Abstract
The cluster Bethe lattice method is extended to study vibrational properties of pseudobinary alloy systems AB1−cCc. As an example, the phonon density of states and the infrared response function of the alloy system In1−cGacP is calculated. The calculations are in agreement with experimental results. In the approach presented here longitudinal modes do not appear and hence there is not any longitudinal–transversal splitting observed in the first order Raman spectrum.