Atomistic study on phase stability and electronic structures of Z phase CrNbNx (x = 1, 2, 3)

The site preference of N in Z-phase CrNbNx (x = 1, 2, 3) is investigated based on the phase stability from density functional theory. The electronic structures and elastic properties of Z-phase CrNbNx are also studied from first principles. The formation enthalpies of the Z phases and related N-rich Z phases with respect to the binary Cr- and Nb-nitride phases are calculated. The formation enthalpy of Z-phase CrNbNx (x = 1, 2, 3) and the Cr-N/Nb-N binary phases are negative, which indicates that these phases are energetic stable. The stability of N-rich Cr-N (Nb-N) binary compound is higher than that corresponding to N-poor binary compound, whereas the stability of the N-rich Z phase is lower than that of the Z-phase CrNbN. The ΔEf of CrNbN is more negative than ([Cr2N] + [Nb2N])/2, which indicates that the Z-phase CrNbN is more stable than ([Cr2N] + [Nb2N])/2. The ΔEf of CrNbN2 is less negative than ([CrN] + [NbN]), which indicates the CrNbN2 phase will spontaneously change into two phases (CrN and NbN). Z-phase CrNbN and II-CrNbN2 are mechanically stable; however, CrNbN3 and I-CrNbN2 are mechanically unstable. Evidently, the mechanical stability of the N-rich Z phases with N (0.5, 0.5, 0.5) is lower than that of the Z phases without N (0.5, 0.5, 0.5). The all results indicate that increasing N can suppress Z phase formation and promote Z phase changing into corresponding binary compounds for a system with sufficient Nb.