Oxide Ion Conductors for Energy Applications: Structure, Dynamics and Properties

Abstract

The work reported in this thesis investigates the relationships between the structural features of oxide ion conductors and the resulting oxide ion conduction mechanisms. This is achieved using a combination of attempted syntheses of new materials with novel structural features, direct observation of oxide ion dynamics via neutron scattering and simulation of oxide diffusion pathways using ab initio molecular dynamics.
The results of a variable temperature solid state 23Na NMR investigation into nominal Sr0.6Na0.4SiO2.8 are reported, showing conclusively that the charge carriers in the material are Na+ ions rather than O2-. The preparation, characterisation and conductivity properties of the Sr1-xLaxSiO3+0.5x and Sr1-xLaxGeO3+0.5x series as well as Y3+ and Ce3+ doped BaZrSi3O9 are also reported.
Quasieleastic and inelastic neutron scattering studies have been carried out, investigating diffusion processes in La2Mo2O9, Bi0.913V0.087O1.587 and La10-xBixGe6O27. These studies extend the body of work reporting the use of neutron scattering techniques on oxide ion conductors significantly. Phonon density of states derived from inelastic neutron scattering provide corroboration of the results gained from ab initio molecular dynamics calculations. The quasielastic neutron scattering results allow direct observation of long range oxide ion dynamics on timescales of nanoseconds, the longest timescales observed in oxide ion conductors reported to date.
The findings from in depth ab initio molecular dynamics (AIMD) investigations into La2Mo2O9 and Bi0.913V0.087O1.587, with a larger simulation boxes and significantly longer simulation times than those previously reported, are also presented. These calculations have allowed the individual conduction mechanisms in these materials to be examined in much greater detail than in previous work. AIMD simulations have also been carried out to probe the dynamics in La10-xBixGe6O27 and explore the effects that Bi3+ doping has on individual oxide conduction mechanisms and overall conductivity. The AIMD simulations are supported by the first explicit electronic calculations of the lone pair locations and orientations in apatite materials, calculated via the use of the electron localisation function (ELF).1