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We examined the formation of the ternary complex hydride phase Mg 2 FeH 6 during the thermal hydrogen absorption of a ball-milled powder mixture of MgH 2 and Fe. Analytical measurements, scanning transmission electron microscopy, and electron energy-loss spectroscopy, with the sample cooled to liquid nitrogen temperature, were utilized to identify the various phases present and to study the features of this phase transformation. The low-loss region of the electron energy-loss spectrum was mainly used to distinguish various constituents. Mg 2 FeH 6 was initially formed during ball milling of MgH 2 and Fe, demonstrating a co-continuous structure with MgH 2 and Mg. Upon the first desorption, this phase was transformed into elemental Mg and Fe. During the initial stages of the subsequent thermal absorption, MgH 2 was first formed with enhanced kinetics because of the presence of Fe. This was followed by the nucleation of Mg 2 FeH 6 between MgH 2 and Fe and its growth with a columnar morphology. This morphology was dictated by the diffusion direction of the atomic hydrogen from the catalytic Fe cap. As the Mg 2 FeH 6 columns grew, the capping Fe particle and the MgH 2 substrate were consumed. When the system was maintained at high temperature for a long time, these columnar Mg 2 FeH 6 structures coalesced into a semispherical morphology.

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