The sensitivity of NMR to the local environment, without the need for any long-range order, makes it an ideal tool for the characterization of disordered materials. Computational prediction of NMR parameters can be of considerable help in the interpretation and assignment of NMR spectra of solids, but the statistical representation of all possible chemical environments for a solid solution is challenging. Here, we illustrate the use of a symmetry-adapted configurational ensemble in the simulation of NMR spectra, in combination with solid-state NMR experiments. We show that for interpretation of the complex and overlapped lineshapes that are typically observed, it is important to go beyond a single-configuration representation or a simple enumeration of local environments. The ensemble method leads to excellent agreement between simulated and experimental spectra for Y2(Sn,Ti)2O7 pyrochlore ceramics, where the overlap of signals from different local environments prevents a simple decomposition of the experimental spectral lineshapes. The inclusion of a Boltzmann weighting confirms that the best agreement with experiment is obtained at higher temperatures, in the limit of full disorder. We also show that to improve agreement with experiment, in particular at low dopant concentrations, larger supercells are needed, which might require alternative simulation approaches as the complexity of the system increases. It is clear that ensemble-based modeling approaches in conjunction with NMR spectroscopy offer great potential for understanding configurational disorder, ultimately aiding the future design of functional materials.

Investigation of the conditions required for quantitative isotopic enrichment and the acquisition of quantitative 17O NMR spectra of ceramic oxides.

An NMR crystallographic approach, involving the combination of 119Sn NMR spectroscopy, XRD, and DFT calculations, is demonstrated for the characterization of La2Sn2–xTixO7 ceramics. A phase change from pyrochlore (La2Sn2O7) to a layered perovskite phase (La2Ti2O7) is predicted (by radius ratio rules) to occur when x ≈ 0.95. However, the sensitivity of NMR spectroscopy to the local environment is able to reveal a significant two-phase region is present, extending from x = 1.8 to ∼0.2, with limited solid solution at the two extremes, in broad agreement with powder XRD measurements. DFT calculations reveal that there is preferential site substitution of Sn in La2Ti2O7, with calculated shifts for Sn substitution onto Ti1 and Ti2 sites (in the “bulk” perovskite layers) in better agreement with experiment than those for Ti3 and Ti4 (“edge” sites). Substitution onto these two sites also produces structural models with lower relative enthalpy. As the Sn content decreases, there is a further preference for substitution onto Sn2. In contrast, the relative intensities of the spectral resonances suggest that Ti substitution into the pyrochlore phase is random, although only a limited solid solution is observed (up to ∼7% Ti). DFT calculations predict very similar 119Sn shifts for Sn substitution into the two proposed models of La2Ti2O7 (monoclinic (P21) and orthorhombic (Pna21)), indicating it is not possible to distinguish between them. However, the relative energy of the Sn-substituted orthorhombic phase was higher than that of substituted monoclinic cells, suggesting that the latter is the more likely structure.

A NMR crystallographic approach, combining 89 Y, 119 Sn and 17 O NMR spectroscopy with X-ray diffraction and first-principles calculations has been used investigate the number and type of phases present, and the local structure and disorder in Y 2 Hf 2-x Sn x O 7 ceramics.Although a phase change is predicted with increasing Hf content, NMR spectra clearly show the presence of a significant two-phase region, with a Sn-rich pyrochlore and relatively Hf-rich defect fluorite phase co-existing for much of the compositional series. A single-phase pyrochlore is found only for the Sn end member, and a single defect fluorite phase only for x = 0 to 0.2. A solid-solution limit of ~10% is seen for the substitution of Hf into Y 2 Sn 2 O 7 , although no evidence is seen for any cation ordering or antisite disorder in this phase. In the defect fluorite phase there is preferential ordering of oxygen vacancies around Sn, which is only ever seen in a six-coordinate environment. The remaining vacancies are more likely to be associated with Hf than with Y, although this distinction is less apparent at higher Sn concentrations. To acquire 17 O NMR spectra samples were postsynthetically exchanged with 17 O 2 (g), although high temperatures (> 900 ºC) were required to ensure uniform enrichment of different chemical species. Although these 17 O NMR spectra confirm the formation of mixed-metal materials and the presence of two phases, more quantitative analysis is hindered by the overlap of signals from pyrochlore and defect fluorite phases. In all cases, DFT calculations play a vital role in the interpretation and assignment of the NMR spectra, and in understanding the local structure and disorder in these complex multi-phase materials.