Metallic to insulating nature of TaN_x: Role of Ta and N vacancies

It has been demonstrated recently that the stoichiometry of rocksalt TaN can be tuned by N₂ pressure and temperature, yielding material that ranges from highly conductive to insulating. Using density functional theory, we investigate the atomic and electronic structure and formation energy of defective TaN structures. The calculations predict that Ta and N vacancies form under N-rich and N-poor conditions, respectively, where the presence of Ta vacancies reduce the density of states (DOS) around the Fermi level (E_F). We also studied the Ta₄N₅ and Ta₃N₅ structures which occur in nature. The former phase, consisting of an ordered arrangement of Ta vacancies, also exhibits a notable decrease in the DOS at E_F, while the latter is a semiconductor with a band gap of 1.5 eV within the local density approximation. Our results suggest that the formation of Tadeficient structures is directly related to the metal-to-insulator transition.