Vacancy ordering in Co3 AlCx alloys: A first-principles study

Ordering of structural vacancies in nonstoichiometric Co3 AlCx alloys has been studied using a combination of first-principles total-energy calculations, a cluster expansion technique, and Monte Carlo simulations. In the proximity of the experimentally observed composition of x∼0.59, our exhaustive ground-state search yields two stable vacancy-ordered structures: a cubic Co3 AlC0.5 phase and a trigonal Co3 AlC0.667 phase. By performing finite-temperature Monte Carlo simulations, the order-disorder transition temperatures of Co3 AlC0.5 and Co3 AlC0.667 are predicted to be ∼1925 and ∼1630 K, respectively.