Scherb, T.; Kimber, S.; Stephan, C.; Henry, P.; Schumacher, G.; Escolastico, S.; Serra, J.; Seeger, J.; Just, J.; Hill, A.; Banhart, J.: Nanoscale order in the frustrated mixed conductor La5.6WO12-δ. Journal of Applied Crystallography 49 (2016), p. 997-1008
This article reports a comprehensive investigation of the average and local structure of La5.6WO12-delta, 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 WO6 octahedra. The local model is confirmed by synchrotron X-ray pair distribution function analysis and EXAFS experiments performed at the La K and W L3 edges. It is shown that ordered domains of about 3.5 nm are found, implying that mixed conduction in La5.6WO12-delta is associated with a defective glassy-like anion sublattice. The origins of this ground state are proposed to lie in the onbipartite nature of the face-centred cubic 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.