A unified model for point-defect formation in B2 intermetallic compounds

With the mean-field first-nearest-neighbour interaction approximation, we derived a unified model for point defect formation in B2-structured intermetallic compounds, which accounts for all kinds of defect structures including triple-defect (TRD) type, antistructure defect (ASD) type and hybrid type. This mathematically simple model clearly manifests the physics underlying the point-defect production and defect types in intermetallics, that is the competition between two entropy production processes (ASD formation and vacancy formation) determines the type of intermetallics and defect concentrations. This model yields excellent quantitative agreement with experimental results both on the composition dependence and on the temperature dependence of vacancy concentrations for three different types of intermetallic. Based on recent experimental results and present model, we conclude that there exist no constitutional vacancies in all intermetallics including the well known NiAl, and the abnormally high vacancy concentration in such TRD compounds can be easily understood in terms of the ASD-vacancy competition. The energy barrier between the forming vacancy configuration and the ASD configuration determines three apparently different types of B2 intermetallic, and there exists no fundamental difference between three apparently different types of intermetallic. Finally, we predict firstly that the vacancy concentration is asymmetric with respect to stoichiometry for all B2 intermetallics including the ASD type and secondly that the vacancy concentration at elevated temperatures may be quite high (0.5-1%) even in ASD-type B2 intermetallics, which have been believed to contain only a negligible amount of vacancies. The physical principle of point-defect formation established in the present study for B2 intermetallics may apply to intermetallics of other structures.