Comments on ”Evidence of the hydrogen release mechanism in bulk MgH2”

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.

Reply to ‘Comments on “Evidence of the hydrogen release mechanism in bulk MgH2”’

“…In fact, several of our previous works have been carried out using this approach289. Reference 16 provided by Surrey et al 1. relates to their interpretation of our data that the transformation we observe is the formation of voids.…”

“…al. 1 on our recently published study2 contains two main assertions:The MgH 2 sample we studied was not MgH 2 at any time but rather MgO.The transformation we observed was the formation of Kirkendall voids due to the outward diffusion of Mg.…”

“…The question that then remains is what was the difference between the samples and observation techniques used by Surrey et al 1. which made the MgH 2 so unstable and the dehydriding reaction so fast as to make observation difficult ?…”

“…which made the MgH 2 so unstable and the dehydriding reaction so fast as to make observation difficult ? The answer of course is that we studied large samples a few microns in size that were relatively free of deformation in UHV-TEM (1,000 kV), while Surrey et al 1. studied small 100 nm samples that were produced by high-energy ball-milling in low voltage TEM (300 kV).…”

Reversible Metal‐Hydride Transformation in Mg‐Ti‐H Nanoparticles at Remarkably Low Temperatures

“…Unfortunately, it is not possible to conduct a high‐resolution TEM investigation of MgH 2 and TiH 2 within the NPs because MgH 2 is highly unstable under high vacuum and electron beam irradiation . X‐ray absorption spectroscopy experiments may help to elucidate the local structure of Ti‐rich NPs with elemental and short‐range‐order sensitivity.…”