Studies of hydride ion conductors using combined spectroscopy techniques

Abstract

This thesis presents research into two superionic conductors: BaH₂ and β-Ca₂NH. The research presented herein explores the diffusion pathway of hydride ions (H⁻) in the solid-state of these materials.

Using novel combined spectroscopic techniques, we show that BaH₂ has a diffusion pathway that involves the partial melting of its crystal lattice. This "liquid-like" sublattice is analysed in detail in order to elucidate the origins of such behaviour as well as provide a physical description of a sublattice manifesting as "liquid-like."

The second material explored in this thesis is a novel superionic conductor not previously published, β-Ca2NH. The analysis shows that Ca₂NH (nitride- hydride) materials come in at least two distinct polymorphs, with dramatically different ionic conductivities. Using in situ neutron powder diffraction along with several other techniques, we explore the unique configurations of the different Ca₂NH polymorphs and identify what gives rise to ionic conductivity in one form and not the other. Furthermore, Ca₂NH is contrasted to the closely related imide (NH₂⁻) and amide (NH₂⁻).

Barium hydride is shown to have an ionic conductivity of 0.32 S/cm at 600 ◦C and its conduction due to the presence of a liquid-like sublattice. Calcium nitride-hydride, on the other hand, is shown to have an ionic conductivity of 0.08 S/cm at 600 ◦C and its conduction defined by an intrinsic vacancy concentration created by anti-Frenkel defects. Thus, the thesis explores two excellent solid-state ionic conductors with dramatically different ionic conduction mechanisms.

The synthesis route for the following compounds are detailed in this thesis: BaH₂, BaD₂, Ca₂NH - both polymorphs, CaNH, and Ca(NH₂)₂.