Lattice locations and diffusion in intermetallic compounds explored through PAC measurements and DFT calculations

Site preferences of 111In/Cd impurity probe atoms were studied as a function of composition in Al3Ni and as a function of temperature in Al3Ti and Al3Zr structures using perturbed angular correlation of gamma rays (PAC). Spectra for Ni-rich Al3Ni exhibit a prominent quadrupole interaction (QI) signal attributed to one of two Al-type sites. For Ni-poor samples, spectra exhibited an ill-defined QI attributed to probes located in grain boundaries. Al3Ti and Al3Zr structures have one TM-site and several Al-sites. At low temperature, probes were determined to occupy an Al-site that has the same local atomic coordination. At higher temperature, probes were observed to transfer partially to other Al-sites. Enthalpy differences of indium solutes at the different sites were determined from equilibrium measurements of ratios of site-fractions as a function of temperature.;To provide additional insight, energies of In-solute atoms and electric field gradients (EFG) at nuclei of daughter 111Cd-solutes were calculated using density functional theory (DFT). It was found for all systems that EFG calculations were not adequate to unambiguously identify the sites occupied. However, site energy calculations helped to identify the sites occupied in all systems studied. Calculated site-energy differences are in good agreement with measurements.;In separate work, jump frequencies of probes were earlier measured at high temperature using PAC for In3R (R = rare-earth) having the L12 structure [Phys. Rev. Lett., 102, 2009]. In that work, comparison of measurements made for samples that were In-rich and In-poor led to the conclusion that the dominant diffusion mechanism involves R-vacancies in light lanthanide-indides (such as In3La) and In-vacancies in heavy lanthanide-indides (such as In3Lu). DFT calculations were carried out to determine whether the observations could be explained by gradual changes in energies of In- and R-vacancies along the In3R series. Instead, calculations showed the opposite behavior: more In-vacancies in In3La and more R-vacancies in In3Lu. This unexpected result indicates that other factors control diffusion behavior, such as differences in migration enthalpies leading to large changes in jump frequencies.