The thermal stability of an equilibrium phase may be tuned due to lattice strain and distortion induced by nanosizing. We apply these effects to destabilize magnesium hydride, a promising hydrogen storage material owing to its high gravimetric hydrogen density but with a too high operating temperature/low supply pressure of hydrogen for most practical applications. The destabilization is attempted with MgH₂ in contact with high entropy alloy (HEA), in which multiple metal atoms lead to lattice strain and distortion. Here, two HEAs, CrMnFeCoNi with a face centered cubic (fcc) structure and TiVZrNbHf with a body centered cubic (bcc) structure, were prepared. Subsequently, they were cosputtered with Mg to synthesize Mg−HEA thin films, respectively. Although, in the Mg−CrMnFeCoNi thin films, miscible metals with Mg as Co and Ni may hamper the formation of independent Mg domains, a small proportion of Mg atoms form destabilized MgH₂. In contrast, Mg and TiVZrNbHf domains are chemically segregated at the nanoscale in the Mg−TiVZrNbHf thin films. The formation of nanometer-sized Mg domains is promoted by atomic rearrangement following the structural change of TiVZrNbHf from a bcc to an fcc structure upon hydrogenation, resulting in distorted and destabilized MgH₂. Our strategy to use HEAs and the structural change upon hydrogenation for the formation of destabilized MgH₂ is effective and opens up the possibility for the development of advanced and low-cost hydrogen storage and supply systems.

Phase segregation in hydride-forming alloys may persist under the action of multiple hydrogenation/dehydrogenation cycles. We use this effect to destabilize metal hydrides in the immiscible Mg-Mn system. Here, in the MgxMn1-x thin films, the Mg and Mn domains are chemically segregated at the nanoscale. In Mn-rich compositions, the desorption pressure of hydrogen from MgH2 is elevated at a given temperature, indicating a thermodynamic destabilization. The increase in the desorption pressure of hydrogen reaches ∼2.5 orders in magnitude for x = 0.30 at moderate temperatures. Such large thermodynamic destabilization allows the MgH2 to reversibly absorb and desorb hydrogen even at room temperature. Our strategy to use immiscible elements for destabilization of MgH2 is effective and opens up the possibility for the development of advanced and low-cost hydrogen storage and supply systems.

Magnesium-based transition-metal hydrides are attractive hydrogen energy materials because of their relatively high gravimetric and volumetric hydrogen storage capacities combined with low material costs. However, most of them are too stable to release the hydrogen under moderate conditions. Here we synthesize the hydride of Mg₂Fe_xSi_1-x, which consists of Mg₂FeH₆ and Mg₂Si with the same cubic structure. For silicon-rich hydrides (x < 0.5), mostly the Mg₂Si phase is observed by X-ray diffraction, and Mössbauer spectroscopy indicates the formation of an octahedral FeH₆ unit. Transmission electron microscopy measurements indicate that Mg₂FeH₆ domains are nanometer-sized and embedded in a Mg₂Si matrix. This synthesized metallographic structure leads to distortion of the Mg₂FeH₆ lattice, resulting in thermal destabilization. Our results indicate that nanometer-sized magnesium-based transition-metal hydrides can be formed into a matrix-forced organization induced by the hydrogenation of nonequilibrium Mg-Fe-Si composites. In this way, the thermodynamics of hydrogen absorption and desorption can be tuned, which allows for the development of lightweight and inexpensive hydrogen storage materials.

Thin films often exhibit fascinating properties, but the understanding of the underlying mechanism behind such properties is not simple. This is partially because of the limited structural information available. The hurdle in obtaining such information is especially high for textured thin films such as Mg-rich Mg_xTi_1-x, a promising switchable smart coating material. Although these metastable thin films are seen as solid solution alloys by conventional crystallographic methods, their hydrogen-induced optical transition is hardly understood by a solid solution model. In this study, we collect atomic pair distribution function (PDF) data for a Mg_0.7Ti_0.3H_y thin film in situ on hydrogenation and successfully resolve TiH₂ clusters of an average size of 30 Å embedded in the Mg matrix. This supports the chemically segregated model previously proposed for this system. We also observe the emergence of a previously unknown intermediate face-centered tetragonal phase during hydrogenation of the Mg matrix. This phase appears between Mg and MgH₂ to reduce lattice mismatch, thereby preventing pulverization and facilitating rapid hydrogen uptake. This work may shed new light on the hydrogen-induced properties of Mg-rich Mg_xTi_1-x thin films. ©