Evidence of Intermediate Hydrogen States in the Formation of a Complex Hydride

A complex hydride (LaMg₂NiH₇) composed of La³⁺, two Mg²⁺, [NiH₄]^4- with a covalently bonded hydrogen, and three H^- was formed from an intermetallic LaMg₂Ni via an intermediate phase (LaMg₂NiH_4.6) composed of La, Mg, NiH₂, NiH₃ units, and H atoms at tetrahedral sites. The NiH₂ and NiH₃ units in LaMg₂NiH_4.6 were reported as precursors for [NiH₄]^4- in LaMg₂NiH₇ [ Miwa et al. J. Phys. Chem. C 2016 , 120 , 5926 - 5931 ]. To further understand the hydrogen states in the precursors (the NiH₂ and NiH₃ units) and H atoms at the tetrahedral sites in the intermediate phase, LaMg₂NiH_4.6, we observed the hydrogen vibrations in LaMg₂NiH_4.6 and LaMg₂NiH₇ by using inelastic neutron scattering. A comparison of the hydrogen vibrations of the NiH₂ and NiH₃ units with that of [NiH₄]^4- shows that the librational modes of the NiH₂ and NiH₃ units were nonexistent; librational modes are characteristic modes for complex anions, such as [NiH₄]^4-. Furthermore, the hydrogen vibrations for the H atoms in the tetrahedral sites showed a narrower wavenumber range than that for H^- and a wider range than that for typical interstitial hydrogen. The results indicated the presence of intermediate hydrogen states before the formation of [NiH₄]^4- and H^-.