Magnesium ions stabilizing solid-state transition metal hydrides

Sammanfattning: This thesis is focused on hydrides containing first-row transition metal–hydrido complexes counterbalanced by magnesium. In particular, fundamental properties of the promising hydrogen storage material Mg2NiH4 have been studied. Results presented in the thesis provide detailed knowledge of the electronic structure and bonding mechanisms and are of significance for improving the hydrogen desorption process of complex transition metal hydrides in general.Two competing stabilization mechanisms in Mg2NiH4 have been identified; presence of microtwinning in the structure and the extra Mg added to the melt-cast Mg2Ni starting alloys, which acts as stabilizing dopant in the Mg2NiH4 system. When eliminating both stabilization mechanisms, the hydrogen release pressure was doubled at 453 K. Mg2NiH4 behaves like a heavily doped semiconductor at lower temperatures, but the conductivity is counteracted by the introduction of microtwinning in the structure at elevated temperatures, which makes the hydride non-conducting at » 400 K. The conductivity can be regained by reducing the amount of microtwinning with an applied mechanical pressure. Size, coordination and type of cation framework have decisive roles in determining the structure type of complex metal hydrides, as shown by ab initio total-energy calculations. Compared to the rich variety of Pd-based complex hydrides, the few Ni hydrido complexes hitherto found only contain Mg2+, solely or in combination with Ca2+, Sr2+, Yb2+, Eu2+ or La3+. Apparently the rather weak Ni–H bond needs a small and polarizing cation, e.g. Mg2+, to be stabilized in the solid state. The rather strong Mg–H interaction makes Mg2NiH4 a hybrid of ionic and complex transition metal hydride. This stabilization mechanism where electron density is redistributed by the polarizable hydrido ligand explains why Mg2NiH4 is very sensitive to disturbances of the crystal ordering, e.g. doping and ball milling, which profoundly affect stability, conductivity, color, structure and phasetransition conditions. Mg2+ also has the ability to stabilize Mn hydrido complexes, as observed in the novel Mg3MnH~6 compound, synthesized at GPa pressure in the solid state.