Ab initio investigations of the phase stability in group IVB and VB transition metal nitrides

Abstract

Using a combination of electronic structure density functional theory and an evolutionary algorithm, we predicted the thermodynamically stable structures of the group IVB and VB transition metal nitrides at 0K. Our results demonstrate that in the group IVB nitrides there is a competition between nitrogen interstitial ordered forms of the hexagonal close packed structure and nitrogen vacancy ordered forms of the B1 structure. The low metal vacancy formation energies in the B1 structure even allowed metal vacancy ordered forms of the B1 HfN structure to be thermodynamically stable. The work also revealed how bond distortion and bond angles allowed the predicted Ti2N and Zr2N P4/mnm phases to be stable as compared to the Pnnm structure for Hf2N. This work also predicted several metal-rich phases to be present in the Zr-N system, which have yet to be observed potentially owing to this system being the least experimentally studied of the group IVB nitrides. In the group VB nitrides, the low formation enthalpies of the ε-TaN prototype and Bh structures, as well as the C6 structure, suppress the dominance of the B1 phase at low temperatures. The collective results provide in-depth and inclusive examination of the similarities and differences for phase stability in these two classes of transitional metal nitrides at 0K.