Exploiting ¹⁷O NMR and first-principles calculations for the study of disorder in ceramic oxides

  • Arantxa Fernandes

Student thesis: Doctoral Thesis (PhD)

Abstract

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 observed 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 well-resolved 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 cost-effective 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.
Date of Award30 Jun 2021
Original languageEnglish
Awarding Institution
  • University of St Andrews
SupervisorSharon E. Ashbrook (Supervisor)

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