Evidence for hydrogen transport in deuteratedLiBH4from low-temperature Raman-scattering measurements and first-principles calculations

Abstract

We provide direct evidence for successive exchange of D atoms in the isotopically pure BH 4 − units of the crystalline solid lithium borohydride. We prove the coexistence of all BH 4−n D n − ͑0 Յ n Յ 4͒ units in the bulk by deconvolution of the D-stretching vibrations band of Raman spectra at 83 and 5 K in partially D-exchanged LiBH 4 and comparison with first-principles Raman intensity calculations. The measured distribution of BH 4−n D n − units is in good agreement with a binomial distribution biased by H-D zero-point motions. This implies breaking the strong D 2 covalent bond and transport of hydrogen in LiBH 4 below the melting temperature.

Mobility and dynamics in the complex hydrides LiAlH4 and LiBH4

“…30 DFT calculations of the solid reproduce this spacing and all the details of the experimental spectra: for the partially exchanged units (BHD 3 , BH 2 D 2 and BH 3 D), the tetrahedral unit is slightly distorted in the solid and substitution at different crystallographic sites results in two different B-D bond lengths and Raman n 1 frequencies. 30 The crystal field further splits all modes, 15 up to four modes per cluster type instead of one for the symmetric, isolated clusters, although this effect becomes visible only at low temperatures. 30 The asymmetric stretching modes of partially exchanged LiBD 4n H 4À4n as displayed in Fig.…”

A thermodynamic assessment of LiBH4

“…3), at 1 bar H 2 pressure, Li 2 B 12 H 12 should be stable above 500 K and a driving force for reaction (15) should be present above that temperature. This driving force could explain the hydrogen transport detected in LiBH 4 below the melting temperature [58], as well as the small hydrogen release detected during melting [35,39,46]. In fact, at these temperatures, there should be no driving force for the decomposition into LiH according to reaction (17) pathway, so that release of H 2 should not be observed.…”

Nuclear magnetic resonance studies of atomic motion in borohydrides

“…Thus, the observed hydrogen diffusion in the high-T solid phase of LiBH 4 with the characteristic jump rate of ∼10 5 s −1 at 500 K (see above) has a negligible contribution from the H exchange processes. The main mechanism of hydrogen diffusion in this phase should be the jump motion of intact BH 4 groups (or BH 3 groups having a hydrogen vacancy [32]). …”