Attempts to improve Mg₂Ni hydrogen storage by aluminium addition

In an attempt to destabilize Mg₂NiH₄, the starting alloy, Mg₂Ni, was doped with aluminium by ball milling before hydrogenation. The structure of Mg₂NiH₄ is based on an electron-rich d¹⁰ [NiH₄]-complex stabilized by a Mg ion framework and from theoretical calculations it had been suggested that a further destabilization of the hydride could be obtained by substituting some Mg for Al. After hydrogenation, the structure, color and phase transition temperature of Mg₂NiH₄ was found to be affected by Al addition. The conventional monoclinic low-temperature (LT) structure of Mg₂NiH₄ was not observed by X-ray diffraction for Mg_2-xAl_xNiH₄, which instead could be indexed with a cubic unit cell, a=6.519 Å, that is very close to the dynamically disordered, cubic, high-temperature (HT) phase of Mg₂NiH₄ (a=6.490 Å). In contrast, the Al-doped Mg₂NiH₄ is stable at room temperature. Furthermore, the typical orange color of Mg₂NiH₄ was not observed and differential scanning calorimetric measurements (DSC) of the Al-containing hydride showed a 10 °C decrease in phase transition temperature. This is interpreted as a transition from a statically disordered pseudo-HT structure to the dynamically disordered HT phase. However, the monoclinic structure of the LT phase of Mg₂NiH₄ could again be observed at room temperature after 10 hydriding/dehydriding cycles, indicating that the Al-doping of Mg₂NiH₄ is counteracted by cycling and only has a temporary influence on the properties.