Local structure and dynamics in proton conducting perovskite oxides: insights from quasielastic neutron scattering and infrared spectroscopy

Licentiate thesis, 2023

Proton conducting oxides have been extensively studied in the past decade due to their potential application in a number of energy related devices such as solid oxide fuel cells, hydrogen sensors, and steam electrolyzers. However, the final industrialisation of these materials depends on unlocking fundamental questions about the relationship between their atomic structure, localized proton dynamics and proton conductivity. The aim of this thesis is to gather insights into these properties in proton conducting oxides based on the perovskite and perovskite-like brownmillerite structures, i.e., BaZr1−xScxO3−x/2 with x = 0.10–0.65, and Ba2In1.85M0.15O5with M = In, Ga, Sc and Y, as both powder samples and films. The investigations are experimental in nature and the main techniques used are quasielastic neutron scattering and infrared spectroscopy.
The studies of powder samples of BaZr1−xScxO3−x/2 (x = 0.10 and 0.50) focused on exploring the nature of localised proton dynamics and how it depends on Sc dopant level via quasielastic neutron scattering. The results confirm earlier studies in which the localised proton dynamics can be described as proton transfers between neighboring oxygens and –OH reorientation motions. Both processes are found to exhibit a relaxation time and activation energy of the order of 1–10 picoseconds and some tens of meV, respectively, and it is found that these values depend insignificantly on the Sc dopant levels as investigated here. Films of BaZr1−xScxO3−x/2, (x = 0.45, 0.54, and 0.65) were investigated with the aim to reveal possible differences in local structure to the corresponding powder samples. These investigations were performed using infrared spectroscopy. Analysis of the infrared spectra focused on the O–H stretch region (2000–3700 cm−1) and revealed the presence of several distinct O–H stretch bands with unsystematic intensity and frequency in terms of Sc dopant level. For x = 0.45, the O–H stretch region was characterized by a broad, weak band between 2500 and 3700 cm−1, suggesting the presence of a wide range of local surroundings of the protons in the material. The O–H stretch regions for x = 0.54 and 0.65 were characterised with a distinct, highly intense band centered at around 3400–3500 cm−1, pointing towards relatively symmetric, weakly hydrogen-bonding, proton configurations. A comparison to the infrared spectra of powder samples of similar compositions suggests a more homogeneous distribution of proton sites in the powder samples compared to the films.
The studies on Ba2In1.85M0.15O5 with M = In, Ga, Sc and Y, focused on the relationship between localised proton dynamics and the type of dopant atom in powder samples, using quasielastic neutron scattering. Since before, it was known that the crystal structure of Ba2In2O6H2 is built up of cubic and “pseudo-cubic” layers of InO6 octahedra, with two main types of proton sites, H(1) and H(2), present. Analysis of the quasielastic neutron scattering data reveals the presence of dynamics on the picosecond timescale, associated with proton transfer and/or –OH rotational motions of H(1) and H(2) protons, respectively, for temperatures above 400 K, quite independent on M. For dopants withsignificantly different ionic radii compared to In, i.e., M = Ga and Y, the combined analysis of the infrared spectra and quasielastic neutron scattering data suggests a larger fraction of protons on the H(2) site, as opposed to M = Sc and In, where there is a larger fraction of protons on the H(1) site. Analysis of the quasielastic neutron scattering data also revealed that doping with M = Ga, Sc and Y resulted in the presence of an additional proton site H(3) at temperatures above 450 K, which feature more complex dynamics, in accordance with a previous study on Ba2In2O6H2.