Formation of the Ternary Complex Hydride Mg2FeH6 from Magnesium Hydride (β-MgH2) and Iron: An Electron Microscopy and Energy-Loss Spectroscopy Study

We examined the formation of the ternary complex hydride phase Mg2FeH6 during the thermal hydrogen absorption of a ball-milled powder mixture of MgH2 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. Mg2FeH6 was initially formed during ball milling of MgH2 and Fe, demonstrating a co-continuous structure with MgH2 and Mg. Upon the first desorption, this phase was transformed into elemental Mg and Fe. During the initial stages of the subsequent thermal absorption, MgH2 was first formed with enhanced kinetics because of the presence of Fe. This was followed by the nucleation of Mg2FeH6 between MgH2 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 Mg2FeH6 columns grew, the capping Fe particle and the MgH2 substrate were consumed. When the system was maintained at high temperature for a long time, these columnar Mg2FeH6 structures coalesced into a semispherical morphology.