Hydride formation in ball-milled and cryomilled Mg–Fe powder mixtures

We report on the formation of the ternary hydride Mg2FeH6 in mechanically activated powders. Powder mixtures with nominal composition Mg67Fe33 were ball-milled in an inert Ar atmosphere either at room temperature or at liquid nitrogen temperature (cryomilling), and subsequently hydrogenated at different pressures at 622 K in a Sieverts-type apparatus. X-ray diffraction data from the hydrided products show Mg2FeH6 is formed directly from 2Mg + Fe + 3H2. Cryomilling improves the hydrogen absorption kinetics, but also results in a decrease of the total yield of Mg2FeH6. This is likely due to MgH2 being kinetically favoured, and in turn acting as a competitor to the formation of the ternary hydride. The absorption data is discussed in the framework of the Johnson–Mehl–Avrami model, which suggests the formation of Mg2FeH6 occurs via interface-controlled growth processes.