Iron fluorides assisted dehydrogenation and hydrogenation of MgH sub(2 studied by Moessbauer spectroscopy)

Mechanically milled MgH sub(2 with the addition of 7 wt.% of either FeF) sub(2) or FeF sub(3 were investigated by means of the X-ray powder diffraction and Moessbauer spectroscopy as prepared and upon dehydrogenation and finally upon subsequent hydrogenation. Mechanical milling leads to the decomposition of iron fluorides. In the case of FeF) sub(2) one obtains magnesium solid solution in metallic BCC iron as the dominant iron-bearing phase, while for FeF sub(3 one gets Mg) sub(2)FeH sub(6 as dominant phase with iron. Dehydrogenation at 325 [deg]C leads to removing defects and formation of magnesium, iron and MgH) sub(2)-xF sub(x phases for both types of dopants, i.e., FeF) sub(2) and FeF sub(3. Subsequent hydrogenation at 325 [deg]C leaves [beta]-MgH) sub(2) as a major phase. However, for original material doped with FeF sub(3 one has iron predominantly in the Mg) sub(2)FeH sub(6 compound, while for FeF) sub(2) dopant iron occurs mostly as nearly pure BCC metallic phase. Moessbauer spectra indicate that Mg sub(2FeH) sub(6) does not order magnetically down to 4.2 K.