Graduate Thesis Or Dissertation
 

Structural properties of indium doped YBa₂Cu₃O₇-x high Tc superconductors

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  • YBa₂Cu₃0₇₋x was doped with In by a stoichiometrically exact substitution of Y by In, doping levels ranging from 0% to 20%. Room temperature X-ray diffraction measurements, Perturbed Angular Correlation spectroscopy using ¹¹¹Cd as nuclear tracer at a variety of temperatures ranging from room temperature to 950°C and magnetic flux exclusion measurements were performed on samples with different In concentrations. X-ray spectroscopy, as the least sensitive, global probe measurement, was used to characterize major and minor phases in the bulk and to investigate the dependence of Bragg spacings and structure factors at doping sites on the doping level. Intensities of Bragg peaks were integrated and normalized by appropriate methods. All phases in the bulk were identified. The major phase was orthorhombic YBa₂Cu₃0₇₋x, secondary phases were identified to be BaCu0₂, CuO and Y₂BaCuO₅ in concentrations between 2 and 3 at% of the sample, independent of the doping level of In. Furthermore, the recently discovered compound InBa₂Cu₃0x was detected at In concentrations above the solubility limit. A linear, positive intensity change for the Bragg peak with Miller indices 003 of the YBa₂Cu₃0₇₋x up to 3.0% and for the strongest peak of the InBa2Cu₃0x spectrum above 3.0% determined the solubility limit of In in the bulk to be 3.0%. A computer simulation was used to calculate X-ray diffraction spectra, assuming a unique and total substitution of the In atoms either at the Y, the Ba, the Cu(1) or the Cu(2) site. The simulation used a point ion model and simply replaced the corresponding site with In atoms in the different concentrations. Integrated, normalized intensities of the Bragg peaks of the simulation were compared with experimental results. PAC spectroscopy as a local probe measurement is very sensitive to distortions of the electric field gradient at the tracer site. PAC spectra for samples which are not In-saturated, show a clear spectrum with three major peaks at 55, 100 and 155 Mrad/sec. This indicates a substitution of the radio-active tracer at a unique site in the lattice and corresponds to preceeding measurements. Saturated samples with In concentrations above 3.0% display a superposition of the regular spectrum with a spectrum generated probably by In substitution at sites in secondary phases. Magnetic flux exclusion measurements show a critical temperature of 92.7°C, indicating a high sample quality. A rather small change of this measure for saturated samples was found and supports the hypothesis of In substitution at the Y site along with PAC and X-ray spectroscopy results.
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