ON THE PROBLEM OF A UNIFIED STUDY OF HCP METALS FROM A PERTURBATIVE APPROACH - A POSSIBLE RESOLUTION

The present work is an attempt to identify the role of pseudopotential third order energy in different lattice mechanical properties of HCP metals. Although it plays an important role in explaining the typical features of the phonon dispersion curves, the third order contribution to the individual elastic constants is found to be unphysical. The ro ot of this unphysical contribution may be identified with the singular ity in the derivative of the g-function appearing in the expression fo r the third order energy. A possible solution to this problem is sugge sted. An alternative approach of a unified study in the framework of a second order pseudopotential theory invoking the consistency conditio n is also examined. This method simulates the effect of some terms bey ond the second order in lattice dynamics and explains typical features of the phonon dispersion curves which are believed to be manifestatio ns of the many-ion effect.