Metallic to insulating nature of (formula presented) Role of Ta and N vacancies

It has been demonstrated recently that the stoichiometry of rocksalt TaN can be tuned by (formula presented) 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 (formula presented) We also studied the (formula presented) and (formula presented) 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 (formula presented) 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 Ta-deficient structures is directly related to the metal-to-insulator transition.