  Two types of solute-solute interactions are investigated in this work.
Quadrupole interactions caused by nearby Ag-solute atoms were measured at
nuclei of 111In/Cd solute probe atoms in the binary compound GdAl2 using the
method of perturbed angular correlation of gamma rays (PAC). Locations of
In-probes and Ag-solutes on both Gd- and Al-sublattices were identified by
comparing site fractions in Gd-poor and Gd-rich GdAl2(Ag) samples. Interaction
enthalpies between solute-atom pairs were determined from temperature
dependences of observed site fractions. Repulsive interactions were observed
for close-neighbor complexes In/Gd/+Ag/Gd/ and In/Gd/+Ag/Al/ pairs, whereas a
slightly attractive interaction was observed for In/Al/+Ag/Al/. Interaction
enthalpies were all in the range +/- 0.15 eV. Temperature dependences of site
fractions of In-probes on locally defect-free Gd- and Al-sites yields a
transfer enthalpy that was found to be 0.343 eV in a previous study of undoped
GdAl2. The corresponding values in GdAl2(Ag) samples are much smaller. This is
attributed to competition of In- and Ag-solutes to occupy sites of the same
sublattice. While the difference in site-enthalpies of In-solutes on Gd- and
Al-sites is temperature independent, it is proposed that the transfer of
Ag-solutes from Gd- to Al-sites leads to a large temperature dependence of
degeneracies of levels available to In-solutes, resulting in an effective
transfer enthalpy that is much smaller than the difference in site-enthalpies.
