The structural stability of ammonia borane NH₃BH₃ originates from a dihydrogen bond between adjacent molecules, in which the hydridic hydrogen bonds to boron (H^δ--B) acts as an acceptor for the protonic hydrogen bonds to nitrogen (H^δ+-N). The length of the dihydrogen bond (H^δ--H^δ+) is shorter than twice of van der Waals radius of hydrogen atom (2.4 Å). In order to investigate an influence of the dihydrogen bond on structural changes of ammonia borane, high-pressure x-ray diffraction (XRD) measurement for NH₃BH₃ and high-pressure neutron diffraction (ND) measurements for deuterated ND₃BD₃ were performed up to 10 GPa.

Powder XRD study was carried out using a diamond-anvil-cell (DAC) with a helium pressure medium at the AR-NE1A beamline in Photon Factory (PF), KEK. Lattice parameters and the axial ratios were calculated from the XRD patterns and a DFT calculation was also carried out with the lattice constants obtained. On the other hand, powder ND study was carried out using a Paris-Edinburgh press at BL11 “PLANET” in J-PARC MLF. No pressure medium was used. Crystallographic parameters and hydrogen-hydrogen (H-H) distances were obtained by Rietveld refinement using the GSAS program.

Regarding to ambient-pressure phase (AP, I4mm), all of the hydrogen atoms form dihydrogen bond between adjacent molecules and the H-H distances are distributed around 2.05 Å with dispersion of less than 2.4 Å. At about 1.2 GPa, the AP transformed to the first high-pressure phase (HP1, Cmc2₁). Structural analysis of HP1 using ND data showed that half of six kinds of H-H distances were larger than 2.4 Å just after the phase transition. It reveals that half of the dihydrogen bonds were broken through the transition from AP to HP1. With increasing the pressure from 1.2 to 10 GPa, the H-H distances monotonously decreased while maintaining the Cmc2₁structure, and above 4 GPa all of the H-H distances became shorter than 2.4 Å. The results of DFT calculation were consistent with those of the ND. Pressure dependence of lattice parameters of HP1 obtained from XRD data apparently changed at about 3 GPa, and then a hysteresis during compression and decompression was also observed. These results suggest that recombination and breaking of dihydrogen bonds took place at about 3-4 GPa during the compression and decompression process, respectively.