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

摘要

We have performed first-principles evolutionary searches for all stabletitanium nitrides and have found, in addition to the well-known rocksalt-typeTiN, new ground states Ti$_3$N$_2$, Ti$_4$N$_3$, Ti$_6$N$_5$ at atmosphericpressure, and Ti$_2$N and TiN$_2$ at higher pressures. The latest nitrogen-richstructure presents encapsulated N$_2$ dumbbells with a N-N distance of 1.348{\AA} at 60 GPa and TiN$_2$ is predicted to be mechanically stable(quenchable). Our calculations of the mechanical properties (bulk modulus,shear modulus, Young's modulus, Poisson's ratio, and hardness) are in excellentagreement with the available experimental data and show that the hardness oftitanium nitrides increases with increasing nitrogen content. The hardness oftitanium nitrides is enhanced by strengthening directional covalent bonds anddisappearance of Ti-Ti metallic bonds. Among the predicted compounds, TiN$_2$has the highest hardness of 27.2 GPa.