Pressure-InducedDehydration and Reversible Recrystallizationof Dihydrogen-Bonded Sodium Borohydride Dihydrate NaBH₄·2H₂O

Sodium borohydride dihydrate (NaBH₄·2H₂O) forms through dihydrogen bonding between the hydridic hydrogen of the BH₄^– ion and the protonic hydrogen of the water molecule. High-pressure structural changes in NaBH₄·2H₂O, observed up to 11 GPa through X-ray diffraction and Raman scattering spectroscopy, were analyzed to assess the influence of dihydrogen bonds on its crystal structure. At approximately 4.6 GPa, certain dihydrogen bonds were broken, leading to the decomposition of NaBH₄·2H₂O into ambient pressure phase of NaBH₄ (α-NaBH₄) and ice VII. Upon further compression beyond 6.6 GPa, NaBH₄ gradually transformed into its high-pressure phase, γ-NaBH₄. During decompression, γ-NaBH₄ reverted to α-NaBH₄ at the pressure between 4.4 and 2.7 GPa and subsequently reacted with ice VII, resulting in the recrystallization of NaBH₄·2H₂O. This recrystallization, occurring during decompression from 4.4 to 2.7 GPa, is identical to the starting sample and can be termed “decompression-induced recrystallization”, highlighting the strength of the dihydrogen bonds in NaBH₄·2H₂O. In addition, density functional theory calculations were used to evaluate the pressure dependence of hydrogen–hydrogen (H–H) distances in NaBH₄·2H₂O. As pressure increased, the number of dihydrogen bonds within the unit cell rose from seven at near-ambient pressure to 12 at approximately 4.5 GPa just before the dehydration, indicating that each hydrogen atom in the water molecule formed dihydrogen bonds with around three hydrogens from the BH₄^– ions just prior to dehydration. Such pressure tuning of dihydrogen bonds may lead to the creation of new energy storage materials.