Diffusion and Equilibration of Site-Preferences Following Transmutation of Tracer Atoms

Using the method of perturbed angular correlation of gamma rays, diffusional jump-frequencies of probe atoms can be measured through relaxation of the nuclear quadrupole interaction. This was first shown in 2004 for jumps of tracer atoms that lead to reorientation of the local electric field-gradient, such as jumps on the connected a-sublattice in the L12 crystal structure. Studies on many such phases using the 111In/Cd PAC probe are reviewed in this paper. A major finding from a 2009 study of indides of rare-earth elements, In3R, was the apparent observation of two diffusional regimes: one dominant for heavy-lanthanide phases, R= Lu, Tm, Er, Dy, Tb, Gd, that was consistent with a simple model of vacancy diffusion on the In a-sublattice, and another for light-lanthanides, R= La, Ce, Pr, Nd, that had no obvious explanation but for which several alternative diffusion mechanisms were suggested. It is herein proposed that the latter regime arises not from a diffusion mechanism but from transfer of Cd-probes from In-sites where they originate to R-sites as a consequence of a change in site-preference of 111Cd-daughter atoms from In-sites to R-sites following transmutation of 111In. Support for this transfer mechanism comes from a study of site-preferences and jump-frequencies of 111In/Cd probes in Pd3R phases. Possible mechanisms for transfer are described, with the most likely mechanism identified as one in which Cd-probes on a-sites transfer to interstitial sites, diffuse interstitially, and then react with vacancies on b-sites. Implications of this proposal are discussed. For indides of heavy-lanthanide elements, the Cd-tracer remains on the In-sublattice and relaxation gives the diffusional jump-frequency.

Параметры ядерного квадрупольного взаимодействия и пространственное распределение электронных дефектов в решетках YBa-=SUB=-2-=/SUB=-Cu-=SUB=-3-=/SUB=-O-=SUB=-7-=/SUB=- и La-=SUB=-2-x-=/SUB=-Sr-=SUB=-x-=/SUB=-CuO-=SUB=-4-=/SUB=-

The comparison of calculated and experimental parameters of nuclear quadrupole interaction in the sites of crystal lattice of YBa_2Cu_3O_7 and La_2– x Sr_ x CuO_4 superconducting copper metal oxides has shown that effective charges of all atoms of superconducting ceramic lattices correspond to standard rates of their oxidation, except the atoms of chain and planar oxygen in YBa_2Cu_3O_7 lattice and atoms of planar oxygen in La_2– x Sr_ x CuO_4 lattice, whose decreased charge is interpreted as a result of hole localization in the corresponding sublattices.