Atomic defects in the ordered compound B2-NiAl:: A combination of ab initio electron theory and statistical mechanics

For an ideal model of a homogeneous thermodynamically stable ordered compound B2-NixAl1-x the effective formation energies and volumes of vacancies and antistructure atoms as well as the Ni and Al activities are calculated by a combination of the nb initio electron theory with a generalized grand canonical statistical approach. For nonstoichiometric compounds the structural defects an Ni vacancies (for x<0.5) or Ni antistructure atoms on the Al sublattice (for x>0.5). At stoichiometry (x=0.5) the calculated effective Ni vacancy formation energy agrees quite well with experimental data. For x<0.5 the theory predicts a shrinkage of the sample with increasing temperature (superimposed to the usual anharmonic lattice expansion) due to thermal annihilations of structural Ni vacancies, in contrast to the experimental observation. The reason for this discrepancy is probably deviations of the structure of real B2-NixAl1-x from the ideal model. [S0163-1829(99)05609-X].