Abstract
The effect of an electron beam induced dehydrogenation of MgH2 in the transmission electron microscope (TEM) is largely underestimated by Nogita et al., and led the authors to a misinterpretation of their TEM observations. Firstly, the selected area diffraction (SAD) pattern is falsely interpreted. A re-evaluation of the SAD pattern reveals that no MgH2 is present in the sample, but that it rather consists of Mg and MgO only. Secondly, the transformation of the sample upon in-situ heating in the TEM cannot be ascribed to dehydrogenation, but is rather to be explained by the (nanoscale) Kirkendall effect, which leads to the formation of hollow MgO shells without any metallic Mg in their cores. Hence, the conclusions drawn from the TEM investigation are invalid, as the authors apparently have never studied MgH2.
Highlights
A re-evaluation of the selected area diffraction (SAD) pattern reveals that no MgH2 is present in the sample, but that it rather consists of Mg and MgO only
After electron irradiation for 1 min, which corresponds to a maximum electron dose of 150 · 103 electrons/nm[2], the specimen is fully dehydrogenated, and no traces of MgH2 are present in the SAD pattern anymore
This observation is identical to the results presented by Nogita et al.[8], with the exception that in the present case, the transformation occurs at temperatures which are roughly 50–60 K higher than in the report of Nogita et al.[8]
Summary
A re-evaluation of the SAD pattern reveals that no MgH2 is present in the sample, but that it rather consists of Mg and MgO only. In order to verify the result of our re-evaluation, we have repeated the electron diffraction experiment on high-energy ball milled MgH2, which was prepared and handled under argon.
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