  Diffusional jumps can produce fluctuating electric field gradients at nuclei
of jumping atoms. Using perturbed angular correlation of gamma rays (PAC),
jumps of probe atoms cause nuclear quadrupole relaxation that can be fitted to
obtain the mean jump frequency. An overview is given of the application of this
approach to highly ordered intermetallic compounds having the L12 (Cu3Au)
crystal structure. New results are then presented for jump frequencies of
111In/Cd probe atoms in pseudo-binary L12 compounds of the forms In3(La1-xPrx)
and (In1-xSnx)3La. For the mixed rare-earth system, jump frequencies are found
to scale with composition between jump frequencies of the end-member phases
In3La and In3Pr. However, for the mixed sp-element system, a large decrease in
jump frequency is observed as Sn atoms substitute for In-atoms. This difference
in behavior appears to depend on whether atomic disorder is on the diffusion
sublattice (In-Sn substitution), as opposed to a neighboring sublattice (La-Pr
substitution), whether or not there is a difference in diffusion mechanism
between end-member phases, and/or whether or not there is a valence difference
between the mixing atoms. All three conditions apply for only (In1-xSnx)3La.

Materials Science