High-Pressure Synthesis and Characterization of Incompressible Titanium Pernitride

A new transition-metal pernitride, TiN₂, was uncovered from the chemical reaction of TiN with N₂ at 73GPa in a laser-heated diamond anvil cell (DAC). The crystal structure of this compound exhibits tetragonal I₄/mcm symmetry and consists of single-bonded nitrogen dimers (N–N dumbbells) embedded in the metal lattice as shown by our ab initio calculations and verified by in situ synchrotron x-ray diffraction measurements. The pressure-volume equation of state determined from the experimental data reveals that TiN₂ is incompressible with bulk modulus in the range of 360-385 GPa which is close to that of cubic boron nitride (382 GPa). Here, the origin of high bulk modulus of TiN₂ (which is metallic) is rooted in the nearly filled anti-bonding states of the pernitride units. TiN₂ is fully recoverable to ambient conditions and represents the lowest-density transition metal pernitride synthesized to date.