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, 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.