Reversible pressure-induced dehydration of NaBH ₄ ·2H ₂ O and the role of dihydrogen bonds: the investigation based on Raman scattering

High-pressure Raman scattering measurements on sodium borohydride dihydrate NaBH4·2H2O were performed up to 8.5 GPa to investigate the structural changes during dehydration and the transformation previously observed by X-ray diffraction. Density functional theory calculations were used to identify the Raman modes of NaBH4·2H2O. Below 4.6 GPa, the wavenumbers of the B-H stretching modes blue-shifted with increasing pressure, whereas the O-H bending modes red-shifted slightly and the O-H stretching modes remained nearly constant. Pressure-induced dehydration of NaBH4·2H2O occurred at 4.6 GPa, producing α-NaBH4 and ice VII. Subsequently, the decomposed NaBH4 underwent a phase transition to γ-NaBH4 at 6.6 GPa. During decompression, a reversible transition from γ-NaBH4 to α-NaBH4 occurred at 5.5 GPa, followed by the regeneration of NaBH4·2H2O at 3.2 GPa. These results are consistent with those of the X-ray diffraction measurements observed previously.