Local structure and dynamics of proton and hydride-ion conducting perovskite type oxides
Abstract: Proton and hydride-ion conducting oxides show potential for application in several technological devices, such as solid oxide fuel cells and batteries. However, fundamental properties pertaining to the local structure and conduction mechanisms in these materials are unclear. Such fundamental knowledge is crucial for the development of novel materials and, ultimately, for their application in technological devices. This thesis reports on investigations of local structure and dynamics in two families of hydrogen containing perovskite structured oxides, namely proton-conducting BaZr xM 1- x O3H x ( M = In, Sc and Y, x ≤ 0.5) and hydride-ion conducting BaTiO3- x H x ( x ≤ 0.15). For the proton conducting BaZr x M1- x O3H x materials, the investigations focused on the nature of the proton sites in polycrystalline powder samples and were performed using inelastic neutron scattering and infrared and Raman spectroscopy combined with computer simulations. The results reveal the presence of a distribution of different types of proton sites, which were virtually the same for all chemical compositions except for a high level ( x ≥ 0.5) of In-doping. It is argued that the high In-doping results in the presence of additional proton sites located in distorted structural arrangements and which resemble those found in the hydrated form of the brownmillerite structured Ba2In2O5 system. It is also shown that the local environment for a specific proton changes over time due to the lattice vibrational dynamics. Additionally, thin-film samples were investigated by means of X-ray and neutron reflectivity and nuclear reaction analysis, with the aim to obtain details about the incorporation and distribution of protons in the samples. A key result is the observation of a thin (3-4 nm) proton-rich layer near to the surface of the films. This layer features proton sites characterized by relatively week hydrogen-bond interactions and a reduced proton mobility compared to the bulk of the film. The studies on hydride-ion conducting BaTiO3- x H x materials focused on revealing the nature of the local environments of the hydride ions and were performed using inelastic neutron scattering techniques and computer simulations. It is found that the presence of oxygen vacancies in the proximity of the hydride ions significantly influences their local environments and the vibrational properties.
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