Theoretical Investigation of Pressure-Induced Structural Transformations of Ethylenediamine Bisborane

Rongfeng Guan, The University of Western Ontario

Abstract

High-pressure experiments on hydrogen-rich compounds provide crucial data for the rational design of hydrogen storage materials. Ethylenediamine bisborane (EDAB), BH₃·NH₂CH₂CH₂NH₂·BH₃, is one of the prime candidates for this role due to its high hydrogen content (10 wt%) and good kinetic stability under ambient conditions. Previous studies of EDAB using in situ Fourier-transform infrared (IR) spectroscopy, Raman spectroscopy, and synchrotron X-ray diffraction (XRD) techniques suggested that EDAB undergoes two possible phase transitions in the pressure range of 0 to 17 GPa. However, the crystal structures of the two new phases arising in these transitions remained unknown due to experimental challenges of in situ structural characterization under high pressures. In this study, we perform Kohn–Sham density functional theory (DFT) calculations and identify the structures of the two high-pressure phases of EDAB. Our results confirm that EDAB undergoes two structural transformations. The first one is at 1 GPa from the orthorhombic Pbca ambient-pressure structure (phase I) to the monoclinic P2₁/c structure (phase II). The second is at 8 GPa from phase II to another structure (phase III) of the monoclinic P2₁/c symmetry, which remains the dominant phase up to at least 17 GPa. The mechanism of these phase transitions is attributed to the formation of dihydrogen bonding frameworks, revealed by DFT calculations.