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

TL;DR

This study reveals that La$_{5.4}$WO$_{12- heta}$ exhibits nanoscale ordered domains within a disordered fluorite structure, with local WO6 octahedral arrangements influencing its mixed conduction properties, suggesting a frustrated lattice mechanism.

Contribution

It provides a detailed local structural model of La$_{5.4}$WO$_{12- heta}$, linking nanoscale order to its mixed conduction behavior and introducing the concept of frustration-driven local order in this material.

Findings

Ordered domains of ~3.5 nm size identified

Short-range order influences mixed conduction

Local structure confirmed by multiple spectroscopic techniques

Abstract

We report a comprehensive investigation of the average and local structure of La$_{5.4}$WO$_{12-\delta}$, 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 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$_{12-\delta}$? 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.