Nitrogen-rich transition metal nitrides

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 non- or 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 Si3N4, Ge3N4 and Sn3N4, and the early transition metals Zr and Hf produce Zr3N4 and Hf3N4. However, the only main example of a highly nitrided transition metal compound known to date is Ta3N5 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. (C) 2013 Elsevier B.V. All rights reserved.