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The main focus of this work is on exploratory investigation of thermoelectric (TE) materials. Thermoelectric devices are solid-state devices that convert thermal energy from a temperature gradient into electrical energy (Seebeck effect), or convert electrical energy into a temperature gradient (Peltier effect). Modifying existing materials and finding new materials with proper thermoelectric properties are the two approaches considered in this research. Good thermoelectric materials are usually narrow band gap semiconductors with large Seebeck coefficient, reasonably high electrical conductivity and low thermal conductivity. Early transition metal antimonides and arsenides, with unique structural features were chosen for finding high performance TE materials. During the investigation of group four antimonides, a series of new ternaries, ZrSi??Sb2-??, ZrGe??Sb2-?? and HfGe??Sb2-?? was developed. Single crystal X-ray diffraction was used for crystal structure determination, and energy depressives X-ray analysis (EDX) was used for compositional analysis. Metallic properties of these compounds were predicted by electronic structure calculations and confirmed by physical property measurements. It was revealed that Mo3Sb7 turns semiconducting by partial Sb/Te exchange. Similarly, isostructural Re3As7 was modified to become semiconducting by partial Ge/As exchange. Crystal structures were determined by single crystal X-ray and powder X-ray diffraction utilizing Rietveld method. Electronic structures were determined by using the LMTO method and confirmed the semiconducting properties of these ternary compounds. Physical property measurements showed exceptional TE properties for these compounds. It was also confirmed by the X-ray single crystal analysis that it is possible to intercalate different cations with the proper size into the existing cubic voids of the structure. The effect of cation intercalation on physical properties of these compounds were investigated and revealed the enhancement of transport properties as a result of this intercalation.

This investigation is based on the topotactic reduction reactions of 3 sets of phases: La₁_-ₓSrₓMnO₃ (0.6 ≤ x ≤ 1), La₁_-ₓCaₓMnO₃ (0.6 ≤ x ≤ 1) and Sr₃Fe₂_-ₓCoₓO₅Cl₂ (0 ≤ x ≤ 1). The topotactic reduction reactions of La₁_-ₓSrₓMnO₃ (0.6 ≤ x ≤ 1) perovskite phases using NaH as a solid state reducing agent result in the formation of reduced phases which contain manganese centres with a constant average oxidation state of +2.5, thus leading to an empirical composition of La₁_-ₓSrₓMnO₃_-(0.5+x)/2. The structures of the reduced phases contain a 6-layer OOTOOT’ stacking sequence of the octahedral (O) and tetrahedral (T) sheets and so are closely related to the 4-layer OTOT’-stacked brownmillerite structure. Close inspection reveals that the structures of the La₁_-ₓSrₓMnO₃_-y (x = 0.67, 0.7 and 0.75) reduced phases contain an intralayer ordered arrangement of twisted tetrahedral chains, and are the first instances of refined 6-layer structures containing such an arrangement. The driving force dictating the arrangement of the twisted tetrahedral chains in these 6-layer structures are shown to be the size of the tetrahedral chain dipole moment and the coupling between neighbouring tetrahedral layers in a similar manner to the brownmillerite structure. This logic has been applied to rationalise the ordering arrangement of the twisted tetrahedral chains in other structures containing tetrahedral layers. Variable temperature neutron diffraction data reveal that the La₁_-ₓSrₓMnO₃_-y (0.67 ≤ x ≤ 0.83) reduced phases adopt magnetically ordered structures incommensurate with the nuclear structures below T_N. In contrast, the structures adopted by reduced phases formed by topotactic reduction reactions of La₁_-ₓCaₓMnO₃ (0.6 ≤ x ≤ 1) perovskite phases using NaH are split into 2 groups as a function of the La:Ca ratio: The reduction of perovskite phases in the range (0.6 ≤ x ≤ 0.8) result in the formation of reduced phases with an empirical composition of La₁_-ₓCaₓMnO₂ which adopt structures containing a disordered arrangement of layers of edge-sharing MnO₆ octahedra and layers of vertex-sharing MnO₄ tetrahedra. In contrast, reduction of the perovskite phases in the range (0.9 ≤ x ≤ 1) result in the formation of reduced phases with an empirical composition of (La₁_-ₓCₓx)_0.5Mn<sub<0.5</sub>O which adopt the rock salt structure with a disordered arrangement of the cations. The (x < 1) reduced phases are the first reported examples of extended oxides which contain manganese centres with an average oxidation state of below 2. Variable temperature neutron diffraction data reveal that the La₁_-ₓCaₓMnO₃_-y (x= 0.6, 0.67 and 0.7) reduced phases adopt G-type antiferromagnetically ordered magnetic structures below approximately T_N ~ 220 K. The structure of Sr₃Fe₂O₅Cl₂ is closely related to that of the A₃B₂O₇ n = 2 Ruddlesden-Popper structure. The topotactic reduction reaction of Sr₃Fe₂O₅Cl₂ with LiH results in the formation of the Fe(II) phase Sr₃Fe₂O₄Cl₂. Neutron powder diffraction data show that Sr₃Fe₂O₄Cl₂ adopts a body-centred tetragonal crystal structure with anion vacancies located within the central SrO layer of the phase. The structure of Sr₃Fe₂O₄Cl₂ is therefore a rare example of a structure consisting of infinite sheets of corner-sharing Fe(II)O₄ square planes. In addition, the effect of cobalt-doping on the magnetic properties of the isostructural Sr₃Fe₂_-ₓCoₓO₄Cl₂ (0 ≤ x ≤ 1) series of reduced phases was investigated. Cobalt-doping was found to frustrate the antiferromagnetic order initially leading to a decrease in the magnetic ordering temperature but ultimately leading to spin glass behaviour.

Feng Xue. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2000. / Includes bibliographical references (p. 142-145). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Mode of access: World Wide Web. / Abstracts in English and Chinese.

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Graduation date: 1995

Thesis (Ph. D.)--University of Missouri-Columbia, 2001. / Typescript. Vita. Includes bibliographical references (leaves 472-488). Also available on the Internet.

Thesis (Ph. D.)--University of Missouri-Columbia, 2001. / Typescript. Vita. Includes bibliographical references (leaves 472-488). Also available on the Internet.

No description available.