Lattice dynamics of ND4I in the NaCl phase (I) at 296°K

Dispersion curves for translational phonons propagating in the high-symmetry directions of sodium-chloridephase N${\mathrm{D}}_4$I have been determined at room temperature by coherent inelastic neutron scattering from single crystals. Acoustic phonons were well defined throughout the Brillouin zone. Optic phonons were considerably more difficult to measure because of the low signal-to-background ratio. The observed phonon energies were fitted to a "simple" and a "breathing" shell model with general repulsive short-range forces out to second neighbors. The results of the model fitting lead to conclusions similar to those reached in previous alkali halide studies. Model predictions of crystal properties are generally in good agreement with observation. The best fit model was used as an interpolation formula to calculate the density of states, $g\left(\nu \right)$. The mean-square displacements are in excellent agreement with those derived from the neutron-diffraction results of Seymour and Pryor. The lattice-dynamics results are compared with similar results obtained recently for the NaCl phases of the alkali cyanides.