Local structure and dynamics in proton- and hydride-ion conducting perovskite-type oxides; bulk and films

Doctoral thesis, 2025

Acceptor doped proton- and hydride-ion conducting perovskite-type oxides are promising candidates for application as electrolytes in solid oxide fuel cells at temperatures below 400 ^◦C. However, to be applied in fuel cells, these electrolytes should have ionic conductivities of at least 10⁻² S/cm which, at present, these materials do not have. Increasing the ionic conductivity relies on a deeper understanding of their local structure and dynamics. Key challenges are related to elucidating how the type and concentration of dopant atoms and the microstructure of these materials influence the structure and proton- or hydride-ion dynamics. In this thesis, these challenges have been approached using quasielastic neutron scattering and infrared spectroscopy. The studies of local structure and dynamics have focused both on microcrystalline bulk samples and on nanocrystalline film samples.


The study of powder samples focused on BaZr_1−xSc_xO_3−x/2 with x = 0.10 and 0.50, with the aim to unravel the effect of doping concentration on localized proton dynamics. A key result of this study is that the localized proton dynamics are largely unaffected by the Sc dopant concentration. Besides contributing to an increased fundamental understanding, this knowledge is of importance for further optimization of these materials in actual applications. The study of films focused on BaZr_1−xSc_xO_3−x/2, (x = 0.45, 0.54, and 0.65), with the aim to elucidate the influence of doping concentration on the local structure and proton environments. The results show that the local proton environments change as a function of increasing Sc dopant concentration, with the creation of preferred proton environments for x > 0.50. In comparison, the results for powder samples show a more homogeneous distribution of proton environments quite independent of x. It follows that the local structure of these proton-conducting oxides depends on the microstructure of the samples, which may be tuned by the fabrication method.


Additionally, the effect on the type of dopant atom on the proton dynamics was investigated for powder samples of the brownmillerite-type proton conducting oxides Ba₂In_1.85M_0.15O₅ with M = In, Ga, Sc and Y. The results show that, compared to Ba₂In₂O₅, introducing a dopant atom effectively lowers the temperature for both localized and long-range proton dynamics, which is associated with the presence of an additional proton site, quite independent of the type of dopant atom. This finding suggests that doping the Ba₂In₂O₅ structure is an effective strategy for promotion of long-range proton diffusion.


Beyond proton-conducting oxides, the relationship between local structure, and hydride-ion incorporation and dynamics was investigated for the perovskite-type oxyhydrides BaTiO_2.88H_0.12 and BaZr_1−xIn_xO_3−x/2 (x =0.10–0.59). This work did not only show that the presence of oxygen vacancies seems critical for proton diffusion to occur, but also that the concentration of oxygen vacancies can be tuned by the selection of precursors used in the synthesis of these novel materials.