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

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.