  Quantum mechanical calculations had been previously applied to predict phase
stability in many ternary and multinary nitride systems. While the predictions
were very accurate for the Ti-Al-N system, some discrepancies between theory
and experiment were obtained in the case of other systems. Namely, in the case
of Ta-Al-N, the calculations tend to overestimate the minimum Al content
necessary to obtain a metastable solid solution with a cubic structure. In this
work, we present a comprehensive study of the impact of vacancies on the phase
fields in quasi-binary TaN-AlN and NbN-AlN systems. Our calculations clearly
show that presence of point defects strongly enlarges the cubic phase field in
the TaN-AlN system, while the effect is less pronounced in the NbN-AlN case.
The present phase stability predictions agree better with experimental
observations of physical vapour deposited thin films reported in the literature
than that based on perfect, non-defected structures. This study shows that a
representative structural model is crucial for a meaningful comparison with
experimental data.
