Force constant and mass disorder in(NH4)cK1−cClalloys in the coherent potential approximation

In a previous paper, the authors described an extension (here called the CPA-F) of the coherent potential approximation (CPA) for vibrational systems to include force-constant changes as well as mass disorder. In the present paper, the CPA-F is used to study the mixed ionic alloy ${\left(\mathrm{N}{\mathrm{H}}_4\right)}_c{\mathrm{K}}_{1-c}\mathrm{Cl}$ for $c=0.07\mathrm{}$ and $c=0.10\mathrm{}$. The theory is found to give reasonably good agreement with the inelastic neutron scattering results of Smith et al. In contrast, the original mass-defect CPA theory of Taylor is in poor agreement with experiment. In particular, the mass-defect CPA predicts a quasilocal mode a full THz lower in frequency than the local-mode peak seen experimentally by neutron scattering and fitted by the CPA-F.