Graduate Thesis Or Dissertation
 

Perturbed angular correlation spectroscopy of oxide ceramics at high temperature

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  • Perturbed angular correlation (PAC) spectroscopy of dilute zirconia alloys and YBa₂Cu₃0₇₋x, ceramic superconductors at elevated temperature have been measured using ¹¹¹Cd as the probe. The oxygen vacancy concentration in the zirconia samples was controlled by doping with yttrium or indium, which introduce vacancies, or with niobium, which removes vacancies. A well-defined distorted site was identified for ¹¹¹Cd in monoclinic pure zirconia above 700 °C; below 700 °C two additional sites appeared. Their populations are larger at lower temperatures, whereas the population of the other site becomes smaller. In the tetragonal pure, yttriumdoped, and indiumdoped zirconia a ¹¹¹Cd traps an oxygen vacancy, causing the PAC frequencies to decrease and the probe nuclei to relax. The association between the Cd and the oxygen vacancy is incomplete between 1000 and 1400°C. The trapping probability of the oxygen vacancies to the ¹¹¹Cd was deduced, and it is found to be a function of oxygen vacancy concentration and temperature. The association energy of the CdVo pair is 0.8(1) eV. When a vacancy is bound to a ¹¹¹Cd impurity, the vacancy hops among 8 nearest-neighboring anionic sites with an energy barrier of 0.8(1) eV. In 0.5 at.% Nbdoped tetragonal zirconia the relaxation of the ¹¹¹Cd probes was suppressed completely, and a small and near axially-symmetric electric field gradient (EFG) was measured. We interpret this as evidence that the lattice relaxes slightly around the ¹¹¹Cd probe to a more cubic-like configuration which accounts for the small EFG seen in this work. The microscopic structure of the YBa₂Cu₃0₇₋x depends on the detailed thermal history of the sample, and is far more complex than the xray or neutron diffraction have indicated. The samples processed in flowing air and cooled slowly after firing showed reproducible PAC spectra with a small, slightly nonaxially-symmetric EFG. These samples displayed standard orthorhombic YBa₂Cu307_x x-ray diffraction patterns at room temperature and have a very sharp Meissner flux exclusion transition at 92.7 K. The local structure of the slowly cooled YBa₂Cu₃0₇₋x was characterized between room temperature and 900°C. The orthorhombic PAC spectra were observed across the entire measured temperature range. We interpret this as evidence that the stable local structure of YBa₂Cu₃0₇₋x is orthorhombic. Between room temperature and 300°C the diffusive motion of oxygen causes detectable relaxation and the bulk activation energy is a few tenths of an eV.
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