Nitrogen-rich transition metal nitrides

Abstract

The solid state chemistry leading to the synthesis and characterization of metal nitrides with N:M ratios > 1 is summarized. Studies of these compounds represent an emerging area of research. Most transition metal nitrides have much lower nitrogen contents, and they often form with nonor sub-stoichiometric compositions. These materials are typically metallic with often superconducting properties, and they provide highly refractory, high hardness materials with many technological applications. The higher metal nitrides should achieve formal oxidation states (OS) attaining those found among corresponding oxides, and they are expected to have useful semiconducting properties. Only a very few examples of such high OS nitrogen-rich compounds are known at present. The main group elements typically form covalently bonded nitride ceramics such as Si 3 N 4 , Ge 3 N 4 and Sn 3 N 4 , and the early transition metals Zr and Hf produce Zr 3 N 4 and Hf 3 N 4 . However, the only main example of a highly nitrided transition metal compound known to date is Ta 3 N 5 , that has a formal oxidation state +5 and is a semiconductor with visible light absorption leading to applications as a pigment and in photocatalysis. New synthesis routes are being explored to study the possible formation of other N-rich materials that are predicted to exist by ab initio calculations. There is a useful interplay between theoretical predictions and experimental synthesis studies at ambient and high pressure conditions, as we explore and establish the existence and structure-property relations of these new nitride compounds and polymorphs. Here we review the state of current investigations and indicate possible new directions for further work.[a] European Synchrotron Radiation Facility,

Nitriding VI-group metals (Cr, Mo and W) in stream of NH3 gas under concentrated solar irradiation in a solar furnace at PSA (Plataforma Solar de Almería)

“…Zerr et al, 2003;Salamat et al, 2013). This line of efforts are certainly of great scientific merit that would contribute to open new horizon for novel nitride synthesis.…”

Thermodynamic Routes to Novel Metastable Nitrogen-Rich Nitrides

“…29 Syntheses of oxides with reactive oxygen precursors, such as peroxide and ozone, have also resulted in highly oxidized transition metal cations, 45 suggesting that the use of high chemical potential anion precursors may be a general strategy to achieve high metal oxidation states. In summary, the low chemical potential of N 2 drives metal−nitride compositions toward the nitrogen-poor "subnitrides", where the nitrogen atom occupies interstitial sites of a metal sublattice (analogous to metal carbides) and does not adopt an anionic charge, leading to metallic electronic structures.…”

“…In nitrides, the low chemical reactivity of the N 2 molecule drives phase equilibrium toward nitrogen-poor compositions, typically leading to reduced nitrides with metallic electronic structure. 29 However, the metastable nitrogen-rich phases should possess cations in the higher oxidation states, manifesting in useful semiconducting properties, and are thus of particular technological interest for electronic and optoelectronic/photovoltaic applications.…”

High-Pressure Annealing of a Prestructured Nanocrystalline Precursor to Obtain Tetragonal and Orthorhombic Polymorphs of Hf3N4

“…This is due to the highly exothermic formation enthalpy of the lower metal nitrides allied with the thermodynamic stability of gaseous nitrogen. Higher nitrides such as Ti 3 N 4 should be stable 1 and could be accessed if a suitable synthesis route is found to achieve them. Lower band gaps are expected relative to the oxides, giving rise to semiconducting and photocatalytic properties, as well as highly colored compounds that can be used as pigments e.g.…”