Lattice dynamics and intrinsic phonon broadening in the alloy Cu0.92Ge0.08

Abstract

Using inelastic scattering of slow neutrons, we have determined the phonon frequencies and intrinsic line widths for the dilute face-centred cubic alloy Cu0.92Ge0.08 (Cu(Ge)) at 296 K. A limited number of measurements have also been made at 96 and 500 K. Spectrometer resolution calculations, using actual instrument and specimen parameters, have been performed and intrinsic phonon widths have been obtained by deconvolution from observed widths. The phonons in Cu(Ge) are well defined and similar to those for copper. Real contributions of the force-constant disorder in the alloy to intrinsic phonon widths are seen. To our knowledge, this is the first time that such effects have been conclusively observed for such dilute alloys. Constituent-atom mass differences are small for this alloy, and no mass disorder effects have been observed. Born–von Kármán model force-constant fits to the frequency data at 296 K are presented and compared with a model for pure copper. The frequency distribution and other thermal data, including the temperature dependence of the specific heat, the Debye–Waller factor, and the Debye temperature, are also given. The Cu(Ge) force constants are compared with those for 14 other iron-group transition elements and alloys. Systematic trends in the force constants as a function of 3d + 4s electron concentration are apparent.