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

MgCo2 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 MgCo2 or MgNiCo and remain thermally stable under normal conditions. The hydrogenation of MgCo2 resulted in its decomposition to a ternary Mg2CoD5 deuteride and metallic cobalt. Phase-structural transformations accompanying the vacuum desorption of deuterium in the temperature range of 27-500 C-omicron were studied using in situ neutron powder diffraction. The investigation showed a complete recovery of the initial MgCo2 intermetallic via a Hydrogenation-Disproportionation-Desorption-Recombination process. At 300 C-omicron, the Mg2CoD5 deuteride first decomposed to elementary Mg and hexagonal Co. At 400 C-omicron, a MgCo phase was formed by interaction between Mg and Co. At the highest processing temperature of 500 C-omicron, a solid-state interaction of MgCo and Co resulted in the recovery of the initial MgCo2. The interaction of MgNiCo with deuterium under the synthesis conditions of 2.8 GPa and 200 C-omicron proceeded in a more complex way. A very stable ternary deuteride MgNi2D3 was leached away while Co was separated in the form of Mg2CoD5 and the remaining nickel formed a solid solution with Co with the approximate composition Ni0.7Co0.3. The thermal desorption of deuterium from MgCo2D2.5 and from MgNiCoD2.5 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. MgCo2 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.