Abstract
The lattice dynamics of a simple Bethe lattice is treated exactly for the single‐defect problem and within the framework of the coherent potential approximation (CPA). There is a critical mass ratio of an impurity for the emergence of localized modes, if the coordination number is greater than two. This simple approach successfully predicts the one‐ and two‐mode behaviour as well as the mixed‐mode behaviour of many III–V and II–VI mixed crystals. The CPA calculation reproduces essential results already obtained by more elaborated theories.