Anomalous Temperature Dependence of the Electric-Field Gradient atTa181Impurities in Holmium: Influence of Host4fCrystalline-Field Levels

Measurements of the electric quadrupole interaction at ${}_{}{}^{181}{}_{}{}^{}\mathrm{Ta}$ impurities in paramagnetic Ho metal using time-differential perturbed angular correlations show marked deviation from the usual $T^{\frac{3}{2}}$ behavior. A phenomenological theory applied to this and other cases of closed-shell probes in rare-earth metals indicates that the deviation is due to the aspherical charge distribution of the host ion arising from the influence of the crystalline electric field on their unfilled $4f$ shells.