Poly-nitrogen compounds have been considered as potential high energy density materials for a long time due to the large number of energetic N–N or N=N bonds. In most cases high nitrogen content and stability at ambient conditions are mutually exclusive, thereby making the synthesis of such materials challenging. One way to stabilize such compounds is the application of high pressure. Here, through a direct reaction between Fe and N₂ in a laser-heated diamond anvil cell, we synthesize three ironnitrogen compounds Fe₃N₂, FeN₂ and FeN₄. Their crystal structures are revealed by single-crystal synchrotron X-ray diffraction. Fe₃N₂, synthesized at 50 GPa, is isostructural to chromium carbide Cr₃C₂. FeN₂ has a marcasite structure type and features covalently bonded dinitrogen units in its crystal structure. FeN₄, synthesized at 106 GPa, features polymeric nitrogen chains of [N₄²⁻]_n units. Based on results of structural studies and theoretical analysis, [N₄²⁻]_n units in this compound reveal catena-poly[tetraz-1-ene-1,4-diyl] anions.

High-pressure chemistry is known to inspire the creation of unexpected new classes of compounds with exceptional properties. Here, we employ the laser-heated diamond anvil cell technique for synthesis of a Dirac material ${\mathrm{BeN}}_4$. A triclinic phase of beryllium tetranitride $\mathrm{tr}\text{−}{\mathrm{BeN}}_4$ was synthesized from elements at $\sim 85\mathrm{GPa}$. Upon decompression to ambient conditions, it transforms into a compound with atomic-thick ${\mathrm{BeN}}_4$ layers interconnected via weak van der Waals bonds and consisting of polyacetylene-like nitrogen chains with conjugated $\pi$ systems and Be atoms in square-planar coordination. Theoretical calculations for a single ${\mathrm{BeN}}_4$ layer show that its electronic lattice is described by a slightly distorted honeycomb structure reminiscent of the graphene lattice and the presence of Dirac points in the electronic band structure at the Fermi level. The ${\mathrm{BeN}}_4$ layer, i.e., beryllonitrene, represents a qualitatively new class of 2D materials that can be built of a metal atom and polymeric nitrogen chains and host anisotropic Dirac fermions.

High-pressure synthesis in diamond anvil cells can yield unique compounds with advanced properties, but often they are either unrecoverable at ambient conditions or produced in quantity insufficient for properties characterization. Here we report the synthesis of metallic, ultraincompressible (K₀ = 428(10) GPa), and very hard (nanoindentation hardness 36.7(8) GPa) rhenium nitride pernitride Re₂(N₂)(N)₂. Unlike known transition metals pernitrides Re₂(N₂)(N)₂ contains both pernitride N₂⁴⁻ and discrete N³⁻ anions, which explains its exceptional properties. Re₂(N₂)(N)₂ can be obtained via a reaction between rhenium and nitrogen in a diamond anvil cell at pressures from 40 to 90 GPa and is recoverable at ambient conditions. We develop a route to scale up its synthesis through a reaction between rhenium and ammonium azide, NH₄N₃, in a large-volume press at 33 GPa. Although metallic bonding is typically seen incompatible with intrinsic hardness, Re₂(N₂)(N)₂ turned to be at a threshold for superhard materials.