Phase Composition and Disorder in La2(SnTi)2O7 Ceramics: New Insights from NMR Crystallography

摘要

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 alimited solid solution is observed (up to ∼7% Ti). DFT calculationspredict very similar ¹¹⁹Sn shifts for Sn substitution intothe two proposed models of La2Ti2O7 (monoclinic (P2₁) and orthorhombic (Pna2₁)), indicating it is not possible to distinguishbetween them. However, the relative energy of the Sn-substituted orthorhombicphase was higher than that of substituted monoclinic cells, suggestingthat the latter is the more likely structure.