Mechanistic aspects of structure and dynamics in perovskite type oxyhydrides and alkali silanides

Licentiate thesis, 2016

This Thesis concerns experimental studies of the two alkali silanides ASiH3 (A = K and Rb) and the recently discovered perovskite type oxyhydrides BaTiO3-xHx (x = 0.14 and 0.40). The alkali silanides ASiH3 (A = K and Rb) are featured by an unusually low enthalpy change over the hydrogen absorption/desorption process, which enables an easy route for hydrogenation and makes them of relevance for hydrogen storage applications. One aim with this thesis is to determine the mechanistic aspects of the structure and hydrogen dynamics that possibly explain this behavior. It is shown that the previously reported structure, featured by a quasi-spherical arrangement with 24 sites of preferred orientations for the hydrogen atoms, can be used as a model for the H dynamics present in the materials. Specifically, the SiH3- species are almost freely rotating, which explains the origin of the low entropy variation. Perovskite type oxyhydrides BaTiO3-xHx (x = 0.14 and 0.40) represent an emerging class of hydride-ion conducting materials, with properties similar to proton conducting equivalents. However, details of the hydride-ion dynamics are still unknown. Accordingly, this thesis focuses also on investigations of the mechanistic aspects of structure and hydride-ion dynamics in perovskite oxyhydrides, with the aim of developing a generic knowledge of hydrogen dynamics in perovskite materials, relevant for both proton and hydride-ion conductors. Structural and dynamical analysis confirm that the hydride-ions are located on vacant oxygen sites of the perovskite lattice and reveal hydride-ion diffusion on the time-scale of nanoseconds. The main means of investigation are neutron scattering techniques, which are very appropriate in order to study hydrogen dynamics in materials. This will be discussed in detail in this thesis and also the basics of dynamics that is at the center of the investigations.