Synthesis and characterization of Magnesium-Iron-Cobalt complex hydrides.

The formation, structure and deuterium desorption properties of Mg2FexCo(1-x)Dy (0 ≤ x ≤ 1 and 5 ≤ y ≤ 6) complex hydrides were investigated. The synthesis was carried out by reactive ball milling, using a mixture of powders of the parent elements in D2 atmosphere. The formation of quaternary deuterides was identified from Rietveld refinements of powder X-Ray diffraction and powder neutron diffraction patterns, and from infrared attenuated total reflectance analysis. It was observed that the crystal structure of deuterides depends on the transition metal fraction. For Co-rich compositions, i.e. up to x = 0.1, hydrides have the tetragonal distorted CaF2-type structure (space group P4/nmm) of Mg2CoD5 at room temperature. For Fe-rich compositions, i.e. x ≥ 0.5, a cubic hydride is observed, with the same K2PtCl6-type structure (space group Fm[Formula: see text]m) as Mg2FeD6 and as Mg2CoD5 at high temperatures. For x = 0.3, both the cubic and the tetragonal deuterides are detected. Differential scanning calorimetry coupled with thermogravimetric and temperature programmed desorption analyses show rather similar deuterium desorption properties for all samples, without significant changes as a function of composition. Finally, hydrogen sorption experiments performed for Mg2Fe0.5Co0.5H5.5 at 30 bar of H2 and 673 K showed reversible reactions, with good kinetic for both absorption and desorption of hydrogen.