Exploring cation disorder in mixed-metal pyrochlore ceramics using ¹⁷O NMR spectroscopy and first-principles calculations

Characterising the local structures (e.g., the cation distribution) of mixed-metal ceramics by NMR spectroscopy is often challenging owing to the unfavourable properties (low γ, large quadrupole moment and/or low abundance) of many metal nuclei. ¹⁷O is an attractive option owing to the prevalence of oxygen within ceramics. The moderate γ and small quadrupole moment of ¹⁷O mean that the greatest barrier to accessing the information available from this nucleus is isotopic enrichment. We explore the challenges of ensuring uniform isotopic enrichment with ¹⁷O₂(g) for the pyrochlore solid solutions, Y₂Sn_xTi₂–_xO₇, La₂Sn_xZr₂–xO₇ and La₂Sn_xHf₂–_xO₇, demonstrating that high enrichment temperatures (900 °C for 12 h) are required. In addition, for sites with very high symmetry (such as the tetrahedral OY4 and OLa4 sites with CQ ≈ 0 present here), we demonstrate that quantitative ¹⁷O NMR spectra require correction for the differing contributions from the centreband of the satellite transitions, which can be as high as a factor of ~3.89. It is common to use first-principles calculations to aid in interpreting NMR spectra of disordered solids. Here, we use an ensemble modelling approach to ensure that all possible cation arrangements are modelled in the minimum possible number of calculations. By combining uniform isotopic enrichment, quantitative NMR spectroscopy and a comprehensive computational approach, we are able to show that the cation distribution in Y₂Sn_xTi₂–_xO₇ is essentially random, whereas in La₂Sn_xZr₂–_xO₇ and La₂Sn_xHf₂–_xO₇, OLa2SnZr and OLa2SnHf sites are slightly energetically disfavoured, leading to a weak preference for clustering of like cations.