Abstract
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.
Highlights
The chemical and structural flexibility of the pyrochlore materials (A2B2O7) has attracted significant interest in recent years
In order to undertake the analysis described and produce the results shown in Figure 7, it must be assumed that the pyrochlore peaks observed can be attributed solely to next-nearest neighbor (NNN) environments of Sn6, Sn5Ti, and Sn4Ti2
An alternative approach is possible, requiring only an accurate determination of the intensity of the peak ascribed to an environment with Sn6 NNN, but does assume that there is a random distribution of the B site cations in the pyrochlore phase
Summary
The chemical and structural flexibility of the pyrochlore materials (A2B2O7) has attracted significant interest in recent years. We use DFT calculations to investigate the preferred position and distribution of the substituted cations in each of the two phases, and consider the consistency of our data with the suggested (i.e., B and C) structural models for La2Ti2O7 This NMR crystallographic study provides more detailed structural information than is available from laboratory X-ray diffraction and demonstrates the future potential of this approach for the investigation of disordered solids. Computed structures and NMR parameters were processed using Python scripts extending the CCP-NC MagresPython module.[34]
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