The formation mechanism and structural characterization of the mixed transition-metal complex hydride Mg2(FeH6)0.5(CoH5)0.5 obtained by reactive milling

Mg-2(Fe0.5Co0.5) elemental powder mixtures were processed by reactive ball milling in a H-2 (or D-2) atmosphere at about 50 bar. The changes of pressure were monitored during milling and hydrogen absorption was detected within the first 10 h of milling. X-ray and neutron powder diffraction analysis, followed by Rietveld refinement, suggests an almost complete conversion of the starting metals into the quaternary hydride Mg-2(FeH6)(0.5)(CoH5)(0.5). The latter has a cubic K2PtCl6-type structure (space group Fm3m) with a cell parameter a approximate to 6.42 angstrom. Fe and Co atoms are disorderly distributed in the 4a position. Hydrogen desorption upon heating was investigated by both in situ x-ray diffraction and temperature programmed desorption analysis. Hydrogen is released in a one-step desorption reaction at temperatures between 500 and 600 K, with Mg and a FeCo solid solution as desorption products. Finally, Mg-2(FeH6)(0.5)(CoH5)(0.5) was also synthesized by ball milling equi-molar mixtures of Mg2FeH6 and Mg2CoH5 in Ar.