Stability domains of Nb(CN) during the carburization/nitridation of metallic niobium

The conditions for the synthesis of a stoichiometric niobium carbonitride (Nb(CN)) are rather ambiguous despite previous experimental studies. Thus, it would be worth to establish the phase domain of Nb(CN) theoretically so that it can be used as a guideline for the successful synthesis of Nb(CN). In this study, Gibbs free energy of the formation of Nb(CN) during the carburization/nitridation of niobium was calculated by the ab initio simulation method. As a result, the phase stability diagram for Nb-NbC-NbN-Nb(CN) system was constructed with respect to the carbon activity and nitrogen pressure. The diagram suggests that only Nb and Nb(CN) would be the stable phases in the real system. In the mean time, the Nb(CN) domain for the real system was found to be somewhat broader than that for the case of ideal mixing of NbC and NbN. The difference can be attributed to the exothermic nature of the mixing reaction between NbC and NbN or the nitriding of NbC.