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.