  Following nuclear decay, a daughter atom in a solid will "stay in place" if
the recoil energy is less than the threshold for displacement. At high
temperature, it may subsequently undergo long-range diffusion or some other
kind of atomic motion. In this paper, motion of 111Cd tracer probe atoms is
reconsidered following electron-capture decay of 111In in the series of In3R
phases (R= rare-earth). The motion produces nuclear relaxation that was
measured using the method of perturbed angular correlation. Previous
measurements along the entire series of In3R phases appeared to show a
crossover between two diffusional regimes. While relaxation for R= Lu-Tb is
consistent with a simple vacancy diffusion mechanism, relaxation for R= Nd-La
is not. More recent measurements in Pd3R phases demonstrate that the
site-preference of the parent In-probe changes along the series and suggests
that the same behavior occurs for daughter Cd-probes. The anomalous motion
observed for R= Nd-La is attributed to "lanthanide expansion" occurring towards
La end-member phases. For In3La, the Cd-tracer is found to jump away from its
original location on the In-sublattice in an extremely short time, of order 0.5
ns at 1000 K and 1.2 ms at room temperature, a residence time too short to be
consistent with defect-mediated diffusion. Several scenarios that can explain
the relaxation are presented based on the hypothesis that daughter Cd-probes
first jump to neighboring interstitial sites and then are either trapped and
immobilized, undergo long-range diffusion, or persist in a localized motion in
a cage.
