Phonon Frequencies and Widths in Dilute Cu-Au Alloys

A detailed study of the effects of force-constant changes on phonon frequencies and widths has been made in dilute copper-gold alloys. In the calculation, a defect perturbation model which includes the change in mass at the impurity site and the changes in the nearest-neighbor central and noncentral force constants around the impurity, has been considered. Numerical calculations have been performed for the three experimentally studied Cu-Au alloys (1, 3, and 9-at.% Au). The calculated frequency shifts and the phonon widths compare well with the results of recently performed inelastic-neutron-scattering experiments on dilute alloys, i.e., copper containing 1- and 3-at.% Au as impurities. In Cu-9.0-at.% Au where interference effects of two or more than two impurities are expected, the resonant frequency calculated in low-concentration theory is found to be smaller than the experimentally measured frequency by about 15%. The effects of the lattice expansion caused by the impurity atoms on the phonon frequencies has been accounted for in a manner which does not involve any arbitrary parameter. The observed discrepancies in the high-frequency region suggest the need of a theory for the lattice expansion effect which properly takes into account the changes in the electron screening in alloys.