High Pressure and High Temperature Synthesis of the Iron Pernitride FeN₂

Abstract

The high pressure chemistry of transition metals and nitrogen was recently discovered to be richer than previously thought, due to the synthesis of several transition metal pernitrides. Here, we explore the pressure-temperature domain of iron with an excess of nitrogen up to 91 GPa and 2200 K. Above 72 GPa and 2200 K, the iron pernitride FeN is produced in a laser-heated diamond anvil cell. This iron-nitrogen compound is the first with a N/Fe ratio greater than 1. The FeN samples were characterized from the maximum observed pressure down to ambient conditions by powder X-ray diffraction and Raman spectroscopy measurements. The crystal structure of FeN is resolved to be a Pnnm marcasite structure, analogously to other transition metal pernitrides. On the basis of the lattice's axial ratios and the recorded N-N vibrational modes of FeN, a bond order of 1.5 for the nitrogen dimer is suggested. The bulk modulus of the iron pernitride is determined to be of K = 344(13) GPa, corresponding to an astounding increase of about 208% from pure iron. Upon decompression to ambient conditions, a partial structural phase transition to the theoretically predicted R3̅ m FeN is detected.

High‐Pressure Synthesis and Phase Stability of Nickel Pernitride

“…[19,20] These peaks are indexed to the orthorhombic symmetry, although a few unidentified peaks remain. The obtained Miller indices and axial ratios are consistent with those of so-far-reported marcasite-type transition metal pernitrides (FeN 2 , [14,15] CoN 2 , [13] OsN 2 , [21] RuN 2 , [22] and RhN 2 [23] ). On the other hand, the arsenopyrite-type structure is also suggested as plausible crystal structure for the late transition metal pernitrides because the arsenopyrite-type structure (monoclinic) is formed via a small distortion from the marcasite-type one (orthorhombic) and the difference of the formation enthalpy between these structures is likely small.…”

“…It is found that the c-and a-axes are the most compressible and incompressible axes, respectively. The order of axial compressibility ( a = 4.4 × 10 -3 GPa -1 , b = 5.3 × 10 -3 GPa -1 , and c = 7.8 × 10 -3 GPa -1 for the a-, b-, and c-axes, respectively) is consistent with that of the other transition-metal marcasite-type pernitrides (FeN 2 , [14,15] CoN 2 , [13] RuN 2 , [22] and RhN 2 [23] ). The experimentally determined K 0 of marcasite-type pernitrides are reported for RhN 2 [K 0 = 235 (13) GPa], [23] CoN 2 [K 0 = 223 (5) GPa], [13] and FeN 2 [K 0 = 250 (16) GPa, 344(13) GPa], [14,15] and OsN 2 (K 0 = 358 GPa).…”

“…TiN 2 consists of the antiprism TiN 8 connected via N‐N dimer and shows remarkable incompressibility with a high bulk modulus . The late transition metal (Fe and Co) coordinates to six nitrogen and forms a marcasite‐type pernitride that consists of an octahedron ( TM N 6 , TM : transition metal) connected via N‐N dimer . The recently discovered CuN 2 consists of the triangular prism CuN 6 connected by a short distanced N‐N dimer along the c ‐axis .…”

“…[8][9][10][11] Recent advanced high-pressure experiments demonstrated that highly coordinated 3d transition metal nitrides and pernitrides are successfully synthesized under high pressure. [12][13][14][15][16] Bhadram et al reported the synthesis of recoverable CuAl 2 -type TiN 2 via a reaction between TiN and molecular nitrogen at 73 GPa. [12] The late transition metals of iron, cobalt, and copper were also discovered to form pernitrides (FeN 2 , CoN 2 , and CuN 2 ) above pressures of approximately 58 [14] or 72, [15] 30, [13] and 50 GPa, [16] respectively.…”

“…transition metal) connected via N-N dimer. [13][14][15] The recently discovered CuN 2 consists of the triangular prism CuN 6 connected by a short distanced N-N dimer along the c-axis. [16] The polyhedral configuration and the alignment of the N-N dimer control the mechanical and electronic properties of the transition metal pernitrides.…”