Structure-dynamics relationships in perovskite oxyhydrides and alkali silanides
Doctoral thesis, 2019
This thesis focuses on investigations of the local structure and dynamics of two classes of hydrogen-containing, energy relevant, materials; perovskite type oxyhydrides BaTiO3-xHx and alkali silanides ASiH3 (A = K and Rb). In the area of oxyhydrides, which are of relevance for the development of electrolytes in fuel cells and batteries, the aim is to elucidate the dynamics and electronic character of the hydrogen species in the materials. This is of great importance for developing new efficient synthesis routes and novel applications for oxyhydrides. The main tools of choice for these investigations are quasielastic and inelastic neutron scattering (QENS and INS, respectively). The results show that the hydride ion exhibits a long-range diffusion with a jump length corresponding to nearest neighbour (NN) jumps at low temperatures (225–250 K) and second nearest neighbour (2NN) jumps at high temperatures (400–700 K). Importantly, the hydride ion diffusivity was shown to be mediated by oxygen vacancies present in the material. Furthermore, the results from INS combined with density functional theory calculations show that the extra electron, originating from the hydride ion, forms a delocalised bandstate, as opposed to a localised polaronic state as suggested elsewhere.
In the area of alkali silanides, which are of interest as hydrogen storage materials, the aim was to investigate the origin of the low entropy variation that these materials exhibit during the absorption/desorption process, using QENS. The results point towards complex dynamics, characterised by a quasi-spherical localised jump diffusion with 24 different preferred sites at high temperatures and slower C3 axis rotations as the dynamical motions starts to "freeze in" closer to the phase transition at lower temperatures. Specifically, at high temperatures the SiH3- ions are almost freely rotating, similar to how the ions behaves in a gas, which explains the origin of the low entropy variation and should be something to strive for when developing new hydrogen storage materials.
hydrogen storage material
long range diffusion
hydride ion conduction