While there are many iron nitrides, none with an N/Fe ratio larger than one had been synthesized so far. Pernitrides of the type MN₂ with other transition metals had been prepared under high pressures and temperatures. Theoretical calculations predict that the iron pernitride FeN₂ should be stable under pressure.

Dominique Laniel, Commissariat à l’énergie atomique et aux énergies alternatives (CEA), Direction des applications militaires Île-de-France (DIF), Arpajon, France, and colleagues have synthesized and characterized FeN₂. The team used laser-heated diamond anvil cells to compress iron powder under a nitrogen atmosphere and obtained the pernitride at pressures above 72 GPa and temperatures above 2200 K. The reaction was monitored using powder X-ray diffraction and Raman spectroscopy.

FeN₂ has a marcasite-type structure (pictured, N in blue, Fe in dark orange).The bond order in the nitrogen dimers was determined to be about 1.5 based on the lattice parameters and Raman frequencies. The compound has a bulk modulus (a measure of resistance to compression) of 344 GPa, which represents a more than 200 % increase over that of pure iron.

• High Pressure and High Temperature Synthesis of the Iron Pernitride FeN₂,
  Dominique Laniel, Agnès Dewaele, Gaston Garbarino,
  Inorg. Chem. 2018.
  https://doi.org/10.1021/acs.inorgchem.7b03272