Search of stable structures in cation deficient (V,Nb)CoSb half-Heusler alloys by an atomic cluster expansion

Abstract

Using a cluster expansion scheme, we evaluate the structural stability of (V,Nb)CoSb half-Heusler alloys over a wide range of V and Nb concentrations when the alloys are simultaneously subjected to the creation of cation vacancies. We found that similar to their V_0.8CoSb and Nb_0.8CoSb counterparts, the alloys can form stable structures at around 20% of vacancy concentration and that the alloys with a high content of Nb present higher stability than those with a high content of V. Moreover, we found that the low-energy structures present a semiconductor gap in the electronic density of states. Monte Carlo simulations at finite temperatures reveal vacancy order coexisting with clusters of Nb or V atoms that have elongated shapes. A complex network of regions containing a rich and poor amount of Nb (V) atoms emerges when the concentration ratio of Nb and V is close to 0.5 with 20% of vacancies. These atomic-vacancy arrangements could reduce or lower the lattice thermal conductivity for thermoelectric applications.