Phase composition and disorder in La₂(Sn,Ti)₂O₇ ceramics: new insights from NMR crystallography

An NMR crystallographic approach, involving the combination of ¹¹⁹Sn NMR spectroscopy, XRD and DFT calculations, is demonstrated for the characterisation of La₂Sn_2–xTi_xO₇ 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.