Exploiting the Chemical Shielding Anisotropy to Probe Structure and Disorder in Ceramics: Y-89 MAS NMR and First-Principles Calculations

The local structure and cation disorder in Y-2(Sn,Ti)(2)O-7 pyrochlores, materials proposed for the encapsulation of lanthanide- and actinide-bearing radioactive waste, is investigated using Y-89 (I = 1/2) NMR spectroscopy and, in particular, measurement of the Y-89 anisotropic shielding. Although known to be a good probe of the local environment, information on the anisotropy of the shielding interaction is removed under magic angle spinning (MAS). Here, we consider the feasibility of experimental measurement of the Y-89 anisotropic shielding interaction using two-dimensional CSA-amplified PASS experiments, implemented for Y-89 for the first time. Despite the challenges associated with the study of low-y nuclei, and those resulting from long T-1 relaxation times, the successful implementation of these experiments is demonstrated for the end member pyrochlores, Y2Sn2O7 and Y2Ti2O7. The accuracy and robustness of the measurement to various experimental parameters is also considered, before the approach is then applied to the disordered materials in the solid solution. The anisotropies extracted for each of the sideband manifolds are compared to those obtained using periodic first-principles calculations, and provide strong support for the assignment of the spectral resonances. The value of the span, Omega, is shown to be a sensitive probe of the next nearest neighbor (NNN) environment, i,e., the number of Sn and Ti on the six surrounding "B" (i.e., six-coordinate) sites, and also provides information on the local geometry directly, through a correlation with the average Y-O-8b distance (where 8b indicates the Wyckoff position of the oxygen).