Fernandes Arantxa, McKay David, Sneddon Scott, Dawson Daniel M, Lawson Sebastian, Veazey Richard, Whittle Karl R, Ashbrook Sharon E
School of Chemistry, EaStCHEM and Centre of Magnetic Resonance, University of St. Andrews , St. Andrews KY16 9ST, United Kingdom.
Department of Materials Science and Engineering, University of Sheffield , Mappin Street, Sheffield S1 3JD, United Kingdom.
J Phys Chem C Nanomater Interfaces. 2016 Sep 15;120(36):20288-20296. doi: 10.1021/acs.jpcc.6b06962. Epub 2016 Aug 24.
An NMR crystallographic approach, involving the combination of Sn NMR spectroscopy, XRD, and DFT calculations, is demonstrated for the characterization of LaSn Ti O ceramics. A phase change from pyrochlore (LaSnO) to a layered perovskite phase (LaTiO) is predicted (by radius ratio rules) to occur when ≈ 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 = 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 LaTiO, 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 Sn shifts for Sn substitution into the two proposed models of LaTiO (monoclinic (2) and orthorhombic (2)), 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.
一种涉及¹¹⁹Sn核磁共振光谱、X射线衍射（XRD）和密度泛函理论（DFT）计算相结合的核磁共振晶体学方法被用于表征LaSnTiO陶瓷。根据半径比规则预测，当x≈0.95时，会发生从烧绿石（LaSnO）到层状钙钛矿相（LaTiO）的相变。然而，核磁共振光谱对局部环境的敏感性能够揭示存在一个显著的两相区域，其范围从x = 1.8延伸到0.2，在两个极端处固溶体有限，这与粉末XRD测量结果大致相符。DFT计算表明，在LaTiO中Sn存在优先的位点取代，计算得出的Sn取代到Ti1和Ti2位点（在“体相”钙钛矿层中）的化学位移比取代到Ti3和Ti4位点（“边缘”位点）的化学位移与实验结果更吻合。取代到这两个位点还会产生相对焓较低的结构模型。随着Sn含量的降低，进一步倾向于取代到Sn2位点。相比之下，光谱共振的相对强度表明，Ti取代到烧绿石相是随机的，尽管只观察到有限的固溶体（高达7%的Ti）。DFT计算预测，对于Sn取代到LaTiO的两种 proposed模型（单斜晶系(2)和正交晶系(2)），¹¹⁹Sn的化学位移非常相似，这表明无法区分它们。然而，Sn取代的正交晶系相的相对能量高于取代的单斜晶系晶胞，这表明后者更可能是结构。
