Abstract
Metal hydrides are of interest for solid-state hydrogen storage and energy applications. Binary alkaline earth hydrides present prototypical systems for understanding the relationship between structure and hydrogen dynamics. Hydrogen transport is a key property that directly relates to the efficiency of devices. We have chosen three different alkaline earth metal hydrides with vastly different hydrogen kinetics: BaH2 [barium hydride], CaH2 [calcium hydride], and MgH¬2 [magnesium hydride]. Hydrogen transport in MgH¬2 is notoriously poor while BaH2 exhibits fast hydride ion conduction. We employ neutron scattering techniques to understand the role that the crystal structure plays in influencing the hydrogen dynamics in these materials. A structural phase transition (orthorhombic to hexagonal) was observed in BaH2 occurring around T = 775 K. Quasielastic neutron scattering measurements showed that the hydrogen diffusion coefficients increase by an order of magnitude following the phase transition. Hydrogen jumps among the shortest hydrogen-hydrogen distances were found to be restricted in the orthorhombic phase but become the preferred jumps in the hexagonal phase. Total neutron scattering measurements show evidence of dynamic structural fluctuations and a splitting of the deuterium sites in the hexagonal phase. High pressure (1.3 GPa) induces the same phase transition and increases the hydrogen dynamics. The orthorhombic structure of CaH2 is isomorphic to BaH2, but the transition to the hexagonal structure is absent. Instead, evidence of a second-order phase transition was found, which influences the hydrogen dynamics. Inelastic neutron scattering indicates that the ionic bonds in CaH2 are stronger than in BaH2, which hampers diffusion. The behavior of MgH¬2 is different due to the stronger, covalent-like bonds. Hydrogen diffusion was not observed preceding the sudden thermal decomposition around 600 K as the hydrogen remains tightly bound. This study served to elucidate the role that a structural phase transition plays in transforming a solid-state material with modest hydrogen kinetics into a fast-ionic conductor. Even small modifications of the structure influence the hydrogen dynamics. The knowledge gained here sheds light on the intricate relationship between the structure and hydrogen diffusion, which can be applied to understand and advance the transport properties in other metal hydride systems.
