Solute-solute interactions in intermetallic compounds

Experiments were carried out on highly ordered GdAl2 samples containing extremely dilute mole fractions of In-111/Cd probe-atom solutes (about 10(-11)), intrinsic antisite atoms Al-Gd having mole fractions of order 0-10(-2), and doped with Ag solutes at mole fractions of order 10(-2). Three types of defect interactions were investigated. (1) Quadrupole interactions caused by Ag-solute atoms neighboring In-111/Cd solute probe atoms were detected using the method of perturbed angular correlation of gamma rays (PAC). Three complexes of pairs of In-probes and Ag-solutes occupying neighboring positions on Gd-and Al-sublattices were identified by comparing site fractions in Gd-poor and Gd-rich GdAl2(Ag) samples and from the symmetry of the quadrupole interactions. 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 small, in the range +/- 0.15 eV. (2) Quadrupole interactions caused by intrinsic antisite atoms Al-Gd neighboring InGd probes were also detected and site fractions measured as a function of temperature, as in previous work on samples not doped with Ag-solutes [Temperature-and composition-driven changes in site occupation of solutes in Gd1+3xAl2-3x, Zacate and Collins (Phys. Rev. B69, 174202 (2004))]. However, the effective binding enthalpy between In-Gd probe and Al-Gd antisite was found to change sign from -0.12 eV (attractive interaction) in undoped samples to +0.24 eV (repulsive) in Ag-doped samples. This may be attributed to an attractive interaction between Al-Gd antisite atoms and Ag-dopants that competes with the attractive interaction between In-Gd and Al-Gd defects observed in undoped samples. Alternatively, it may be attributed to competing flows of Ag and Al atoms that, in effect, change the numbers of available sites on the two sublattices (termed degeneracies). (3) The site preference of In-probes to occupy Gd- and Al-sublattices, without nearby defects, in Ag-doped samples was measured. Effective transfer enthalpies between the two sublattices were found in doped samples that were much smaller than the value 0.343(3) eV found in the previous study in undoped GdAl2. Two approaches to understanding why the measured enthalpies in doped and undoped samples differ are discussed.