Graduate Thesis Or Dissertation
 

Structure-Property Relationships of Disordered Electronic Materials : From Crystalline to Amorphous

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/wm117v26w

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  • Two perovskite materials (Bi0.5(A)0.5)(Sc0.5Nb0.5)O3, where A = K+ and Na+ were synthesized phase pure for the first time. Their structures were determined by indexing synchrotron X-ray diffraction data and subsequent combined Rietveld refinement on neutron and synchrotron X-ray diffraction data revealing significant disorder on the A-sites of these materials. A = K+ was indexed to the cubic 𝑃𝑚3𝑚 space group, but a distorted cubic model with an A-site displacement in the [100] directions was needed to account for disorder. A distorted cubic model has been used to solve the structure of related materials, such as Pb(Sc0.5Nb0.5)O3, and is consistent with relaxor properties that have been previously reported. A = Na+ was indexed to the space group P4/mbm, which is a high temperature phase of the related compound NaNbO3. A-site cation disorder was less than in A = K+, but the tilting of BO6 octahedra along the c-axis was found to be disordered. A 15% tilt disorder was used to model observed peak intensities, but diffuse scattering in the neutron diffraction data indicate that there may be local-scale tilting that is more complicated and beyond the limit of detection for average structural techniques. Three known Aurivillius phases Bi2Sr(A)TiNb2O12 where A = Ca2+, Sr2+, and Ba2+ have had their physical properties characterized for the first time. The lack of ferroelectricity observed was at odds with previous assumptions made in structural studies. The structures were revaluated using combined Rietveld refinement on neutron and synchrotron X-ray diffraction data and by selected area electron diffraction. The structure of A = Ba2+ was reassigned to the centrosymmetric space group I4/mmm, which lacks ferroelectric properties. The structures for A = Ca2+ and Sr2+ were found to be consistent with pervious structural studies, regardless of the lack of observed properties. It is determined that the cation disorder found on every crystallographic site in the structure explains the properties observed. This disorder leads to the local-scale destruction of macroscopic ferroelectric properties, but result in low temperature relaxor properties. Three characteristic hard carbon materials have been synthesized with increasing pyrolysis temperature to differentiate their defect concentration. The physical and electrochemical properties have been characterized and show a clear trend of decreasing defect concentration with increasing pyrolysis temperature, which results in lowering the capacity for Na ion storage. The most defective sample is modeled using molecular dynamics (MD) simulated quench methods and those models are fit to neutron pair distribution function (PDF) data using the reverse Monte Carlo (RMC) method. Analysis of the structure shows that it is made of graphene sheets with mostly 6-memberd rings, but other sized rings, edge sites, pore sites, and undercoordinated methyl and methylene sites also exist. These sites are then discussed in the regard to their contributions to the galvanostatic charge and discharge data, allowing for a structural explanation of the enhanced electrochemical storage in the most defective structure.
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  • Ongoing Research
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  • 2018-03-30 to 2019-04-30

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