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      X-ray Transitions in Broad Band Materials

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      MCLEOD-DISSERTATION.pdf (5.423Mb)
      Date
      2013-08-28
      Author
      McLeod, John
      Type
      Thesis
      Degree Level
      Doctoral
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      Abstract
      The general application of soft X-ray spectroscopy in the study of the electronic structure of materials is discussed, with particular emphasis on broad band materials. Several materials are studied using both soft X-ray spectroscopy and density functional theory to provide experimental and theoretical electronic structures, respectively. In particular, bonding, cation hybridization, and band gaps for several binary oxides (the alkali oxides: BeO, MgO, CaO, SrO, BaO; the post-transition metal oxides: ZnO, CdO, HgO; and the period 5 oxides In2O3, SnO, SnO2, Sb2O3, Sb2O5, and TeO2) are studied. The technique of using the peaks in the second derivatives of an X-ray emission and an X-ray absorption spectrum to estimate the band gap of a material is critically analyzed, and a more accurate ``semi-empirical'' method that involves both measured spectra and theoretical calculations is proposed. The techniques used in the study of binary oxides are then applied to a more interesting (and industrially relevant) group of ternary oxides based on TiO2 (PbTiO3, Sn2TiO4, Bi2Ti4O11, Bi4Ti3O12, and ZnTiO3), and a general rule for the band gaps of these materials is suggested based on empirical data. This research may help direct efforts in synthesizing a hydrogen-producing photocatalyst with a band gap that can efficiently harness the bulk of the solar spectrum. Finally, several layered pnictide superconductors and related compounds (CaFe2As2, Co-, Ni- and Cu-doped BaFe2As2, LiFeAs, LiMnAs, CaCu1.7As2, SrCu2As2, SrCu2(As0.84Sb0.16)2, SrCu2Sb2, and BaCu2Sb2) are studied. The X-ray spectra provide rather strong evidence that these materials lack strong on-site Hubbard-like correlations, and that their electronic structures are almost entirely like those of a broad band metal. In particular, it is shown that the notion that the transition metals are all divalent is completely wrong for copper in a layered pnictide, and that at best in these systems the copper is monovalent.
      Degree
      Doctor of Philosophy (Ph.D.)
      Department
      Physics and Engineering Physics
      Program
      Physics
      Supervisor
      Moewes, Alexander
      Committee
      Tse, John; Chang, Gap-Soo; Grosvenor, Andrew; Sham, Tsun-Kong
      Copyright Date
      August 2013
      URI
      http://hdl.handle.net/10388/ETD-2013-08-1141
      Subject
      X-ray Emission Spectroscopy
      X-ray Absorption Spectroscopy
      Materials Science
      Band gap
      Electronic Structure
      Density Functional Theory
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