First-principles study of ground-state properties and phase stability of vanadium nitrides

Using a series of density functional electronic structure total energy calculations, we have systematically studied the ground-state properties and phase stability of vanadium nitrides. Comparison of enthalpy of formation shows that V 2N is equally stable (polymorphic) in ϵ -Fe 2 N , ζ -Fe 2 N and Fe2C phases within a few meV. Formation enthalpy of the various phases considered for perfect stoichiometric V N1.0 shows that it has enhanced stability in hexagonal WC and NiAs structures in relation to NaCl-type δ -phase. The TiAs phase of VN has nearly same energy as NaCl structure. Comparison of energetics of ϵ -Fe 2 N -type β V 2 N 1 − x □ x , for x = 0 and 0.3333 and of δ VN 1 − x □ x , for x = 0 , 0.0625, 0.125 and 0.25 shows that vacancies on the nitrogen sublattice lowers the formation enthalpy in relation to respective stoichiometric phases which is in agreement with experiments, as bulk vanadium nitrides are known to be generally non-stoichiometric. The calculated dilute heat of solution for the interstitial nitrogen is found to be in good agreement with experimental values and shows that nitrogen prefers to occupy the octahedral sites in bcc vanadium. The α ′ -FeN and α ″ -Fe 16 N 2 martensite structures, considered for the metastable phases of vanadium nitrides, have higher formation enthalpy in relation to equilibrium phases. Analysis of electronic density of states of V 2N shows that the low energy ϵ -Fe 2 N , ζ -Fe 2 N and Fe2C phases are characterized by broad V 3d-N 2p and V 3d bonding bands. Density of states of VN shows that in the low energy WC and NiAs phases some of the antibonding states are made empty, leading to a minimum near the Fermi level. For β V 2 N 1 − x □ x and δ VN 1 − x □ x , density of states shows that vacancies on the nitrogen sublattice introduce additional filled states in the 3d band below Fermi level enabling enhanced bonding. Comparison between bulk moduli and atomic volumes for the various phases of vanadium nitrides shows that higher bulk moduli are dominated by increased V–N bonds combined with low atomic volumes.