Distortion and Destabilization\nof Mg Hydride Facing\nHigh Entropy Alloy Matrix

The thermal stability of an equilibrium phase may be\ntuned due\nto lattice strain and distortion induced by nanosizing. We apply these\neffects to destabilize magnesium hydride, a promising hydrogen storage\nmaterial owing to its high gravimetric hydrogen density but with a\ntoo high operating temperature/low supply pressure of hydrogen for\nmost practical applications. The destabilization is attempted with\n MgH<sub>2</sub> in contact with high entropy alloy (HEA), in which\nmultiple metal atoms lead to lattice strain and distortion. Here,\ntwo HEAs, CrMnFeCoNi with a face centered cubic (fcc) structure and\nTiVZrNbHf with a body centered cubic (bcc) structure, were prepared.\nSubsequently, they were cosputtered with Mg to synthesize Mg–HEA\nthin films, respectively. Although, in the Mg–CrMnFeCoNi thin\nfilms, miscible metals with Mg as Co and Ni may hamper the formation\nof independent Mg domains, a small proportion of Mg atoms form destabilized\nMgH<sub>2</sub>. In contrast, Mg and TiVZrNbHf domains are chemically\nsegregated at the nanoscale in the Mg–TiVZrNbHf thin films.\nThe formation of nanometer-sized Mg domains is promoted by atomic\nrearrangement following the structural change of TiVZrNbHf from a\nbcc to an fcc structure upon hydrogenation, resulting in distorted\nand destabilized MgH<sub>2</sub>. Our strategy to use HEAs and the\nstructural change upon hydrogenation for the formation of destabilized\nMgH<sub>2</sub> is effective and opens up the possibility for the\ndevelopment of advanced and low-cost hydrogen storage and supply systems.