Vibrational and optical properties of the mixed crystal NaCℓ1−xBrx

A one-dimensional model is developed for the [111] transverse modes of the mixed crystal NaC?_1?xBr_x. The frequencies and eigenvectors of the normal modes of vibration are calculated for ten random configurations at each of a number of compositions. The imaginary part of the dielectric function is calculated for the average configuration at each composition. The behavior of the system cannot be classified in terms of the usual one-mode or two-mode categories. Representative eigenvectors are displayed, and the random element isodisplacement model is discussed in the light of our results.