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

TL;DR

This study uses first-principles calculations to identify new stable titanium nitrides, analyze their bonding and mechanical properties, and predict that nitrogen-rich compounds like TiN2 are particularly hard and mechanically stable.

Contribution

The paper discovers new stable titanium nitride phases and provides detailed insights into their bonding and mechanical properties using first-principles methods.

Findings

New stable titanium nitrides identified, including Ti3N2, Ti4N3, Ti6N5, Ti2N, and TiN2.

TiN2 predicted to be mechanically stable and possess the highest hardness among studied compounds.

Hardness increases with nitrogen content, driven by covalent bonding and loss of metallic bonds.

Abstract

We have performed first-principles evolutionary searches for all stable titanium nitrides and have found, in addition to the well-known rocksalt-type TiN, new ground states Ti$_3$N$_2$, Ti$_4$N$_3$, Ti$_6$N$_5$ at atmospheric pressure, and Ti$_2$N and TiN$_2$ at higher pressures. The latest nitrogen-rich structure 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 excellent agreement with the available experimental data and show that the hardness of titanium nitrides increases with increasing nitrogen content. The hardness of titanium nitrides is enhanced by strengthening directional covalent bonds and disappearance of Ti-Ti metallic bonds. Among the predicted compounds, TiN$_2$ has the highest hardness of 27.2 GPa.