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Updated: Jun 2, 2025
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Published on: May 15, 2015
Pressure-Induced Dehydration and Reversible Recrystallization of Dihydrogen-Bonded Sodium Borohydride Dihydrate
Satoshi Nakano1, Hiroshi Fujihisa2, Hiroshi Yamawaki2
1National Institute for Materials Science (NIMS), Tsukuba, Ibaraki 305-0044, Japan.
Abstract:
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.
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