unit cell of β-Mg₂N₄ (the light green and dark green spheres represent the four distinct nitrogen atoms forming the a-N₄⁴⁻ and b-N₄⁴⁻ units, respectively, the orange spheres represent Mg atoms); e a projection of the β-Mg₂N₄ structure along the b-axis allowing to see the alternating layers of isolated a-N₄⁴⁻ and b-N₄⁴⁻ units, intercalated with Mg²⁺ ions. f The a-N₄⁴⁻ (left) and b-N₄⁴⁻ (right) entities with bond lengths and angles indicated

Fig. 2

Evolution of the Raman modes of the Mg₂N₄ compounds with pressure. a Typical Raman spectra of Mg₂N₄ taken during decompression from 53.5 GPa to ambient conditions. The spectra at ambient conditions is markedly different than those at higher pressures, evidencing a phase transition. b Pressure dependence of the Raman modes' frequencies, which evolve smoothly and continuously down to 2.3 GPa. Red dots indicate the modes at ambient pressure. The spectra are offset along the y-axis for clarity and ×5 indicates that the spectrum is five-times magnified

Fig. 3

Unit cell volume of the Mg₂N₄ compounds as a function of pressure. All solid and open symbols are experimental and theoretical data points, respectively, while the dashed line is the fit of the experimental PV data of the β-Mg₂N₄ compound using the second order Birch-Murnaghan equation of state (BM2 EoS) (V₀ = 461(9) ų and K₀ = 121(17) GPa). Red and blue symbols are the experimental and theoretical, respectively, unit cell volume of α-Mg₂N₄ at ambient pressure. The extrapolation of the experimental equation of state suggests a volume jump of 9.7% between the high pressure β-Mg₂N₄ and the ambient pressure α-Mg₂N₄ phases. The fit of the theoretical PV data using the BM2 EoS gives for β-Mg₂N₄ V₀ = 470.23 ų and K₀ = 110.83 GPa. The theoretical volume difference is thus of 10.2% between the calculated β-Mg₂N₄ and α-Mg₂N₄. The insets show the dependence of the unit cell parameters on pressure. The full (open) black and full red (open blue) symbols represent experimental (theoretical) data from the β and α phases of Mg₂N₄. The slightly higher volume obtained from the DFT calculations, compared to the experimental values, shows the underbinding in GGA

Fig. 4

Crystal structure of the α-Mg₂N₄ salt at ambient conditions. a The unit cell of α-Mg₂N₄ (see also Supplementary Fig. 16). b The bond lengths and angles in the two distinct N₄⁴⁻ entities: the a’-N₄⁴⁻ (light green, top) and b’-N₄⁴⁻ (dark green, bottom). The orange and green spheres represent Mg and N atoms, respectively