We provide direct evidence for successive exchange of D atoms in the isotopically pure ${\text{BH}}_4^{-}$ units of the crystalline solid lithium borohydride. We prove the coexistence of all ${\text{BH}}_{4-n}{\text{D}}_n^{-}$ $\left(0\le n\le 4\right)$ units in the bulk by deconvolution of the D-stretching vibrations band of Raman spectra at 83 and 5 K in partially D-exchanged ${\text{LiBH}}_4$ and comparison with first-principles Raman intensity calculations. The measured distribution of ${\text{BH}}_{4-n}{\text{D}}_n^{-}$ units is in good agreement with a binomial distribution biased by H-D zero-point motions. This implies breaking the strong ${\text{D}}_2$ covalent bond and transport of hydrogen in ${\text{LiBH}}_4$ below the melting temperature.

• Received 12 August 2009

DOI:https://doi.org/10.1103/PhysRevB.80.100301