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THE EFFECTS OF POTASSIUM AND SODIUM CATIONS ON THE PHASE AND MORPHOLOGY OF BISMUTH IRON OXIDE CRYSTALS

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Abstract

Bismuth iron oxide (BFO) crystals have potential for applications ranging from next-generation, magnetoelectricity-based electronics to photocatalysts for the breakdown of toxic chemicals. Hydrothermal synthesis (HTS) has been shown to be an attractive solution growth method for producing well-defined, phase-pure BFO crystals. Until now, research in this field has focused on understanding the relationships between external experimental parameters in HTS and resultant crystalline phases of BFO, neglecting the role of the solution chemistry in crystal phase determination. Two spectator cations, Na+ and K+, commonly used in HTS of BFOs were investigated for their potential roles in determining the phase outcome of the final products in relation to a recently discovered, time-dependent transformation of an intermediate phase that was found to strongly influence the final phase outcome. Synthesis of BFOs was performed using wet chemistry techniques and reaction in a high-pressure, low-temperature environment. Characterization was performed using powder X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. Phase-pure BiFeO3 was confirmed to form in the presence of K+ only when the intermediate underwent transformation while phase-pure Bi2Fe4O9 could be formed in the presence of Na+ regardless of the status of the intermediate. A new synthetic method was developed to elucidate whether the cations affected the transformation of the intermediate, and whether this potential transformation was the cause of the phase control. It was found that the intermediate was not crystallographically affected by the cations and the presence of the cations themselves only during heat treatment determined the final phase of the product. Finally, a thought experiment was conducted that suggests a stabilization role for spectator anions and a formation mechanism for Bi2Fe4O9. The effects of cations on crystal morphology were inconclusive. The methods developed for these investigations and subsequent results lay the groundwork for a better understanding of the role of other ions in the HTS of BFOs.

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2018-12-30

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bismuth iron oxide; HTS; hydrothermal synthesis; spectator ions; Materials Science; Nanoscience; BFO; bismuth ferrite

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Estroff, Lara A

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Van Dover, Robert B.

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Materials Science and Engineering

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M.S., Materials Science and Engineering

Degree Level

Master of Science

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dissertation or thesis

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