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 Mg2FexSi1-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.@en

Metal hydrides may play a paramount role in a future hydrogen economy. While most applications are based on nanostructured and confined materials, studies considering the structural response of these materials to hydrogen concentrate on bulk material. Here, using in situ in- and out-of-plane X-ray diffraction and reflectometry, we study the fcc ↔fct transition in Hf thin films, an optical hydrogen-sensing material. We show that the confinement of Hf affects this transition: compared to bulk Hf, the transition is pushed to a higher hydrogen-to-metal ratio, the tetragonality of the fct phase is reduced, and phase coexistence is suppressed. These nanoconfinement effects ensure the hysteresis- free response of hafnium to hydrogen, enabling its remarkable performance as a hydrogen-sensing material. In a wider perspective, the results highlight the profound influences of the nanostructuring and nanoconfinement of metal hydrides on their structural response to hydrogen with a significant impact on their applicability in future devices.@en

Nanostructured metal hydrides could play a key role in a hydrogen economy. The nanostructuring or confinement of these materials as, e.g., thin films significantly affects the structural and functional properties. For tantalum hydride, a versatile hydrogen sensing material, we show that the confinement of tantalum as a thin film extends the solubility limit by suppressing the phase transition observed in bulk upon hydrogenation. Different from bulk, the body centered cubic unit cell continuously deforms with unequal lattice constants and angles between lattice vectors. This deformation ensures that the volumetric expansion is realized in the out-of-plane direction, and surprisingly, completely elastic in nature. The first-order phase transition suppression combined with the continuous elastic deformation of the tantalum unit cell over an extraordinary wide solubility range ensures the superb performance of tantalum and its alloys as a hysteresis-free optical hydrogen sensing range over a hydrogen pressure/concentration range of over 7 orders of magnitude.@en

Hydrogen is playing a key role in the transition to a sustainable economy and in a variety of industrial processes. For its safe handling, the detection of hydrogen in a fast, reliable and accurate manner is crucial. Thin film metal hydride based optical hydrogen sensors provide an attractive option to sense hydrogen in a variety of conditions and have an attractive safety benefit over other methods of detection: They do not require the presence of electrical leads near the sensing area. These sensors rely on a change of the optical properties arising from a change in the hydrogenation of the metal hydride sensing layer in response to a different partial hydrogen pressure in the environment of the sensor. In this paper, we review how the performance and characteristics of an optical hydrogen sensor are related to the material properties of the metal hydride sensing layer and we discuss previously considered materials and challenges and opportunities left for the future.@en

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. @en

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 MgxTi1-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 Mg0.7Ti0.3Hy 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 MgxTi1-x thin films. ©@en