MgCo2-D2 and MgCoNi-D2 systems synthesized at high pressures and interaction mechanism during the HDDR processing

MgCo 2 and MgNiCo crystallize with hexagonal Laves type intermetallic structures of the C14 type and do not form hydrides at ambient hydrogen pressures. However, applying high hydrogen pressures in the GPa range forces the hydrogen absorption and leads to the formation of multi-phase compositions, which contain approximately 2.5 atoms H per formula unit of MgCo 2 or MgNiCo and remain thermally stable under normal conditions. The hydrogenation of MgCo 2 resulted in its decomposition to a ternary Mg 2 CoD 5 deuteride and metallic cobalt. Phase-structural transformations accompanying the vacuum desorption of deuterium in the temperature range of 27–500 °C were studied using in situ neutron powder diffraction. The investigation showed a complete recovery of the initial MgCo 2 intermetallic via a Hydrogenation-Disproportionation-Desorption-Recombination process. At 300 °C, the Mg 2 CoD 5 deuteride first decomposed to elementary Mg and hexagonal Co. At 400 °C, a MgCo phase was formed by interaction between Mg and Co. At the highest processing temperature of 500 °C, a solid-state interaction of MgCo and Co resulted in the recovery of the initial MgCo 2 . The interaction of MgNiCo with deuterium under the synthesis conditions of 2.8 GPa and 200 °C proceeded in a more complex way. A very stable ternary deuteride MgNi 2 D 3 was leached away while Co was separated in the form of Mg 2 CoD 5 and the remaining nickel formed a solid solution with Co with the approximate composition Ni 0.7 Co 0.3 . The thermal desorption of deuterium from MgCo 2 D 2.5 and from MgNiCoD 2.5 has been studied by Thermal Desorption Spectroscopy with deuterium released into a closed volume. The observed effects nicely correlate with changes in the phase structural composition of the hydrides formed. MgCo 2 is a new example of the hydrogen storage alloy, in which a successful HDDR processing results in the reversible formation of the initial intermetallic at much lower temperatures than in the equilibrium phase diagram of the Mg-Co system.