Influence of pressure-induced formation of dihydrogen bonds on lattice parameters, volume, and vibrational modes of ammonia borane

The high-pressure phase of ammonia borane (NH₃BH₃) observed at ∼1.2 GPa has been reported to result in pressure-induced formation of dihydrogen bonds at ∼4 GPa. In this study, we performed high-pressure x-ray diffraction measurements on the high-pressure phase (up to ∼10.2 GPa) using a He hydrostatic pressure medium to examine the influence of the formation of dihydrogen bonds on the lattice parameters and unit cell volume of NH₃BH₃. We observed a unique behavior in the pressure dependence of lattice parameters close to the pressure at which the dihydrogen bond was formed. The lattice parameters demonstrated hysteresis curves under compression and decompression conditions but the unit cell volume did not. Moreover, the pressure dependence of the unit cell volume could not be expressed using a single Birch–Murnaghan equation within an acceptable margin of error, thus suggesting a change in bulk modulus under compression. These results are considered to have originated from the pressure-induced formation of dihydrogen bonds. Moreover, high-pressure Raman scattering measurements and a simulation using density functional theory calculations revealed the vibrational modes of the high-pressure phase of NH₃BH₃. The results demonstrated that librational modes were enhanced by forming dihydrogen bonds. Moreover, the intramolecular stretching modes of BN, BH, and NH monotonically shifted with pressure, while the symmetrical in-plane bending modes of BH₃ and NH₃ split irrespective of the formation of dihydrogen bonds.