Site occupation in the Ni-Nb μ phase

The site occupation in the Ni-Nb μ phase is computed from first principles as a function of temperature and composition. It is shown that an empirical rule of thumb based on atomic size, as well as a recently formulated rule based on the approximate point group and the degeneracy of the electronic d-like states, can explain the gross features of the site occupations. However, neither rule explains the distinct behavior of the 12-fold-coordinated 3a and 18h sites in the μ phase. The strong Ni preference of the 18h site and the mixed occupancy of the 3a site is explained on the basis of an analysis of ordering tendencies. Predicted site occupations qualitatively agree with recent Rietveld measurements. A comparison with the compound energy model is made and the limitations of the latter are discussed.