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

A complex hydride (LaMg 2 NiH 7 ) composed of La 3+ , two Mg 2+ , [NiH 4 ] 4- with a covalently bonded hydrogen, and three H - was formed from an intermetallic LaMg 2 Ni via an intermediate phase (LaMg 2 NiH 4.6 ) composed of La, Mg, NiH 2 , NiH 3 units, and H atoms at tetrahedral sites. The NiH 2 and NiH 3 units in LaMg 2 NiH 4.6 were reported as precursors for [NiH 4 ] 4- in LaMg 2 NiH 7 [ Miwa et al. J. Phys. Chem. C 2016 , 120 , 5926 - 5931 ]. To further understand the hydrogen states in the precursors (the NiH 2 and NiH 3 units) and H atoms at the tetrahedral sites in the intermediate phase, LaMg 2 NiH 4.6 , we observed the hydrogen vibrations in LaMg 2 NiH 4.6 and LaMg 2 NiH 7 by using inelastic neutron scattering. A comparison of the hydrogen vibrations of the NiH 2 and NiH 3 units with that of [NiH 4 ] 4- shows that the librational modes of the NiH 2 and NiH 3 units were nonexistent; librational modes are characteristic modes for complex anions, such as [NiH 4 ] 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 [NiH 4 ] 4- and H - .