Washington, DC, 18 March 2016—Geophysical Laboratory team Tim Strobel, Venkat Bhadram, and alum DuckYoung Kim, has discovered a new transition metal, titanium pernitride, TiN2, which is ultraincompressible (bulk modulus ~360-385GPa) and could be a potential superhard material. Using a laser-heated diamond anvil cell (LHDAC) titanium nitride (TiN) and nitrogen (N2) were compressed to 73 gigapascals and heated up to 2400K. At these extreme conditions titanium nitride reacted with nitrogen to form a new compound, titanium pernitride (TiN2).

This discovery is a part of an on-going search for new semiconductor materials in the Ti-O-N system that could possibly be useful for photocatalytic water splitting reactions to produce H2 from water. TiO2 is a well-known semiconductor photocatalyst which works in the UV region of the solar spectrum due to its wide bandgap (~3 eV). It is established that nitrogen doping within TiO2 can reduce its band gap and enhances photocatalytic properties in the visible light region.  But what would happen if all of the oxygen in TiO2 is replaced with nitrogen? The result would be TiN2, a new material. Although there were a few theoretical reports that predicted the crystal structure and properties of TiN2 before, there was no prior experimental evidence to validate these predictions.

                           Caption: Schematic that represents the reaction conditions and crystal structure of TiN2

While high-pressure and high-temperature conditions were used for its synthesis, TiN2 is a dynamically stable at ambient conditions and exhibits a unique crystal structure with single-bonded dinitrogen units (N24-–pernitride ions) embedded in a metal lattice. The appearance of single N–N bonds in materials is rare as it requires filling of high-energy anti-bonding molecular orbitals of dinitrogen. The N–N bond length in TiN2 is ~1.383 Å and is comparable to the F–F bond length (1.42 Å) in the F2 molecule, which is isoelectronic to the pernitride ion. The filling of anti-bonding orbitals serves to elongate the nitrogen covalent bonds and makes them more resistant to external stress. Several pernitride compounds were discovered previously, but the transition metals belong to the noble metal group. TiN2 is the first non-noble metal pernitride and the lowest-density transition metal pernitride synthesized to date.

This work, which is published in Chemistry of Materials, has important implications for the synthesis of new metal nitrides and the understanding their structure–property relationships. TiN2 turns out to be metallic, but it places important end-member constraints on new photocatalysts in the ternary Ti-O-N system.    


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