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 (,,) and with the density of combined states J(,,). 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.

The normal modes of vibration propagating along the unique c axis of stretch-oriented polytetrafluoroethylene at -II, 25, and 40·C have been studied by means of coherent neutron inelastic scattering methods. No significant effects associated with the 19 and 30·C phase transitions have been definitely observed. The results, together with related information obtained from Raman scattering and infrared absorption measurements, have been analyzed in terms of various models of the forces both within and between the polymer chains. The effects of interchain forces on the shapes of the dispersion curves and on the selection rules for radiation scattering and absorption are discussed. It has not been possible to establish a satifactory model for describing all the available data, mainly because of the mathematical complexity involved in treating the dynamical behavior of the coupled chains. The calculations nevertheless indicate that this type of model is capable of providing at least qualitative agreement with experiment. The elastic constant C 33 may be estimated from the limiting slope of the longitudinal acoustic branch: we obtain C 33= 1.50±O.IO (10 12 dyn/cm~, a constant for all three temperatures within experimental error.

Dispersion curves and a frequency distribution were calculated for isotactic polypropylene on the basis of a single-chain model using a valence force field previously reported in the literature. The obtained results provide useful information for the interpretation of the inelastic neutron scattering spectrum published by other authors. It is shown that calculations on the single-chain model account for practically all peaks in the experimentally determined frequency distribution. Most of these peaks arise from phase values which do not correspond to spectroscopically active motions.

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