Impurity-host force-constant changes fromAu197Mössbauer recoilles-fraction measurements for dilute Cu(Au) and Ag(Au) alloys

The absolute ${}_{}{}^{197}{}_{}{}^{}\mathrm{Au}$ Mössbauer absorber recoilless fraction $f_a$ has been measured for two dilute gold-alloy systems. These systems were 2-at.% Au in Cu and 5-at.% Au in Ag. For the Cu(Au) alloy, measurements were made over the temperature range 4.2-100 K. For the Ag(Au) system, the recoilless fraction was measured only at 4.2 K. For example, at 4.2 K, $f_a=0.239\mathrm{}\pm 0.003$ for Cu(Au), and $f_a=0.195\mathrm{}\pm 0.008$ for Ag(Au). The recoilless-fraction measurements are interpreted in terms of the impurity-atom lattice-dynamical model of Mannheim. In terms of this model, we have obtained values for the effective nearest-neighbor force constant ratios $\frac{{\gamma }_{\mathrm{A}\mathrm{u}-\mathrm{C}\mathrm{u}}}{{\gamma }_{\mathrm{C}\mathrm{u}-\mathrm{C}\mathrm{u}}}$ and $\frac{{\gamma }_{\mathrm{A}\mathrm{u}-\mathrm{A}\mathrm{g}}}{{\gamma }_{\mathrm{A}\mathrm{g}-\mathrm{A}\mathrm{g}}}$. For both alloy systems, the atomic coupling between the gold atom and the host metal is found to be enhanced relative to the host-host coupling. In obtaining these estimates, we have used host phonon density-of-states functions determined for neutron inelastic scattering measurements.