Exploring cation disorder in mixed-metal pyrochlore ceramics using O 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. O is an attractive option owing to the prevalence of oxygen within ceramics. The moderate γ and small quadrupole moment of O 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 O (g) for the pyrochlore solid solutions, Y Sn Ti O , La Sn Zr O and La Sn Hf O , demonstrating that high enrichment temperatures (900 °C for 12 hr) are required. In addition, for sites with very high symmetry (such as the tetrahedral OY4 and OLa4 sites with C  ≈ 0 present here), we demonstrate that quantitative O 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 Y Sn Ti O is essentially random, whereas in La Sn Zr O and La Sn Hf O , OLa2SnZr and OLa2SnHf sites are slightly energetically disfavoured, leading to a weak preference for clustering of like cations.