Atomic defects in the ordered compound B2-CoAl: A combination ofab initioelectron theory and statistical mechanics

Abstract

For an ideal model of a homogeneous thermodynamically stable ordered compound $B2\ensuremath{-}{\mathrm{Co}}_{x}{\mathrm{Al}}_{1\ensuremath{-}x},$ the effective formation energies of vacancies and antistructure atoms on both sublattices are calculated by a combination of ab initio electron theory and statistical mechanics. The results are compared with data for FeAl and NiAl obtained by the same procedure and with experimental results. The structural defects in ${\mathrm{Co}}_{x}{\mathrm{Al}}_{1\ensuremath{-}x}$ are Co antistructure atoms (for $x>0.5)$ and Co vacancies (for $x<0.5),$ albeit these latter defects are only slightly more favorable than Al antistructure atoms. This near balance possibly is responsible for conflicting experimental results obtained in real Al-rich samples. The dominant thermal excitations are also discussed.