Rare-Earth Impurities in Metals: The Configurational Transition of Ho in Liquid AgAl Solvents

We report measurements of the Knight shifts $K$ and susceptibilities $\chi$ introduced by Ho impurities in liquid AlAg alloys at 1100°C. The susceptibilities correspond to values of ${\mu }_{\mathrm{eff}}$ in good accordance with those predicted by Van Vleck's theory for free ions. The value of $\Gamma =K^{-1\mathrm{}}\frac{\partial K}{\partial c}$ caused by Ho impurities exhibits an abrupt transition, as a function of solvent composition, very similar to that previously observed by Blodgett and Flynn for Gd in AgAl and CuAl liquid-alloy solvents. An analysis of these data and the Ho solubility indicates that the rare earths undergo a transition in which the impurities thermally populate two available many-electron configurations that vary in relative energy with solvent composition. It appears probable that the large $\Gamma$ in Al-rich solutions originate in a degenerate mixing of impurity and orbitals with host band states near $E_F$; this conclusion cannot, however, be reached with complete certainty. Similar results for intermetallic compounds containing rare-earth components are also discussed.