Graduate Thesis Or Dissertation
 

NMR study of novel oxides

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  • We report ²⁷Al and ⁶⁹,⁷¹Ga impurity and ⁶⁷Zn host species NMR experiments on ZnO powders containing Al or Ga dopants in the range 0.03 to 3 at. %. Spectral analysis of the ⁶⁷Zn and ⁶⁹Ga quadrupole powder patterns indicate gallium is substitutional for zinc and the electric quadrupole coupling of dilute ⁶⁹Ga is determined to be (e²qQ/h)₆₉ = 3.9±0.3MHz. Resonant shift data and spin-lattice relaxation rates are reduced to yield values for impurity chemical and Knight shifts. The data are correlated by the Korringa relation throughout the impurity concentration range indicating nearly ideal metallic behavior, where exchange and correlation effects are absent in the conduction electron system. Temperature dependent studies of the spin-lattice relaxation rate are consistent with coupling of a degenerate electron system to nuclear magnetic moments. Knight shift data therefore indicate a rapid development of an impurity band below about 0.5 at. %. Spin-echo experiments show the presence of a second distorted gallium site in nonstoichiometric samples containing up to 1.0 at. % excess oxygen, the limit of the range studied. The fraction of substitutional sites decreases with increasing oxygen content as does the carrier concentration at these sites. Knight shifts and spin-lattice relaxation rates remain correlated by the Korringa relation for the substitutional sites. Relaxation studies reveal that the distorted site is essentially decoupled from the conduction electrons. These results imply that the distorted sites are part of a diamagnetic carrier trap involving gallium complexed with a defect. We suggest that the complex consists of two gallium ions and an oxygen interstitial, i.e. Ga₂³⁺Oᵢ²⁻. Additionally, we report ⁵¹V and ³¹P NMR experiments in the series ZrP₂₋ₓVₓO₇ for x=2.0, 1.6, 1.0, 0.3, and 0.0. Analysis of the ⁵¹V and ³¹P spectra for ZrV₂O₇ and ZrP₂O₇ show conplex patterns consistent with the proposed Pa3 space group and the 3x3x3 superstructure in which there exist 11 crystallographic distinct sites for phosphorous. 'V variable temperature MAS NMR in ZrV₂O₇ exhibits evidence of the 3x3x3 superstructure at room temperature and shows that the ideal structure is reached by 130°C. The ³¹P spectrum in ZrVPO₇ shows no evidence of superstructure.
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