  Measurements were made of jump frequencies of 111In/Cd tracer atoms on the
Sn-sublattice in rare-earth tri-stannides having the L12 crystal structure via
perturbed angular correlation spectroscopy (PAC). Phases studied were Sn3R (R=
La, Ce, Pr, Nd, Sm and Gd). Earlier measurements on isostructural rare-earth
tri-indides showed that the dominant diffusion mechanism changed along that
series. The dominant mechanism was determined by comparing jump frequencies
measured at opposing phase boundary compositions (that is, more In-rich and
more In-poor). Jump frequencies were observed to be greater at the In-rich
boundary composition in light lanthanide indides and greater at the In-poor
boundary composition in heavy-lanthanide indides. These observations were
attributed to predominance of diffusion via rare-earth vacancies in the former
case and indium vacancies in the latter. Contrary to results for the indides,
jump frequencies found in the present work are greater for the Sn-poor boundary
compositions of the stannides, signaling that diffusive jumps are controlled by
Sn-vacancies. Possible origins of these differences in diffusion mechanisms are
discussed.
