Nanoscale order in the frustrated mixed conductor La$_{5.4}WO$_{12-δ}$

Tobias Scherb; Simon A. J. Kimber; Christiane Stephan; Paul F. Henry; Gerhard Schumacher; Justus Just; Sonia Escolastico; Jose M. Serra; Janka Seeger; Adrian H. Hill; John Banhart

arXiv · 1305.3385

Nanoscale order in the frustrated mixed conductor La$_{5.4}WO$_{12-δ}$

Tobias Scherb·Simon A. J. Kimber·Christiane Stephan·Paul F. Henry·Gerhard Schumacher·Justus Just·Sonia Escolastico·Jose M. Serra·Janka Seeger·Adrian H. Hill·John Banhart

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Abstract

We report a comprehensive investigation of the average and local structure of La$_{5.4}$WO$_{12-δ}$, 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 six-fold coordinated, and we detect a trace (4.4(2) %) of anti-site disorder. In addition to sharp Bragg reflections, strong diffuse neutron scattering is observed, which hints at short-range order. We consider plausible local configurations, and show that the defect chemistry implies a simple 'chemical exchange' interaction that favours ordered WO6 octahedra. Our local model is confirmed by synchrotron x-ray pair distribution function analysis and EXAFS experiments performed at the La K and W L3-edges. We show that ordered domains of around ca. 3.5 nm are found, implying that mixed conduction in La$_{5.4}$WO$_{12-δ}$? 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 fcc lattice and the pairwise interactions which link the orientation of neighbouring octahedral WO6 sites. This 'function through frustration' could provide a means of designing new mixed conductors.

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Tobias Scherb, Simon A. J. Kimber, Christiane Stephan, Paul F. Henry, Gerhard Schumacher, Justus Just, Sonia Escolastico, Jose M. Serra, Janka Seeger, Adrian H. Hill, John Banhart. 2013-05-15. Nanoscale order in the frustrated mixed conductor La$_{5.4}WO$_{12-δ}$. https://arxiv.org/abs/1305.3385

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