Electronic origin of the isostructural decomposition in cubic M 1− xAl xN (M=Ti, Cr, Sc, Hf): A first-principles study

Abstract

We have used first-principles calculations to investigate the mixing enthalpies, lattice parameters and electronic density of states of the ternary nitride systems Ti 1− x Al x N, Cr 1− x Al x N, Sc 1− x Al x N and Hf 1− x Al x N in the cubic B1 structure where the transition metals and aluminium form a solid solution on the metal sublattice. We discuss the electronic origins of the possible isostructural decomposition in these materials relevant for hard coatings applications. We find that in the systems Ti 1− x Al x N and Hf 1− x Al x N the electronic structure effects strongly influences the phase stability as d-states are localised at the Fermi level in AlN-rich samples. This leads to a strongly asymmetric contribution to the mixing enthalpy, an effect not present in Cr 1− x Al x N and Sc 1− x Al x N. The lattice mismatch is large in Sc 1− x Al x N and Hf 1− x Al x N, giving a symmetric contribution to the mixing enthalpies in those systems.