First-principles study of phase stability of stoichiometric vanadium nitrides

First-principles pseudopotential total-energy and phonon-spectrum calculations of various phases of stoichiometric vanadium nitrides with a small number of vanadium and nitrogen vacancies, as well as with small carbon and oxygen admixtures, were carried out. It was found that the stoichiometric hexagonal and tetragonal vanadium nitrides (VN) were dynamically stable. The NaCl-type VN exhibited lattice instability. The total energy for the computed ground-state phases of VN increased in the sequence hexagonal (WC type, P6̄m2)--hexagonal (AsNi type, P6₃/mmc)--tetragonal (P4₂/mcm)--cubic (NaCl type, Fm3̄m)--cubic (ZnS type, F4̄3m)--cubic (CsCl type, Pm3̄m). At low temperatures, the vacancies in both sublattices stabilize the triclinic phases of VₓNₓ for $x<0.94$, which is consistent with tetragonal symmetry. At high temperatures, the stability of the NaCl- and WC-type VN structures was estimated using an approach based on band-energy smearing. The results obtained indicate that the cubic structure of VN with the fully occupied sublattices will be stable only at high temperatures. The small carbon and oxygen admixtures do not stabilize the cubic phase compared to the hexagonal one. The phase stability of the cubic and hexagonal structures of VₓNₓ was explained on the basis of the peculiarities of their electronic densities of states near the Fermi level. The predicted structural transformations in stoichiometric vanadium nitrides are compatible with those observed experimentally.