The hydrogen-release reaction of a complex transition metal hydride, LaMg 2 NiH 7 , composed of La 3+ , 2×Mg 2+ , [NiH 4 ] 4− and 3×H − , was studied by thermal analyses, powder X-ray, and neutron diffraction and inelastic neutron scattering. Upon heating, LaMg 2 NiH 7 released hydrogen at approximately 567 K and decomposed into LaH 2−3 and Mg 2 Ni. Before the reaction, covalently bound hydrogen (H c ° v. ) in [NiH 4 ] 4− exhibited a larger atomic displacement than H − , although a weakening of the chemical bonds around [NiH 4 ] 4− and H − was observed. These results indicate the precursor phenomenon of a hydrogen-release reaction, wherein there is a large atomic displacement of H c ° v. that induces the hydrogen-release reaction rather than H − . As an isothermal reaction, LaMg 2 NiH 7 formed LaMg 2 NiH 2.4 at 503 K in vacuum for 48 h, and LaMg 2 NiH 2.4 reacted with hydrogen to reform LaMg 2 NiH 7 at 473 K under 1 MPa of H 2 gas pressure for 10 h. These results revealed that LaMg 2 NiH 7 exhibited partially reversible hydrogen-release and uptake reactions.
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