On the shear homeotypism in the MgCu2 family

It is well known that there are several homeotypes of MgCu₂ which are distinguished by different shears between double layers of atoms parallel to (111) MgCu₂ (Friauf-Laves phases) and that they are stable at specific valence electron concentrations (rule of Komura). This phenomenon may be interpreted by the two-correlations model, a valence model for metallic phases. The correlation of the peripheral core electrons (c correlation) and its commensurability to the crystal cell a is the same for all quasi-homologous shear homeotypes of MgCu₂, but the correlation of the valence electrons (b correlation) depends on the valence-electron concentration. The b correlation exhibits different discrete commensurabilities with respect to the c correlation in the hexagonal basal plane of a. Also, the distances of the b correlation planes parallel to the hexagonal basal plane exhibit energetically favorable commensurabilities to a. These different commensurabilities correspond to observed phases. The b electron correlation induces transitory electrical dipole vectors at the atoms with component in a₃ direction (the hexagonal axis) and the interactions of the dipoles determine the stacking sequence of the double layers.