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

Abstract

The local structure and cation disorder in Y₂(Sn,Ti)₂O₇ pyrochlores, materials proposed for the encapsulation of lanthanide- and actinide-bearing radioactive waste, is investigated using ⁸⁹Y (<i>I</i> = 1/2) NMR spectroscopy and, in particular, measurement of the ⁸⁹Y 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 ⁸⁹Y anisotropic shielding interaction using two-dimensional CSA-amplified PASS experiments, implemented for ⁸⁹Y for the first time. Despite the challenges associated with the study of low-γ nuclei, and those resulting from long T₁ 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, Ω, 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)