Local modes in Rb/sub 1-c/K/sub c/ alloys: A neutron-scattering study

The lattice dynamics of Rb/sub 1-c/K/sub c/ alloys with c = 0.06, 0.18, and 0.29 have been investigated by neutron scattering, using crystals grown for the experiment by a modified Bridgman method. Local modes associated with the K atoms are observed which, as c is increased, rise in frequency, increase in intensity and begin to exhibit noticeable dispersion. The local-mode intensity exhibits a strong wave-vector dependence whose form does not change much with c. At the zone-boundary point N, (2π/a)(2.5, 2.5, 0) in Rb/sub 0.71/K/sub 0.29/, the local mode is about three times as intense as the band mode. It is apparent that, as c → 1, the local mode must evolve into the band mode of pure K. A substantial broadening of the local mode is also apparent at this wave vector and concentration, probably indicating the presence of local environment effects. We compare our results with coherent-potential-approximation calculations, both with and without inclusion of force-constant disorder. The calculations, carried out by Mostoller and Kaplan, and independently by Gruenewald and Scharnberg, indicate a softening in the nearest-neighbor Rb-K force constants of about 15% relative to the Rb-Rb force constants. We also compare our results with molecular-dynamics calculations by Jacucci, Klein, and Taylor