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Lattice dynamics and the vibrational spectra of diamond and diamondlike crystals of the elements of Group IV (Si, Ge, and gray Sn) are treated on the basis of a valence force potential. Experimental phonon dispersion curves from the literature are satisfactorily fitted by a six-parameter valence force potential. The one- and two-phonon absorptions are compared with the calculated density of states g(v) and with the density of combined states J(v). The results are satisfactory. The success in reproducing the spectroscopic properties of all four elements with only these parameters is consistent with the highly directional character of the covalent bonds in the crystal and the lack of ionic character. The different information derived from the widely used shell model and the valence force potential are briefly discussed.
Tubino et al. (Tue,) studied this question.