Phase stability, chemical bonding and mechanical properties of titanium nitrides: A first-principles study

We have performed first-principles evolutionary searches for stable Ti-N compounds and have found, in addition to the well-known rock-salt TiN, new ground states Ti₃N₂, Ti₄N₃, Ti₆N₅ at atmospheric pressure, and Ti₂N and TiN₂ at higher pressures. The latter nitrogen-rich structure contains encapsulated N₂ dumbbells with a N-N distance of 1.348 Å at 60 GPa. TiN₂ is predicted to be mechanically stable and quenchable. Our calculations on the mechanical properties (bulk modulus, shear modulus, Young's modulus, Poisson's ratio, and hardness) are in excellent agreement with the available experimental data. Further analyses of the electronic density of states, crystal orbital Hamilton population and the electron localization function reveal that the hardness is enhanced by strengthening directional covalent bonds and disappearance of Ti-Ti metallic bonding.