Neutron spectroscopy and computational studies of hydride-ion dynamics in oxide- and nitride-hydride materials

Licentiate thesis, 2024

This thesis presents studies of the hydride-ion dynamics in the nitride- and oxide­hydride materials Ca3CrN3H and BaTiO3-xHx (with x < 0.2). These mixed-anion compounds are of great interest because they show capabilities for hydrogen transport and catalysis, but fundamental questions concerning the hydride-ion dynamics in these materials remain to be answered. Here, these questions have been tackled using inelastic and quasielastic neutron scattering techniques, as well as density functional theory calculations.

For Ca3CrN3H, it is found that hydride-ions undergo one-dimensional diffusion, which supposedly mediated by the presence of vacancies in the hydride-ion sub­lattice. The measured diffusion coefficient is comparable to the three-dimensional hydride-ion diffusion in BaTiO3-xHx. This result suggests that reducing dimensionality might be used to optimize the hydride-ion diffusivity in oxide- and nitride-hydride materials. In addition, it was demonstrated that hydride-ion vacancies are present in the material, which impact the vibrational frequencies of hydride-ions and thus their jump rate. This implies that the hydride-ion diffusivity may be tailored by tuning the hydride-ion vacancy.

For BaTiO3-xHx, the results corroborated earlier findings which showed that hydride-ion diffusion relies on the presence of oxygen vacancies serving as jumping sites for the hydride-ions. Specifically, two time scales of diffusion were observed, indicating significant inhomogeneity in the local diffusion environment of the hydride-ions. This new finding suggests that understanding and controlling these inhomogeneities may be used to optimize the hydride-ion diffusivity. Finally, it was found that the presence of valence electrons localised near the hydride-ion-oxygen vacancy pair hinders the jump diffusion mechanism, indicating a correlation of electronic and ionic mobility in BaTiO3-xHx. This implies that tuning the hydride-ion conductivity will impact the electronic conductivity of the material, an effect which should be considered for technological applications of BaTiO3-xHx such as its use in fuel cells.