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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., Comment: 17 pages, 6 figures, 2 tables. To be presented at the International Conference on Hyperfine Interactions and Their Applications, July 3-8, 2016, Leuven, Belgium; www.hyperfine2016.be/

Lattice locations of 111In impurity probe atoms in intermetallic GdNi2 were studied as a function of alloy composition and temperature using perturbed angular correlation spectroscopy (PAC). Three nuclear quadrupole interaction signals were detected and their equilibrium site fractions were measured up to 700 oC. Two signals have well-defined electric field gradients (EFGs) and are attributed to In-probes on Gd- and Ni-sites in a well-ordered lattice. A third, inhomogeneously broadened signal was observed at low temperature. This is attributed to trapping, or segregation, of In-probes to lattice sinks such as grain boundaries (GB) that have a large multiplicity of local environments and EFGs. Changes in site fractions were reversible above 300oC. Measurements were made on a pair of samples that were richer and poorer in Gd. Remarkably, the GB-site was populated only in the more Gd-rich sample. This is explained by the hypothesis that excess Gd segregates to the grain boundaries and provides a lower enthalpy environment for In-probe atoms. Observations are discussed in relation to a three-level quantum system. Enthalpy differences between levels were determined from measurements of temperature dependences of ratios of site fractions. The enthalpy of transfer of In-probes from the Gd- to Ni-sublattice was found to be much smaller in the Gd-rich sample. This is attributed to a large temperature-dependence in the degeneracies of levels available to In-solutes in the phase, leading to an effective transfer enthalpy that differs greatly from the difference in site-enthalpies. A possible scenario is discussed. Different segregation enthalpies were measured for In-solute transferring from GB sites to Gd- and Ni-sites, whereas only an average value can be determined through macroscopic measurements., Comment: 13 pages, 6 figures, 1 table. To be presented at the International Conference on Hyperfine Interactions and Their Applications, Leuven, July 3-8, 2015. http://www.hyperfine2016.be/