Vibrations of a mixed crystal: Neutron scattering fromNi55Pd45

Abstract

The crystal dynamics of an alloy with a large amount of mass disorder (Ni and Pd have masses 58.7 and 106.5 amu, respectively) have been investigated by inelastic neutron scattering. A triple-axis spectrometer was used to measure the phonon dispersion curves and widths in a crystal of the fcc random alloy ${\mathrm{Ni}}_{55}$${\mathrm{Pd}}_{45}$. Significant broadening was observed in each branch for neutron groups corresponding to phonons of large wave vector. For transverse modes, many of the widths are large and indicate behavior characteristic of resonance modes. Neutron groups for [111] transverse vibrations at the zone boundary had the largest widths (2.1 THz, compared with a mean frequency of 3.3 THz). The widths of the longitudinal modes are generally smaller than those of transverse modes, and no definite indications of resonance behavior are apparent. The results are compared with the predictions of several theories, notably the coherent-potential approximation with some qualitative success. The form of the discrepancy between theory and experiment, and the similarity of the results to those of an experiment on crystals containing relatively dilute concentrations of Au in Cu, suggest the need to consider force-constant changes as well as mass disorder. Some new measurements for Cu(Au) are also reported.