School of Chemistry (ETDs)

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Browsing School of Chemistry (ETDs) by Author "Billing, Caren"

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    Investigation of rhombohedral 𝑩𝒊𝟐𝑶𝟑 as an oxide conducting electrolyte for solid oxide fuel cell applications
    (University of the Witwatersrand, Johannesburg, 2023-09) Kerspuy, Tanner Royele Rowan; Billing, Caren; Erasmus, Rudolph M.; Billing, Dave Gordon
    The synthesis of a bismuth system co-doped with neodymium (Nd3+) and yttrium (Y3+) was at the core of this project. The focus was placed on the synthesis of the rhombohedral phase of bismuth oxide, which has not been observed in pure bismuth oxide. Neodymium was selected as the main dopant (the one used in highest dopant concentration), due to its Shannon ionic radii. Upon doping with Nd3+ as a single dopant, it is observed that a mixture of the rhombohedral and monoclinic phases is obtained, thus noting that the single dopant system using Nd3+ does not stabilise the rhombohedral phase. When using a co-doped system of 15 mol % Nd3+ and 5 mol % Y3+ (15Nd5YSB), it is observed that we are able to obtain a stable phase pure rhombohedral phase, with a total dopant concentration of 20 mol%. The total dopant concentration % ranges selected ranged between 8.5-10 mol %, 20 mol % and 22.5 mol %. The Rietveld refinement of the X-ray diffraction data obtained for both the laboratory and synchrotron-based techniques indicate sample phase purity and phase stability for the samples under investigation. The refinements obtained for the samples indicated that not only one structure model was used to fit the experimental data. The structural models which fit the Rietveld refinements of the experimental data resulted in the observation of pure phase and mixed phase rhombohedral samples being observed. The Nd0.15Y0.05-Bi2O3 (15Nd5YSB)sample resulted in a phase pure rhombohedral structural model. Hereafter all samples will be referred to with the shorthand notation. The thermal analysis techniques are used to indicate the thermal dependence of the samples, this analysis also indicated phase stability across the temperature range of investigation as no phase transitions occurred throughout the heating and cooling cycles, and minimal weight loss is observed. The samples of importance in this study were the 12.5Nd10YSB sample which obtained a conductivity of 2.4511×10-5 S.cm-1 at 500 ℃, and the 15Nd5Y2.5TbSB sample which obtained a conductivity of 2.1725×10-5 S.cm-1 at 500 ℃. The Arrhenius plots obtained indicated stability 3 of these samples across the 200-500 ℃ temperature range with no discontinuities, which suggests no phase transitions, or order-to-disorder transitions. Variable temperature Raman spectroscopy indicated that the behaviour for all the samples analysed using Raman spectroscopy is consistent, however, a deviation was observed for the 15Nd5Y2.5ScSB sample which has a distinctive spot which exhibits different Raman shift behaviour as compared to all other samples. The VT-Raman spectroscopy spectra indicate a distinctive signature Raman peak at ~250 cm-1, which can be concluded to be the Raman peak which is indicative of the rhombohedral 𝐵𝑖2𝑂3, this peak also appears in the low cubic phase % sample after cooling back to room temperature. This assignment of the Raman spectral peak is confirmed through this peak being evident throughout all the spectra obtained and it being consistent throughout all the spectra observed.
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    Structural Characterization of Bimetal-Phosphate Based Solid-State Electrolytes: A PXRD, PDF and XAS Study
    (University of the Witwatersrand, Johannesburg, 2024) Nkala, Gugulethu Charmaine; Billing, David G.; Billing, Caren; Vila, Fernando D.; Forbes, Roy P.
    In this work, NASICON-type lithium titanium phosphate (LiTi2(PO4)3, LTP) was synthesized following the conventional solid-state reaction methodology. Single and double-doped formulations of LTP were made, with the primary objective of improving the room-temperature ionic conductivity, for their application as potential solid-state electrolytes for all-solid-state Li ion batteries. The primary characterization technique applied was ambient-temperature powder X-ray diffraction (PXRD) at both laboratory and synchrotron experimental conditions. The Rietveld refinement approach was used to determine the qualitative and quantitative phase compositions of each sample, revealing the rhombohedral (R-3c, space group #167) main phase, with phosphate-based secondary phases. Total scattering data, through the pair distribution function (PDF) was applied, revealing lattice site preference during the substitution of Ti with Al, Sn and Dy at the 12c site. Further analysis through small-box modelling indicated the local structure deviation below 10 Å, from rhombohedral (R-3c) to monoclinic (P21/n, space group #14). The application of experimental X-ray absorption spectroscopy (XAS) revealed a stable 4+ oxidation state for Ti regardless of doping. However, the extended X-ray absorption fine structure (EXAFS) data showed that the replacement of Ti with Sn results in heavy disorder and subsequent changes in the PO4 tetrahedra, corroborating the findings from Raman spectroscopy. Theoretical XAS spectra were computed using FEFF, providing insights into the origins of experimentally observed XAS features from first-principles. Applying electrochemical impedance spectroscopy (EIS) to assess the ambient-temperature ionic conductivity, co-doped systems showed an improvement in the conductivity. The application of characterization techniques at various length scales has been demonstrated to provide insights into the mechanisms governing the performance of the solid-state electrolytes.