Structure-property relationships of oxides with hexagonal AMO₃ and brownmillerite related structures Public Deposited

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  • Transition metal oxides exhibit potential in various application fields due to the special d-electrons. Solid state chemistry focuses on discovering the structure-property relationships. The work in this thesis mainly discusses compounds with hexagonal or brownmillerite-type structure and their practical properties. Hexagonal YIn[subscript 1-x]Fe[subscript x]O₃ (x = 0-0.3, 0.7-1.0) phases have been prepared and characterized. All phases appear to have the ferroelectric structure known for YInO₃. The color of the phases changes from yellow to orange to dark red with increasing Fe content. Magnetic measurements confirm high-spin Fe³⁺ for all phases. Similarly, solid solution YAl[subscript 1-x]Fe[subscript x]O₃ (x = 0-0.4, 0.7-1.0) phases were successfully synthesized through the sol-gel method. The Al-rich compounds present paraelectric YAlO₃ structure while the Fe-rich side samples exhibit YFeO₃ structure. The color of the compounds appear to be yellow with small Fe content and change to brown which has higher Fe content. Brownmillerite-type oxides Ba₂In[subscript 2-x]Mn[subscript x]O[subscript 5+x] (x = 0.1-0.7) have been prepared and characterized. Magnetic measurements confirm that Mn in as prepared samples is substituting as Mn⁵⁺ for all values of x with observed paramagnetic spin-only moments close to values expected for two unpaired electrons. Neutron diffraction structure refinements show Mn⁵⁺ occupies tetrahedral sites for orthorhombic (x = 0.1) and tetragonal (x = 0.2) phases. For Mn ≥ 0.3 samples, neutron refinements show the phases are cubic with disordered cations and oxygen vacancies. The colors of the phases change from light yellow (x = 0) to intense turquoise (x =0.1), to green (x = 0.2, 0.3) or dark green (x ≥ 0.4). Solid solution Ba₂In[subscript 2-x]Fe[subscript x]O[subscript 5+y] (x = 0.1-1.5) also exhibit brownmillerite-type structure. The color of the compounds appear to be green with small Fe content and change to black with higher Fe content (x ≥ 0.3). Magnetic measurements and Mössbauer spectroscopy conclude the mixed valence of Fe³⁺/Fe⁴⁺ for all the phases. Nonstoichiometry compound YCu₀.₆Ti₀.₄O[subscript 3-δ] has been prepared and characterized. Structure study indicates that oxygen vacancy is favored under the synthesis condition. This change in oxygen content was further studied in the Mn-doped system. And the effect of stoichiometric difference in the Mn-doped samples was not as obvious as the initial compound. The disorder in the cation site enhanced the tolerance of the structure in the aspect of oxygen content. The hexagonal phases LnCu₀.₅Ti₀.₅O₃ (Ln = Y, Tb-Lu) phases were prepared by the traditional solid state reactions. The prepared compounds were reduced at high temperature in the reduction atmosphere created by the H₂/N₂ gas mixture. Study on the structure and properties changes by reduction was conducted by X-ray diffraction, optical measurement, magnetic measurement and thermalgravimetric analysis. And we observed some evidence of the presence of Cu⁺ in the reduced phase by these characterization methods. Solid solution YMn[subscript x]Ti[subscript y]O[subscript 3-δ] (y = 0.1-0.4) was successfully prepared through conventional solid state approach. All the samples showed hexagonal structure. But the structure transition from ferroelectric P6₃cm to paraelectric P6₃/mmc occurred when Ti amount is higher than 0.2. Based on the neutron diffraction refinement, the lattice expanded in the ab plane but contracted along the c axis direction.
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