Determination of an unambiguous parameter for the impurity-lattice interaction

Experimental data analyzed with harmonic theories have established that generally the force constants between impurities and pure lattice differ from those of the host. However, the meaning of most reported impurity-host force-constant ratios is ambiguous for various reasons, one of which is that anharmonic effects are not properly accounted for. A method is outlined which leads to a well-defined interaction parameter, provided sufficient data are available for the impurity system. For highly dilute Fe in Cu and Fe in Pd we find an effective nearest-neighbor impurity host to host-host force-constant ratio $\frac{{A^{\prime }}_H\left(a_{\mathrm{eff}}\right)}{A_H\mathrm{}\left(a\right)\mathrm{}}=1.31\mathrm{}\pm 0.03 \mathrm{and} 0.60\pm 0.01$, respectively.