Prediction of Stable Iron Nitrides at Ambient and High Pressures with Progressive Formation of New Polynitrogen Species

Abstract

Nitride materials are of considerable interest due to their fundamental importance and practical applications. However, synthesis of transition metal nitrides often requires extreme conditions, e.g., high temperature and/or high pressure, slowing down the experimental discovery. Using global structure search methods in combination with first-principles calculations, we systematically explore the stoichiometric phase space of iron–nitrogen compounds on the nitrogen-rich side at ambient and high pressures up to 100 GPa. Diverse stoichiometries in the Fe–N system are found to emerge in the phase diagram at high pressures. Significantly, FeN4 is found to be stable already at ambient pressure. It undergoes a polymerization near 20 GPa which results in a high energy density. Accompanying the polymerization, FeN4 transforms from a direct band gap semiconductor to ferromagnetic metal. We also predict several phase transitions in FeN and FeN2 at high pressure, and the results explain the previous experimental observations by comparing the X-ray diffraction patterns. Stepwise formation of polynitrogen species is observed following the increment of nitrogen content in the stoichiometry, from isolated N atoms in FeN, to the N2 unit in FeN2 and Fe3N8, to the N6 unit in Fe3N8 and FeN3, and to the N∞ chain in FeN4, FeN6, and FeN8. Ultra-incompressibility is found in marcasite-FeN2, FeN3, and FeN4 along particular crystalline directions, while high energy density, 1.37–2.02 kJ g–1, is expected for FeN4, FeN6, and FeN8. Our results shed light on understanding the chemistry of transition metal polynitrides under pressure and encourage experimental synthesis of newly predicted iron nitrides in the near future.

In-situ growth of iron mononitride thin films studied using x-ray absorption spectroscopy and nuclear resonant scattering

“…In fact such N−N or N≡N bonds are also expected to be present as polymeric N chains in N rich FeN x (x > 1), evidenced recently under HPHT by Bykov et al [16]. Recently Wu et al [17] predicted the existence of FeN 4 phase even under ambient pressure and temperature. The occurrence of such a sharp N K-edge feature may also stem from the presence of polymeric N chains in ultrathin FeN films but this needs to be further confirmed.…”

“…the spinel nitride Fe 3 N 4 [13,14] and the pernitride FeN 2 [15], only very recently the FeN 2 phase has been synthesized experimentally under HPHT (T ≈ 2000 K, P ≈ 50 GPa) [12,16] and by raising the pressure above 100 GPa, the FeN 4 phase was also evidenced by Bykov et al [16]. Very recently even higher N phases like FeN 6 and FeN 8 have been predicted [17]. Polymeric nitrogen chains present in N rich FeN x compounds makes them very attractive as high energy density materials (HEDMs), however, their synthesis under ambient temperature and pressure remains a challenge.…”

“…Subsequently, FeN thin films were synthesized using ion beam sputtering [27], dc/rf magnetron sputtering [28][29][30][31][32][33][34][35][36][37][38], pulsed laser deposition (PLD) [39][40][41][42], high power impulse magnetron sputtering [43], nitrogen plasma assisted molecular beam epitaxy (MBE) [44][45][46][47][48] and very recently under HPHT [12,16,[49][50][51]. From applications points of view, the mononitride FeN is also very interesting as its oxidation resistance makes it a effective catalyst in chemical reactions [52,53], it can be used as a precursor to yield magnetic phases in a controlled way [38,44,54,55] and also in biomedical applications [17]. However, the fundamental understanding of mononitride FeN compound is still not well understood and variances can be seen between theory and experiments in terms of the lattice parameter (LP), structure and the magnetic ground state.…”

“…Considering recent theoretical works [17,[56][57][58][59][60][61] and comparing with experimental works, various possibilities emerge about the structure and the magnetic ground state of FeN (i) γ -FeN with zinc-blende (ZB)-type structure (LP ≈ 4.3Å) and a nonmagnetic (NM) ground state (ii) γ -FeN with rock-salt (RS)-type structure (LP ≈ 4.5Å) and a ferromagnetic (FM) or antiferromagnetic (AFM) ground state (iii) NiAs-type structure with a FM ground state (iv) CsCl-type structure with NM ground state (iv) wurtzite-type structure with a NM ground state (v) MnP-type structure at very high pressure. Among these, FeN in CsCl, wurtzite and MnP structures has not yet been synthesized experimentally, NiAs-type structure was experientially evidenced recently under HPHT [12,16,[49][50][51].…”

Solid Nitrogen and Nitrogen‐Rich Compounds as High‐Energy‐Density Materials

“…[27] Moreover, theoretical structure search reveals that there are other FeN 4 and FeN 6 entities at low-pressure regions that are potentially synthesizable. [114] Recently, the same group synthesized a series of metal-inorganic frameworks (MIFs) with polymeric nitrogen linkers at pressures exceeding 100 GPa, including [115,116] The porous frameworks (with molecular N 2 enclosed) are built from transitionmetal atoms linked either by nitrogen chains or through dinitrogen units. The chains structures presented in these compounds are being seen as a common building block of nitrogen-rich compounds under high pressure.…”