3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Investigating novel acyclic lanthanide complexes suitable for magnetic resonance imaging(2014-07-04) Padayachy, KamentherenThe ligands 2-(bis(2-(octahydro-2-oxobenzo[b][1,4]oxazin-4-yl)ethyl)amino)acetic acid trihydrochloride and bis(2-(octahydro-2-oxobenzo[b][1,4]oxazin-4-yl)ethyl)amine trihydrochloride are synthesized for the formation of Gd(III) complexes, potentially useful as MRI contrast agents. Speciation and hydration numbers of the Gd(III) complexes of 1,7- bis(2-hydroxycyclohexyl)-1,4,7-tris(carboxymethyl)-1,4,7-triazaheptane (Cy2DETA.3A) and 1,7-bis(2-hydroxycyclohexyl)-1,7-bis(carboxymethyl)-1,4,7-triazaheptane (Cy2DETA.2A) can be inferred from spectroscopic studies of the analogous Eu(III) complexes, due to their similar ionic radii, charge and coordination geometry. The phosphorescence emission spectra for the Eu(III) complexes are recorded as function of pH, in buffered and aqueous media. Modulation of the relative intensities of the emission bands is observed for both Eu(III) complexes. Spectroscopically determined pKa values are used to determine potential solution speciation. Ligand pKa values were determined potentiometrically. Attempts were made to correlate results from potentiometric titrations with that of lanthanide luminescent titrations of these complexes. Hydration states (q) were determined for the Eu(III) complexes at spectroscopically significant pH values, in buffered and aqueous media. The results confirm the formation of a octadentate complex between Eu(III) and Cy2DETA.3A, with the inclusion of one H2O molecule in the inner sphere. The complex has high stability and is responsive towards changes in pH and analyte concentration (o-phthalate); potentially suitable as a luminescent sensor. The heptadentate complex formed between Cy2DETA.2A and Eu(III) is substantially weaker, with precipitation of Eu(OH)3 observed at neutral pH, that limits its potential application as a luminescent sensor.Item An investigation of the structural and magnetic properties of Ho substituted BiFeO3(2012-09-18) Ncube, MehluliThe doping of BiFeO3 with lanthanide elements like Ho, with a radius smaller than Bi, is ideal to improve the ferroelectric and magnetic properties of BiFeO3, which in principle can cause structural distortions of the lattice that improve the electrical and magnetic properties. In this work, we report on the temperature dependence of the structural and magnetic properties of Ho substituted BiFeO3 (BHFO) samples, which have been investigated by X-ray diffraction (XRD) and Mössbauer spectroscopic techniques. The XRD and Mössbauer measurements were done at room temperature on the as-synthesized BHFO samples and after annealing the samples in Argon up to 1073 K. The resultant XRD patterns have shown that BHFO is of rhombohedral R3m space group, with a majority Bi25FeO90 phase and a minority Bi2Fe4O9 phase. These two phases are attributed to the local stoichometry fluctuations in BiFeO3 (BFO). A new phase was evident in the XRD spectra after annealing the sample between 673 – 873 K; this has been assigned to the octahedral B-site of Fe3O4. The Mössbauer spectra were characterized by broadened features and the magnetic hyperfine splitting patterns were indicative of magnetic ordering mostly probably screwed or slightly antiferromagnetic ordering. The spectra were fitted with two symmetric sextets (S1 & S2) which were present in all annealed samples, a symmetric sextet (S3) which was observable at annealing temperatures greater than 673 K, a Lorentzian doublet (D) and a single line (SL) which were present in all spectra. The extracted hyperfine parameters of sextet S1 are consistent with those of rhombohedral BiFeO3 and are characteristic of magnetically ordered Fe3+. At TA > 673 K, a third sextet S3 was assigned to the high symmetry cubic spinel phase. The paramagnetic doublet D was attributed to the Bi25FeO40 phase and the singlet line SL to the Bi2Fe4O9 phase which has been observed previously in the studies of BiFeO3 and other BiFeO3 doped systems. The isomer shift and quadrupole splitting values of the paramagnetic doublet D corresponds to an oxidation state of Fe3+, while the isomer shift of S1 remained fairly constant up to TA = 623 K then decreased gradually after the appearance of S3 indicating an increase of the s-electron density at the Fe nucleus. The quadrupole splitting of S2 showed no systematic change with annealing temperature, however at TA > 623 K this parameter changed dramatically to a negative value with a slightly larger magnetic field. The distribution of the isomer shift and the difference in the quadrupole splitting values and signs are due the variation in the angles between the principal axis of the electric field gradient (EFG) and the spin direction. The hyperfine fields of S1 and S2 remained fairly constant for all measured samples, however at TA > 623 K the hyperfine field of S3 showed a slight increase which could be due to Ho being substituted at the Fe site in BiFeO3. In addition, in-situ Mössbauer measurements at temperatures in the range 300 – 748 K were made on the BHFO samples. The room temperature spectrum showed similar features as observed on the annealing series of measurements. The hyperfine magnetic fields of the two sextet components (S1 and S2) decreased with increasing temperature and finally collapsed at T > 588 K. The hyperfine fields of both the S1 and S2 components decreased systematically with temperature to a field distribution just below the Néel temperature. From our measurements, we estimated the Néel temperature for BHFO to be in the range 598 – 617 K. The isomer shift for all spectral components showed a linear decrease with increasing temperature which closely followed the usual second order Doppler shift variation with temperature. The S1 and S2 spectral components present at room temperature disappeared just before the Néel temperature resulting in the area fraction of the paramagnetic doublet D dominating the spectrum. From the site populations, an average Debye temperature (θD) was estimated to be 240 ± 81 K for BHFO which is lower than the value of 340 ± 50 K cited for BiFeO3.