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 = 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–O8b distance (where 8b indicates the Wyckoff position of the oxygen).

Color and Brightness Tuning in Heteronuclear Lanthanide Terephthalate Coordination Polymers

“…[31] Recent papers proved that 89 Y NMR spectroscopy is a useful probe for local ordering in solid-state compounds. [38][39][40][41] Consequently, we performed 13 (Figures 2 and 3) show no evidence of local ordering and support the interpretation of randomly distributed yttrium and lanthanum atoms. Figure 2 clearly shows that regardless of the lanthanum doping rate, the 89 Y NMR signal remains almost identical.…”

Columnar self-assembly of N,N′,N′′-trihexylbenzene-1,3,5-tricarboxamides investigated by means of NMR spectroscopy and computational methods in solution and the solid state

“…The 13 C CP CSA Amplification [39,40] spectrum of the BTA considered in this study is shown in Figure 7 each analyzed carbon environment is given by the 95% confidence interval as extracted from the rms deviation 2D contour plots shown in (e-g). [41,42,69] The parameters extracted at the minima of these 2D surfaces are given in Table 3 for the three sites of interest.…”

“…With respect to the anisotropy of the shift interaction, methods have been introduced that are not only capable of recoupling the inhomogeneity averaged out by the fast mechanical rotation, but also amplifying it, as if the experiment was performed at a slower and fictitious spinning rate. [37][38][39][40][41][42] This allows for more precise measurements of the full shielding tensor than those that one can achieve under static conditions or by spinning the sample at lower rates. [38] With respect to the chemical shift interaction, the following convention of NMR parameters is adopted in this study: [43] …”

Investigation of zeolitic imidazolate frameworks using 13 C and 15 N solid-state NMR spectroscopy

“…For nuclear sites that possess high local symmetry, resulting in small anisotropies, "amplification" experiments have also recently been developed [27], increasing the apparent magnitude of the interaction (by a user-defined scaling factor) and enabling more accurate measurement. The experiment used in this work, the CSA-amplified PASS approach [28,29], has been used successfully to investigate 1 H, 13 C, 31 P and 89 Y local environments in a range of materials, and correlations with local structural parameters, e.g., bond distances or point symmetry, demonstrated [29][30][31][32][33]. For ZIFs, solid-state NMR spectroscopy can provide information on the type and relative proportions of linkers present, and the number of crystallographically-distinct molecules present in the asymmetric unit.…”

“…Although these are typically removed under the sample rotation used to achieve high resolution, they can be measured either from a slow MAS spectrum or from the sideband pattern in the indirect dimension of a two-dimensional experiment that recouples, or selectively reintroduces, the anisotropic shielding [26,27]. It has been shown that the anisotropic shielding, the asymmetry and the tensor orientation can provide information on the local structure including, e.g., bond distances and point symmetry [15,16,24,30]. Given the complexity of the 13 C spectra of most ZIFs, slow MAS experiments result in considerable spectral overlap, and CSA-amplified PASS experiments have been used to reintroduce and measure the anisotropic shielding.…”