On the lattice dynamics of alkali metals

The phonon frequencies of alkali metals calculated on the basis of a valence force field model (used by Mishra and Singh) are compared with those previously deduced on the basis of the present author's CGW model, to examine the claim that the former model produces a crossover in lithium along the [ζ00] direction. It is shown that this model which is neither selfconsistent nor in conformity with the symmetry requirements of the lattice, is incapable of producing a crossover in any alkali metal. The distortions introduced by the deficiencies of this model into the elastic constants and phonon frequencies of alkali metals, the dubious claims regarding the electron-ion interactions and the reproduction of the observed crossover in lithium as well as the apparent agreement between the calculated and measured dispersion curves, are discussed. (orig.)