Localized Moments on Rhodium inPd-Rh Alloys

The Knight shifts of ${}_{}{}^{100}{}_{}{}^{}\mathrm{Rh}$ in very diluted Pd-Rh alloys were measured over a temperature range between 4.2 and 1053°K by $\gamma -\gamma$ perturbed angular-correlation techniques. The Knight shifts were negative, very large (up to -15%), and strongly temperature-dependent. The Curie-Weiss behavior of the Knight shift and the difference between its temperature dependence in a dilute Pd-Rh alloy and in pure palladium, establish the existence of a local moment on the rhodium atom. From existing susceptibility data, the impurity susceptibility ${\chi }_i$ was calculated. Its temperature dependence is conspicuously different from that of the Knight shift. The partial derivative $\frac{\partial K_i\mathrm{}\left(T\right)\mathrm{}}{\partial {\chi }_i\mathrm{}\left(T\right)\mathrm{}}$ was used to obtain the apparent core-$d$ hyperfine field, $H_{\mathrm{hf}}^{}{}_{}{}^{\left(\mathrm{d}\right)}$. The result is $H_{\mathrm{hf}}^{}{}_{}{}^{\left(\mathrm{d}\right)}\simeq -700\mathrm{}$ kG for $T>\mathrm{}400\mathrm{}$°K, and $H_{\mathrm{hf}}^{}{}_{}{}^{\left(\mathrm{d}\right)}\approx -43\mathrm{}\pm 1$ kG for $T<\mathrm{}400\mathrm{}$°K. The drastic change in the field at about 400°K indicates that above this temperature the localized moment is concentrated on the Rh atom, while at lower temperatures the polarization of the lattice by the impurity atom seems to be rather long ranged, suggesting the formation of a quasibound state. Knight shifts for ${}_{}{}^{100}{}_{}{}^{}\mathrm{Rh}$ in Ag and Pt hosts are also reported.