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

Abstract

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

First-principles investigation of LaMg2Ni and its hydrides

“…In the present study, the impact of Mg-Ni and Ni-H interactions on the hydrogenation of ternary compound www.nature.com/scientificreports/ phase, showing a covalent nature with the bond order between Ni-H being positive (BO Ni-H > 0) 31,32,34,38 . Miwa et al 17 and Sato et al 19 (Fig. 4) [11][12][13] .…”

“…In our studies, because the host compound LaMg 2 Ni and the intermediate hydride LaMg 2 NiH 4.5 are both metallic, the dehydrogenation reaction between them may be free from the energy barrier associated with the metal-insulator transition. In fact, the hydrogenation reaction of LaMg 2 Ni to LaMg 2 NiH 4.5 process even at room temperature 17,19 (Fig. 2).…”

“…Referring to the Ni–H bonds, it is formed as the hydrogenation reaction to the intermediate hydride phase, and becomes stronger to the full hydride phase, showing a covalent nature with the bond order between Ni–H being positive (BO Ni–H > 0) 31 , 32 , 34 , 38 . Miwa et al 17 and Sato et al 19 had proposed that the intermediate hydride phase LaMg 2 NiH 4.5 may play as precursor state for the following complex hydride LaMg 2 NiH 7 formation as the Ni–H bonds in LaMg 2 NiH 4.5 are essentially covalent nature similar to those in LaMg 2 NiH 7 , which subsequently provides the reduction of energy barrier for LaMg 2 Ni hydrogenation. According to this viewpoint, our intermediate hydrides LaMg 2 NiH 4.5 , LaMg 2 NiH 4.5 –Co and LaMg 2 NiH 4.5 –Pd with Ni–H covalent bonds may also act as precursor states for the following hydrogenation reaction to LaMg 2 NiH 7 , LaMg 2 NiH 7 –Co and LaMg 2 NiH 7 –Pd, respectively.…”

“…In our studies, because the host compound LaMg 2 Ni and the intermediate hydride LaMg 2 NiH 4.5 are both metallic, the dehydrogenation reaction between them may be free from the energy barrier associated with the metal–insulator transition. In fact, the hydrogenation reaction of LaMg 2 Ni to LaMg 2 NiH 4.5 process even at room temperature 17 , 19 . Once LaMg 2 Ni is hydrogenated to form LaMg 2 NiH 4.5 , this metallic intermediate hydride LaMg 2 NiH 4.5 with Ni–H covalent bonds may act as precursor state for the following complex nonmetallic hydride LaMg 2 NiH 7 formation (as described above).…”

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

“…This characteristic feature opens a window for new insight into the formation of complex transition metal hydrides. Using complementary studies of X–ray and neutron diffraction, inelastic neutron scattering and theoretical calculations, the formation process of LaMg 2 NiH 7 was evaluated in detail . These studies have clarified the existence of an intermediate hydrogen state in an intermediate phase LaMg 2 NiH 4.6 that occurs before the formation of the complex anion and hydride ions in LaMg 2 NiH 7 via an interstitial hydrogen in LaMg 2 NiH 0.05 (Figure ).…”