Among the various functionalities of hydrides, their use in hydrogen storage has been the most intensively studied because hydrides can store hydrogen compactly and safely. Thus, hydrides are key materials for the hydrogen economy. Here, the hydrogen storage material Al3FeH3.9 has been synthesized from cost-effective earth-abundant metals, Fe and Al. Hydrides consisting of Al and transition metals with low hydrogen affinities are rare because such alloys are unstable. However, it is expected that appropriate mixing of the chemical states of hydrogen atoms would allow synthesis of Al-Fe hydrides. The experimentally determined crystal structure of Al3FeD3.9 suggests realization of the mixing of the chemical state of hydrogen. Al3FeH3.9 is more thermodynamically stable than AlH3, and it is likely that the mixing of the chemical state of hydrogen atoms is the source of increased stability. The results of this study confirm that by controlling the chemical states of hydrogen, it is possible to tune the thermodynamic stability of hydrides and thus realize novel functional hydrides.
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