Structural evolution and metastable phase detection in MgH2-5%NbH nanocomposite during in-situ H-desorption in a synchrotron beam

Hydrogen sorption in magnesium and magnesium hydrides has been shown to accelerate in sub-micron structures generated by mechanical attrition. It further accelerates when transition metal nanoparticle hydrides such as Nb are added. These hydrides are reported to trigger the desorption during heating. Metastable orthorhombic gamma-MgH2 also forms during attrition and has been shown to desorb first during heating. In this work, using a high brilliance high energy beam for diffraction in transmission, we find that indeed the desorption sequence during heating of mechanically milled MgH2-5%NbH nanocomposite proceeds through at least two major steps. First the desorption of the metastable orthorhombic gamma-MgH2 into hexagonal Mg triggers the decomposition of part of the tetragonal beta-MgH2 and the orthorhombic beta-NbH. The second stage involving the desorption of hydrogen from the remaining beta-MgH2 appears to be linked to a NbHx metastable phase previously reported to be NbH0.6. The present results however, using more extensive diffraction data, indicate that this metastable phase is a supersaturated Nb solid solution as seen likely from the phase diagram. (C) 2002 Elsevier Science B.V. All rights reserved.