Hydrogen−Deuterium Exchange in Bulk LiBH₄

Abstract

Because of its apparent simplicity, diffusion of hydrogen in solids can be regarded as a general model system for diffusion. However, only rudimentary knowledge exists for the dynamics of hydrogen in complex hydrides. Insight into the specific diffusion process is given by hydrogen-deuterium exchange experiments. Thermogravimetry and Raman spectroscopy are used to measure the hydrogen-deuterium exchange during the decomposition of LiBH4. At a temperature of 523 K the self-diffusion constant of deuterium in LiBH4 is estimated to be D approximately 7 x 10(-14) m(2) s(-1). A careful analysis of the Raman spectra shows that hydrogen is statistically exchanged by deuterium in LiBH4; i.e., the diffusing species is assumed to be the single hydrogen atom.

Atomic Motions in LiBH4 by NMR

“…We consider later (see below) the question of whether the diffusing units are BH 4 molecular anions or H anions, as has been suggested. 15 Here we simply remark that either motion will fully average to zero all of the hydrogen dipolar interactions. Related two-step narrowing appears 24 in LiH.…”

Nuclear magnetic resonance studies of atomic motion in borohydrides

“…The idea that B atoms are involved in the translational motion on the NMR time scale is supported by the observation [15,24] that the 11 B line width drops below the value expected for the B-B contribution to the 'rigid lattice' width. It should be noted that recent Raman scattering measurements of the H-D exchange in the high-T solid phase of LiBH 4 [30] have been interpreted in terms of individual hydrogen atoms moving from one BH 4 group to another. However, estimates of the local H exchange rate between BH 4 groups based on the 11 B NMR experiments in the liquid phase of LiBH 4 [31] have shown that near the melting point this rate is lower than 1 s −1 .…”

Hydrogen Dynamics in Lightweight Tetrahydroborates

“…[82]), in liquid LiBH 4 (stretching vibrations measured by Raman spectroscopy, Ref. [83]), in LiBH 4 dissolved in organic solvents (stretching vibrations measured by IR spectroscopy, Ref. [84]) and in solid LiBH 4 [62].…”

“…Hydrogen.deuterium exchange probed by vibrational spectroscopy is also used to probe the hydrogen transport mechanism in tetrahydroborates: recently we evidenced the presence of all partially exchanged BH 4-n D n units (0 < n < 4) in LiBH 4 exposed to deuterium gas below the melting temperature [83,88].…”