17O solid-state NMR spectroscopy of A2B2O7 oxides: quantitative isotopic enrichment and spectral acquisition?

Abstract

The potential of ¹⁷O NMR spectroscopy for the investigation of A₂B₂O₇ ceramic oxides important in the encapsulation of radioactive waste is demonstrated, with post-synthetic enrichment by exchange with ¹⁷O₂ gas. For Y₂Sn₂O₇, Y₂Ti₂O₇ and La₂Sn₂O₇ pyrochlores, enrichment of the two distinct O species is clearly non quantitative at lower temperatures (∼700 °C and below) and at shorter times, despite these being used in prior work, with preferential enrichment of OA₂B₂ favoured over that of OA₄. At higher temperatures, the ¹⁷O NMR spectra suggest that quantitative enrichment has been achieved, but the integrated signal intensities do not reflect the crystallographic 1 : 6 (O1 : O2) ratio until corrected for differences in T₁ relaxation rates and, more importantly, the contribution of the satellite transitions. ¹⁷O NMR spectra of Y₂Zr₂O₇ and Y₂Hf₂O₇ defect fluorites showed little difference with any variation in enrichment temperature or time, although an increase in the absolute level of enrichment (up to ∼7.5%) was observed at higher temperature. DFT calculations show that the six distinct resonances observed cannot be assigned unambiguously, as each has contributions from more than one of the five possible next nearest neighbour environments. For La₂Ti₂O₇, which adopts a layered perovskite-like structure, little difference in the spectral intensities is observed with enrichment time or temperature, although the highest absolute levels of enrichment (∼13%) were obtained at higher temperature. This work demonstrates that ¹⁷O NMR has the potential to be a powerful probe of local structure and disorder in oxides, but that considerable care must be taken both in choosing the conditions for ¹⁷O enrichment and the experimental acquisition parameters if the necessary quantitative measurements are to be obtained for more complex systems.