New Metastable Nitrogen Rich Nitrides of Titanium

The high-pressure and temperature synthesis technique has previously been used for discovering many nitrogen-bearing compounds that exhibit unique properties like high hardness, superconductivity and elctrocatalytic activity etc. Here, we report our experimental discovery of two new compounds in Ti-N system; 1) TiN₂ (titanium pernitride) and 2) Cubic-Ti₃N₄. Both the compounds were synthesized at pressures of a few tens of GPa and temperatures >2300K using diamond anvil cells (DACs) and characterized using in situ X-ray diffraction and Raman scattering. Our first principles calculations suggest that both the compounds exhibit distinct electronic and mechanical properties as compared to corresponding mononitride, TiN. The first non-noble metal pernitride,TiN₂ exhibits crystal structure that consists of single-bonded nitrogen dimers (N–N dumbbells) and It is an ultraincompressible material (bulk modulus ~385 GPa). TiN₂ is metallic and fully recoverable to ambient conditions and is stable in air. On the other hand Cubic-Ti₃N₄ is dynamically unstable below 5 GPa but it is the first known semiconducting nitride of titanium and exhibits a crystal structure with coordination numbers of Ti and N that are much higher than usual metal mononitrides with the rocksalt structure.