Nanoscale order in the frustrated mixed conductor La_5.6WO_12−δ

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• 作者：Tobias Scherb ; Simon A. J. Kimber ; Christiane Stephan ; Paul F. Henry ; Gerhard Schumacher ; Sonia Escolá ; stico ; José ; M. Serra ; Janka Seeger ; Justus Just ; Adrian H. Hill and John Banhart
• 关键词：neutron diffraction ; X-ray diffraction ; pair distribution function ; EXAFS ; proton conductors ; La5.6WO12&minus ; &delta
• 刊名：Journal of Applied Crystallography
• 出版年：2016
• 出版时间：June 2016
• 年：2016
• 卷：49
• 期：3
• 页码：997-1008
• 全文大小：1719K
• ISSN：1600-5767

文摘

This article reports a comprehensive investigation of the average and local structure of La_5.6WO_12−δ, which has excellent mixed proton, electron and oxide ion conduction suitable for device applications. Synchrotron X-ray and neutron powder diffraction show that a cubic fluorite supercell describes the average structure, with highly disordered lanthanum and oxide positions. On average, the tungsten sites are sixfold coordinated and a trace [3.7 (1.3)%] of anti-site disorder is detected. In addition to sharp Bragg reflections, strong diffuse neutron scattering is observed, which hints at short-range order. Plausible local configurations are considered and it is shown that the defect chemistry implies a simple `chemical exchange' interaction that favours ordered WO<sub>6</sub> octahedra. The local model is confirmed by synchrotron X-ray pair distribution function analysis and EXAFS experiments performed at the La K and W L<sub>3</sub> edges. It is shown that ordered domains of ∼3.5 nm are found, implying that mixed conduction in La<sub>5.6</sub>WO<sub>12−δ</sub> is associated with a defective glassy-like anion sublattice. The origins of this ground state are proposed to lie in the non-bipartite nature of the face-centred cubic lattice and the pairwise interactions which link the orientation of neighbouring octahedral WO<sub>6</sub> sites. This `function through frustration' could provide a means of designing new mixed conductors.