Perturbed γ−γ angular correlations: A spectroscopy for point defects in metals and alloys |
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Authors: | Gary S Collins Steven L Shropshire Jiawen Fan |
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Institution: | (1) Department of Physics, Washington State University, 99164-2814 Pullman, WA, USA |
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Abstract: | Atomic defects which migrate and trap at impurity probe atoms can be labelled by the changes they induce in the hyperfine
interactions of the probe nuclei. Many studies have been made using perturbed γ−γ angular correlations (PAC) and the111In probe because of the excellent resolution of different sites. Identification of the bound states is the key problem in
applying hyperfine interactions methods to point defects studies. In this study three structure-sensitive methods are applied
to help identify the atomic structures of various multivacancy complexes in Pt and Au: (1) Quadrupole interaction parameters
are compared with results of point-charge calculations of electric-field gradients for 20 structures containing 1–4 vacancies
in the fcc lattice. (2) Hyperfine interactions induced by decorating vacancy complexes with hydrogen atoms are measured and
interpreted with the assistance of point-charge calculations. (3) Transformations between complexes observed by annihilation
of vacancies by mobile self-interstitials are used to test the consistency of the identifications. Using these methods in
conjunction with analysis of the trapping behavior which occurs during annealing of damaged samples, structural models are
presented for divacancy (2V), 3V and 4V complexes in Pt, and 3V and 4V complexes in Au. The activation temperatures of the
3V defect in Au and Pt are determined to be 162 K and 390 K, respectively, and activation temperatures of defects in Ni, Cu,
Pt and Au are compared. For Pt, trapping of H at 1V and 2V complexes is observed to lead to small changes in the quadrupole
interactions, consistent with well shielded protonic charges. However, trapping at 3V and 4V complexes leads to very large
changes which we attribute to atomic restructuring to the defect complexes. Finally, the application of the same methodology
to interpret recent experiments on NiAl, an ordered alloy, is described. |
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