The two-correlations model, a valence model for metallic phases

Abstract

To build up a theory of metallic phase stability the conventional arguments for the discussion of crystal structures have to be completed by the concept of spatial correlation of electrons. The parameters of the spatial correlations may be analyzed from the ample empirical material of determined crystal structures by means of several evident rules. The surprising result is that for many crystal structures two correlations are essential for understanding the special features of the crystal structures. This two-correlations model makes possible an easy survey of metallic structures. Two examples of crystal structure-type families are considered, the Cu-family and the W-family. In the Cu-family substitutional variants, simple shear variants, multiple shear variants, and displacive variants may be distinguished. In the W-family cubic substitutional variants, deformed substitutional variants, vacancy variants with weak deformations, variants with stronger deformations, and variants with row shear may be distinguished. The essential feature of the present exposition is that the structural types are not only organized in a geometrical manner but that the arrangement of the structural types is put on an energetic base. This energetic base gives an answer to the fundamental question why certain phases are formed in certain mixtures.