Structural investigations of some ternary metal hydrides

Sammanfattning: This work was undertaken with the intention of describing the metal atom environment of hydrogen in a selection of ternary metal hydrides that could be of interest as hydrogen sorption materials. Mainly X-ray and neutron diffraction techniques were used for the purpose.The new Laves-phase hydride (Hf,Ti)(Ni,V)2H2 and also CaPd3H consist of close packed metal atom layers with interstitial hydrogen atoms. The hydrogen sorption ability could be explained from geometrical criteria and also, in the case of CaPd3H, by electronic structure considerations. The Laves phase improves the activation and kinetics in a negative electrode of a vanadium-based solid solution by forming a second hydride phase.The low-temperature phase of Mg2NiH4has some features of basic interest which will be clarified in this survey, namely how lattice faults can stabilise the hydride with respect to hydrogen desorption, leading to slower hydrogen desorption kinetics, and nucleation of the transition between the low-(LT) and high-(HT) temperature phase. A microtwinned sample moreover seems to have the ability to seal the surface, thereby counteracting further oxidation/decomposition on exposure to the air. This phenomenon is observed as a colour change from orange to brown. The LT phase was also mechanically ground under argon, in order to improve the hydrogen sorption characteristics. The obtained structure is disordered and similar to the HT phase, previously only observed above the phase transition temperature (510 K).The perovskite-related structures of NaMgH3 and Na3AlH6, isostructural with the corresponding fluorides, are interesting as light-weight, high-capacity alternatives to the conventional AB2 and AB5 type alloys presently used in rechargeable batteries. Investigation with respect to the degree of distortion from the ideal structure reveals that the hydrides have more regular structures than the fluorides.