Synthesis and characterization of Magnesium-Iron-Cobalt complex hydrides

The formation, structure and deuterium desorption properties of Mg₂Fe_xCo_(1-x)D_y (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 Mg₂Fe_0.5Co_0.5H_5.5 at 30 bar of H₂ and 673 K showed reversible reactions, with good kinetic for both absorption and desorption of hydrogen.