title: Phase Composition and Disorder in La2(Sn,Ti)2O7Ceramics: New Insights from NMR Crystallography.
abstract: An NMR crystallographic approach, involving the combination of119Sn NMR spectroscopy, XRD, and DFT calculations, is demonstrated for the characterization of La2Sn2-xTixO7ceramics. A phase change from pyrochlore (La2Sn2O7) to a layered perovskite phase (La2Ti2O7) is predicted (by radius ratio rules) to occur whenx≈ 0.95. However, the sensitivity of NMR spectroscopy to the local environment is able to reveal a significant two-phase region is present, extending fromx= 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 similar119Sn 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.
authors.affiliation: School of Chemistry, EaStCHEM and Centre of Magnetic Resonance, University of St. Andrews , St. Andrews KY16 9ST, United Kingdom.
authors.forename: Arantxa
authors.initials: A
authors.lastname: Fernandes
authors.affiliation: School of Chemistry, EaStCHEM and Centre of Magnetic Resonance, University of St. Andrews , St. Andrews KY16 9ST, United Kingdom.
authors.forename: David
authors.initials: D
authors.lastname: McKay
authors.affiliation: School of Chemistry, EaStCHEM and Centre of Magnetic Resonance, University of St. Andrews , St. Andrews KY16 9ST, United Kingdom.
authors.forename: Scott
authors.initials: S
authors.lastname: Sneddon
authors.affiliation: School of Chemistry, EaStCHEM and Centre of Magnetic Resonance, University of St. Andrews , St. Andrews KY16 9ST, United Kingdom.
authors.forename: Daniel M
authors.initials: DM
authors.lastname: Dawson
authors.affiliation: Department of Materials Science and Engineering, University of Sheffield , Mappin Street, Sheffield S1 3JD, United Kingdom.
authors.forename: Sebastian
authors.initials: S
authors.lastname: Lawson
authors.affiliation: Department of Materials Science and Engineering, University of Sheffield , Mappin Street, Sheffield S1 3JD, United Kingdom.
authors.forename: Richard
authors.initials: R
authors.lastname: Veazey
authors.affiliation: Department of Materials Science and Engineering, University of Sheffield, Mappin Street, Sheffield S1 3JD, United Kingdom; School of Engineering, University of Liverpool, Brownlow Hill, Liverpool, L69 3GH, United Kingdom.
authors.forename: Karl R
authors.initials: KR
authors.lastname: Whittle
authors.affiliation: School of Chemistry, EaStCHEM and Centre of Magnetic Resonance, University of St. Andrews , St. Andrews KY16 9ST, United Kingdom.
authors.forename: Sharon E
authors.initials: SE
authors.lastname: Ashbrook