The effective formation energies and volumes of vacancies and antistructure atoms, as well as Ni and Al activities, were calculated for an ideal model of an homogeneous thermodynamically-stable ordered B2 compound by using a combination of ab initio electron theory and generalized grand canonical statistics. In non-stoichiometric compounds, the structural defects were Ni vacancies (for Ni molar fractions of less than 0.5) or Ni antistructure atoms on the Al sub-lattice (for Ni molar fractions of more than 0.5). At stoichiometry, the calculated effective Ni vacancy formation energy agreed quite well with experimental data. For lower Ni contents, the theory predicted a shrinkage of the sample with increasing temperature (superposed on the usual anharmonic lattice expansion) due to the thermal annihilation of structural Ni vacancies. This was in contrast to experimental observations. This discrepancy was attributed to deviations, of the structure of real B2-NiAl, from the ideal model.

Atomic Defects in the Ordered Compound B2-NiAl: a Combination of ab initio Electron Theory and Statistical Mechanics. B.Meyer, M.Fähnle: Physical Review B, 1999, 59[9], 6072-82