Trapping of solute atoms at grain boundaries in GdNi2

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.