Metallurgical Synthesis of Mg₂Fe_xSi_1–x Hydride: Destabilization of Mg₂FeH₆ Nanostructured in Templated Mg₂Si

Abstract

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 Mg 2 Fe x Si 1−x , which consists of Mg 2 FeH 6 and Mg 2 Si with the same cubic structure. For silicon-rich hydrides (x < 0.5), mostly the Mg 2 Si phase is observed by X-ray diffraction, and Mossbauer spectroscopy indicates the formation of an octahedral FeH 6 unit. Transmission electron microscopy measurements indicate that Mg 2 FeH 6 domains are nanometer-sized and embedded in a Mg 2 Si matrix. This synthesized metallographic structure leads to distortion of the Mg 2 FeH 6 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.