Graduate Thesis Or Dissertation
 

Functional transition metal oxides : structure-property relationships

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/ng451m43d

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  • Transition metal oxides are an important class of materials for the wide variety optical, electrical, dielectric, magnetic, and thermal properties observed. Their unique structure-property relationships allow for property tuning and often provide insight into the fundamentals of chemistry. The work in this thesis was focused on the design, synthesis and physical property characterization of novel materials with applications in mind. In particular, magneto-electric multiferroics and spintronics are a promising technology for applications in many areas of electronics where either the magnetic or electric properties can be manipulated with electric or magnetic fields respectively. La₂MnRhO₆ thin films were found to exhibit a change in the defect structure, surface morphology, and magnetic properties for films grown at different oxygen pressures revealing the importance of deposition methods. Bulk ceramic of La₂MRhO₆ (M = Cr, Mn, Fe, Co, Ni and Cu) were characterized and all were found to be orthorhombic p-type semiconductors. Unique magnetism was found for M = Fe and Cr with both showing magnetic hysteresis at 5K. The Cu analogue is likely to be in a 2+/3+ oxidation state, and showed a favorable power factor up to ~575 K. LaCo[subscript 1-x]Rh[subscript x]O₃ compositions were found to exhibit an interesting lattice crossover at x = 0.5. Strong evidence for at least some conversion of Rh³⁺/Co³⁺ to Rh⁴⁺/Co²⁺ is found in both structural and electrical transport data. A thermoelectric figure-of-merit (ZT) of about 0.075 has been achieved for LaCo₀.₅Rh₀.₅O₃ at 775 K YMnO₃ is a well known magneto-electric material, and in our study, we have prepared complete or nearly complete solid solutions with YInO₃, YAlO₃, and YCu₀.₅Ti₀.₅O₃ systems in order to better understand the structure-property relationships. Our results indicate exciting optical properties as a wide range of blue was observed in the YInO₃ solid solution, the intensity of which was critically dependent on the apical bond distances and the crystal field splitting. The YInO₃ and YCu₀.₅Ti₀.₅O₃ systems showed magnetic and magneto-electric suppression for increasing values as expected, but the field dependent capacitance for YInO₃ showed a unique improvement and sign swap indicating a strong magneto-electric coupling. We determined the YAlO₃ structure to have been improperly reported based on bond valence calculations. Our investigation into the YAlO₃-YMnO₃ system is underway, however we believe a carbonate group exists within the basal plane resulting in a chemical formula such as YAl[subscript 1-x]Mn[subscript x]O[subscript 3-y](CO₃)[subscript y]. For the high-k dielectric CaCu[subscript 3-x]Mn[subscript x]Ti[subscript 4-y]Fe[subscript y]O₁₂ and CaCu₃Ti₄O[subscript 12-x]F[subscript x], the limits of x, y were determined. A structural study on ceramic samples determined single phase materials, and dielectric measurements showed improvement of properties for fluorinated samples with the dielectric loss lowered while maintaining a high dielectric constant. It is likely that F may be concentrating on important planar defect sites. A colossal decrease in the dielectric constant was observed for Mn-Fe co-doped samples and provides insight into dual site occupations of dopants.
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