A combined experimental and computational approach to understanding and developing new solid-state ionic conductors

Abstract

Materials that exhibit significant mobility of several different types of charge carriers (e.g., oxide ions, protons and electrons) have diverse potential applications as fuel-cell membranes, electrodes, batteries, and sensors. A thorough understanding of the fundamental atomic-scale mechanisms of the conduction processes in these materials and their relationship to chemical composition, crystal structure and lattice dynamics is necessary to suggest ways in which the local chemistry and structure of these materials can be modified to lower activation barriers and optimise pathways for conduction. This project set out to thoroughly characterise the crystal structures and dynamics of ionic conductivity in complex solid-state materials, at a level that would permit the rational design of new and improved versions, using a combined experimental and computational approach. On the experimental side, powder samples and single crystals of promising materials were synthesised. These materials were characterised structurally using a variety of diffraction techniques combined with X-ray absorption spectroscopy measurements, to understand the relationship between the average structure and local coordination environments. The central techniques were then inelastic and quasielastic neutron scattering (INS, QENS). Being sensitive to all dynamic processes – the incoherent diffusive motions and coherent lattice vibrations – INS and QENS are able to provide an overview of the entire dynamic processes in the material over the time scale of several femtoseconds to a few picoseconds. Finally, ab initio density functional theory calculations, in the form of single point energy calculations, geometry optimisations and molecular dynamics simulations were used to supplement our experiments and help correlate physical properties with structure and dynamics.