Synthesis and characterization of Magnesium-Iron-Cobalt complex hydrides

The formation, structure and deuterium desorption properties of Mg(2)Fe(x)Co((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 D(2) 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 CaF(2)-type structure (space group P4/nmm) of Mg(2)CoD(5) at room temperature. For Fe-rich compositions, i.e. x ≥ 0.5, a cubic hydride is observed, with the same K(2)PtCl(6)-type structure (space group Fm[Formula: see text] m) as Mg(2)FeD(6) and as Mg(2)CoD(5) 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 Mg(2)Fe(0.5)Co(0.5)H(5.5) at 30 bar of H(2) and 673 K showed reversible reactions, with good kinetic for both absorption and desorption of hydrogen.