Lattice vibrations inRb1−cKcalloys: Shortcomings of a single-site coherent potential approximation

A single-site coherent potential approximation (CPA) including force constant changes in the additive limit (designated the CPA-F) is applied to phonons in ${\mathrm{Rb}}_{1-c}{\mathrm{K}}_c$ alloys. The CPA-F results are in better overall agreement with the neutron scattering measurements of Kamitakahara and Copley than mass-defect CPA results. However, marked discrepancies in the concentration, branch, and wave-vector dependence and in the line shapes remain between CPA-F and experimental neutron scattering cross sections. These discrepancies suggest the need to include multiple scattering from pairs and larger clusters in the theoretical treatment.