First-principles study of phase stability of stoichiometric vanadium nitrides

Abstract

First-principles pseudo-potential total-energy and phonon spectrum calculations of various phases of stoichiometric vanadium nitrides with a small number of vanadium and nitrogen vacancies, as well 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, P6m2) -hexagonal (AsNi-type, P 63/mmc) -tetragonal (P 42/mcm) -cubic (NaCl-type, F m3m) -cubic (ZnS-type, F43m) -cubic (CsCl-type, P m3m). 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 in experimentally.

Phase stability and mechanical response of tantalum nitrides to electronic excitation effect

“…In particular, the PDOS of TaN 0.83 coincides with the results of θ-TaN structure, as highlighted in the inset of figure 1. We observed that these data are similar with those obtained for the first-principles study of phase stability of hexagonal and cubic structures of VN and NbN [26,27]. Compared with the acoustic branch, the optical phonon frequencies change very slightly and can be neglected.…”

“…A relatively small vacancy concentration stabilized the cubic WN phase and induced the mechanical stability transition from the NaCl phase to the NbO phase [24]. Ivashchenko et al verified that an increase in nitrogen vacancies can increase the frequency of soft mode and induce the stabilization of VN x and NbN x lattice structures [25][26][27][28][29][30][31]. In addition, the θ-TaN structure was stable at 1773-2073 K, while θ-TaN 0.8-0.9 materials have been obtained at a lower temperature (773-2073 K) due to improved nonmetal vacancy [9][10][11].…”

Dynamic and structural stability of cubic vanadium nitride

“…Based on our measured ρ o value, we establish an upper limit, f vac < 0.005, to the vacancy fraction in our VN layers which is an order of magnitude smaller than the value f vac ∼ 0.06 predicted in Ref. [8] to influence the structural stability of VN. Thus, VN has the NaCl structure at room temperature despite having a much lower vacancy concentration.…”

“…Three decades ago, Kubel et al [7] reported a cubic-totetragonal phase transition upon cooling bulk polycrystalline stoichiometric VN samples below 204 K. Lattice symmetry breaking was observed by x-ray diffraction and confirmed by heat capacity measurements. Recently, Ivashchenko et al [8] found, using density-functional perturbation theory, that the low-temperature tetragonal-VN structure is dynamically stable at 0 K, with all phonon modes exhibiting positive, real energies.…”

First-principles calculation and molecular dynamics simulation of fracture behavior of VN layers under uniaxial tension

“…The results are compared with those obtained using the first-principles calculations.a Reference[25]. b Reference[20]. c Reference[41].…”