Cation Disorder in Pyrochlore Ceramics: ⁸⁹Y MAS NMR and First-Principles Calculations

The use of first-principles DFT calculations to interpret solid-state Y-89 MAS NMR spectra of Y2Ti2-xSnxO7 pyrochlores, materials with applications for the encapsulation of actinide-bearing radioactive waste, is investigated. Although NMR is a sensitive probe of local structure, which does not rely on the presence of long-range order, spectra of disordered materials are often complex and difficult to interpret. We show how calculations can be used alongside experiment to confirm that Y occupies only the eight-coordinate pyrochlore A site in these materials and that the Y-89 isotropic chemical shift is primarily affected by the number of Sn/Ti on the neighboring B sites. Small changes in local geometry and more distant B-site cation substitutions are shown to have a smaller effect on the chemical shift, and will result in broadening, shoulders, and small splittings in the NMR spectrum, In general, the results confirm the validity of the assumptions made in the previous spectral analysis, although in a very small minority of cases, chemical shifts are calculated which lie outside the expected ranges. However, these are shown to result from significant deviations in local geometry (O-Y-O bond angles and Y-O bond distances) and are thought to arise from the periodicity (and, therefore, long-range order) which is imposed in the calculations. Using our calculated results we can confirm that there is a random distribution of Sn/Ti on the six-coordinated pyrochlore B sites in Y2Ti2-xSnxO7, and also demonstrate that an equilibrium structure has been achieved by studying materials which have been annealed for different durations.