Oxide Ion Conductors
Oxide ion conductors are technologically important materials, essential for applications such as oxygen sensors and pumps, ceramic membranes for oxygen separation and partial oxidation of light hydrocarbons and solid oxide fuel cells (SOFCs), where they act as electrolytes transporting O2- ions to react with a fuel such as hydrogen in the direct clean conversion of chemical to electrical energy. Our current research focusses on the discovery of new materials with high ionic conductivity, particularly at at lower temperatures (400-600oC). Ionic mobility in solids is of fundamental significance; however, if suitable materials are found, the SOFC operating temperatures could be lowered, thus alleviating the two main problems (reliability and cost) which currently limit their widerspread use in energy generation.
The first insight into oxide ion diffusion pathways in lone pair-containing apatites
Our recent work on the La10-xBixGe6O27 (x = 0, 2, 4) series provided the first experimenatally- and computationally-derived insight into lone pair-containing apatite-type oxide ion conductors. We identified four types of oxide ion exchange mechanisms and described the effects that the introduction of lone-pair cations has on the O2- migration pathways and on the conductivity.
The analysis of the dynamics within the La10-xBixGe6O27 (x = 0, 2, 4) series also revealed how the anisotropic nature of conductivity in these materials changes as a function of temperature and composition.
Structural complexity in triclinic apatite-type ionic conductors
Complementary use of high-resolution powder neutorn diffraction, synchrotron dffraction and electron microscopy allowed us to determine the structure of Bi2La8[(GeO4)6]O3, which adopts a triclinic variant of the apatite structure type. Combining annular bright‐field scanning transmission electron microscopy experiments with frozen‐phonon multislice simulations enabled direct imaging of the crucial interstitial oxygen atoms present at very low levels (8 out of 1030 electrons per formula unit) in the material.
Best-in-class low-temperature oxide ion conductor
We reported remarkably high low-temperature ionic conductivity in Bi1-xVxO1.5+x (x = 0.087, 0.095; σ~3.9 ×10-2 S/cm at 450oC) and the roles of the different structural building blocks in this process.We have attributed this remarkable behaviour to the simultaneous presence of three key factors: a highly polarisable sublattice with vacancies, central atoms able to support variable coordination numbers and geometries, and the rotational flexibility of the these coordination polyhedra. Importantly, the structure is a stable, pseudo-cubic 3 × 3 × 3 fluorite-based superstructure.
Oxide ion migration pathways in Bi1-xVxO1.5+x obtained by AIMD simulations are shown below; white displacement clouds represent space visited by oxide ions; V coordination polyhedra VOx shown in red; OBi4 groups and Bi atoms shown in yellow. (a) direct O2-exchange between VOx groups; (b) O2- exchange between VOx groups via a OBi4 tetrahedron; (c) O2- vacancy-hopping through the Bi-O subllatice.