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Title: Exploiting ¹⁷O NMR and first-principles calculations for the study of disorder in ceramic oxides
Author: Fernandes, Arantxa
ISNI:       0000 0004 7656 9500
Awarding Body: University of St Andrews
Current Institution: University of St Andrews
Date of Award: 2019
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Ceramic oxides of the type A2B2O7 (A = La, Y and B = Sn, Ti, Zr and Hf) were investigated in this thesis. Initial work was concerned with the 119Sn NMR study of phase transitions and cation distribution in La2(Sn,Ti)2O7 ceramics, supported by DFT calculations. This study suggested a random distribution of Ti cations in the pyrochlore phase, while a preferential substitution of Sn on to the two bulk perovskite-like sites was obsreved for La2Ti2O7. However, for most starting compositions a two phase mixture was obtained. 119Sn was also employed to study cation disorder in La2(Sn,Zr)2O7 and La2(Sn,Hf)2O7 pyrochlores. Although wellresolved resonances were obtained these proved difficult to assign and interpret owing to the overlap of signals from different local environments, suggesting an alternative approach is required. 17O NMR spectroscopy offers an alternative or additional approach for the study structure and disorder, and would be of particular use in systems that lack appropriate spin I = ½ nuclei, such as e.g., La2Ti2O7, La2Zr2O7 and La2Hf2O7. Owing to the low natural abundance of 17O (~0.037%), samples in this work were enriched post-synthetically with 70% 17O2(g). A systematic study of the conditions required to obtain uniform enrichment was performed for a series of end member compositions, before 17O NMR was applied to more complex materials (e.g., Y2Hf2-xSnxO7, La2Sn2-xTixO7). This work explains in detail how quantitative spectral acquisition can be achieved for 17O, with emphasis on differences in nutation rates of different O species, differences in longitudinal relaxation (T1) and additional contributions from quadrupolar satellite transitions to the central transition signal. The O sites in the pyrochlore materials showed uniform enrichment with heating at 900 °C for 12 h, while defect fluorite and layered perovskite-like materials enriched uniformly at a relatively lower temperature (i.e., 600 °C for 12h). However, the absolute levels of enrichment in all materials were found to be better at the highest enrichment temperatures. This study proves that 17O quantitative enrichment can be achieved in a costeffective manner and 17O NMR is an informative probe for investigating local structure and disorder in oxides. Although complex spectra can be obtained in some cases, e.g., multi-phase systems, 17O NMR can provide important information, which would have been difficult to obtain using other approaches and offers great potential for the future.
Supervisor: Ashbrook, Sharon E. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral