Local modes in Al0.1Cu0.9 and (NH4)0.1K0.9Cl in the coherent-potential approximation

Abstract

Numerical techniques are described to apply the coherent-potential approximation (CPA) to mass defects in polyatomic cubic crystals, and to facilitate CPA calculations for light-mass defects in monatomic cubic crystals. Results are given for two alloys in which local modes have been observed by inelastic neutron scattering, ${\mathrm{Al}}_{0.1}$ ${\mathrm{Cu}}_{0.9}$ and (N${\mathrm{H}}_4$ ${)}_{0.1}$ ${\mathrm{K}}_{0.9}$Cl. The CPA is in good agreement with experiment for the local-mode peak position in the Al-Cu system, but the observed peak is much sharper. For the (N${\mathrm{H}}_4$-K)Cl alloy, the mass-defect CPA predicts a quasilocal resonant mode much lower in frequency than the experimental local-mode peak, showing the need to include force-constant changes in the theoretical model.