Hydrogen-Release Reaction of a Complex Transition Metal Hydride with Covalently Bound Hydrogen and Hydride Ions.

The hydrogen-release reaction of a complex transition metal hydride, LaMg2 NiH7 , composed of La3+ , 2×Mg2+ , [NiH4 ]4- and 3×H- , was studied by thermal analyses, powder X-ray, and neutron diffraction and inelastic neutron scattering. Upon heating, LaMg2 NiH7 released hydrogen at approximately 567 K and decomposed into LaH2-3 and Mg2 Ni. Before the reaction, covalently bound hydrogen (Hc °v. ) in [NiH4 ]4- exhibited a larger atomic displacement than H- , although a weakening of the chemical bonds around [NiH4 ]4- and H- was observed. These results indicate the precursor phenomenon of a hydrogen-release reaction, wherein there is a large atomic displacement of Hc °v. that induces the hydrogen-release reaction rather than H- . As an isothermal reaction, LaMg2 NiH7 formed LaMg2 NiH2.4 at 503 K in vacuum for 48 h, and LaMg2 NiH2.4 reacted with hydrogen to reform LaMg2 NiH7 at 473 K under 1 MPa of H2 gas pressure for 10 h. These results revealed that LaMg2 NiH7 exhibited partially reversible hydrogen-release and uptake reactions.