ABSTRACT
For binary lanthanide compounds, a theory has been proposed which postulates that the heat of formation of a binary phase H involves contributions bound to the interactions among the neighboring atoms in the particular crystal structure. Three kinds of contributions were distinguished: the difference in electronegativity of the two component elements; the difference in component atom electron density at the Wigner-Seitz cell boundary. The crystal structure data collected up to now for the lanthanide ternary phases indicate that they crystallize in a large number of different structure types. An attempt to explain this phenomenon turned out to be difficult, since the stability of a particular structure is dependent on many factors, such as the geometrical, the electron, the electrochemical, and the chemical bond system. However, the discovery of the superconductive and, in particular, the heavy fermion effect in a number of them has attracted considerable attention to the lanthanide ternaries.
