Lattice Vibrations in Deuterated Ammonium Chloride at 85 °K. II. Theoretical

A shell model is shown to provide a good fit to the phonon dispersion curves in ND₄Cl. The model contains 22 parameters of which 13 are fitted to the experimental measurements. The ionic charge on the chlorine, and the electronic polarizabilities of the ions, are not fitted to the experiments. The ND₄ group is not treated as rigid, but it is shown that an effective dynamical matrix can be set up, which does not contain the internal coordinates explicity, but which describes their effect on the external dispersion curves to a very good approximation. The dispersion curves in NH₄Cl are also calculated. A group-theoretical analysis of the symmetry of the normal modes is given in the Appendix.