Motional averaging of quadrupole interactions in Al$_{4}$Sr and Ga$_{4}$Sr studied using PAC*

Perturbed angular correlation of gamma rays (PAC) was used to study diffusion of $^{111}$In/Cd probe atoms in Al$_{4}$Sr and Ga$_{4}$Sr phases. These phases have the tetragonal Al$_{4}$Ba structure, with two inequivalent Al sublattices that have axially symmetric and collinear electric field gradients. Measurements of nuclear quadrupole interactions in each phase revealed three dynamical regimes. Static quadrupole interactions were observed below 500$^{\circ}$C. Between 500 and 700$^{\circ}$C, the two quadrupole interaction signals were observed to approach each other and merge at 700$^{\circ}$C (slow fluctuation regime). Only a single quadrupole interaction signal was observed above 700$^{\circ}$C (fast fluctuation regime). These changes are attributed to jumping of probe atoms between the two sublattices, leading to motional averaging of the two electric field gradients as temperature and jump frequency increase. The two signals merge at a temperature at which the jump frequency between the sublattices is equal to the difference in the static quadrupole interaction frequencies. Temperature dependences of the quadrupole interaction frequencies will be contrasted with those measured earlier on phases including Al$_{4}$Ba and In$_{4}$Ba.