Effect of interaction energies on the vacancy behaviour in B2 ordered intermetallics

We examine the correlation between the interaction energies used in a nearest neighbour mean field model and xV, the vacancy concentration, in B2 intermetallic alloys. The range of vacancy behaviour has been classified into four generic categories. In particular, a condition on interaction energies is derived for the alloys to exhibit constitutional vacancies (Type III behaviour in our classification). The existing theoretical results for NiAl, FeAl, CoAl, NiGa, CoGa and AuCd have been analysed within the framework of our classification scheme, and rationalized using the interaction energies used in those studies. Since different sets of interaction energies may predict the same vacancy behaviour, we conclude that experimental verification should not only use measurements of xV, but also the measurement of other point defects. An examination of the relative atomic size of B in comparison with the nearest neighbour distance in the B2 structure reveals that it could be an important reason for the aluminides to exhibit constitutional vacancies. However, this size effect is not a significant factor in NiGa, CoGa and AuCd.