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 Ti3N2, Ti4N3, Ti6N5 at atmospheric pressure, and Ti2N and TiN2 at higher pressures. The latest nitrogen-rich structure presents encapsulated N2 dumbbells with a N-N distance of 1.348 {\AA} at 60 GPa and TiN2 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, TiN2 has the highest hardness of 27.2 GPa.
@article{arxiv.1411.1485,
title = {Phase stability, chemical bonding and mechanical properties of titanium nitrides: A first-principles study},
author = {Shuyin Yu and Qingfeng Zeng and Artem R. Oganov and Gilles Frapper and Litong Zhang},
journal= {arXiv preprint arXiv:1411.1485},
year = {2014}
}