Lattice locations and diffusion in intermetallic compounds explored through PAC measurements and DFT calculations
Bevington, John Paul
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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-sites. For Nipoor 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-solutes 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.