Structure and stability of phases within the NbN–AlN system

The outstanding electronic and physical properties of NbN and the continuous need for advanced materials, e.g. in the field of hard coatings, motivated us to investigate the Nb1−xAlxN pseudo-binary alloy by means of first principles calculations and experimental studies. The experimentally reported phases for NbN and AlN, cubic B1, hexagonal Bk, B81 (δ'-NbN) and Bi (ε-NbN) and wurtzite B4 were considered for the calculations. The phase stability analysis based on the comparison of formation energies of various phases yields that the Nb-rich alloys favour the hexagonal B81-like structure (up to an AlN fraction of x ≈ 0.14), the Al-rich side favours the wurtzite B4 structure (for x ⪆ 0.7) and the intermediate compositions favour the cubic B1 phase. The available hybridization schemes together with the analysis of the density of states and the (difference) charge density maps provide reasoning for the obtained results. Experimental investigations confirm preferred B1 structure for Nb1−xAlxN coatings with x below 0.45, a mixed B1, Bk and B4 structure for x in the range 0.45–0.71 and a preferred B4 structure for x above 0.71.