Experimental and Theoretical Studies on a High Pressure Monoclinic Phase of Ammonia Borane

Abstract

The effect of pressure on the structure of ammonia borane NH3BH3 (AB) was investigated using a combination of high pressure X-ray diffraction (XRD) and density functional theory (DFT). In situ XRD was performed up to 15.0 GPa at room temperature in a diamond anvil cell, and two first-order phase transitions were observed at 1.6 and 12.9 GPa. The ambient pressure I4mm structure transformed into the high pressure Cmc21 phase at 1.6 GPa, and then experienced a second-order isostructural phase transition at 5 GPa, and further developed into a monoclinic P21 (Z = 4) phase at 12.9 GPa. The structure of the high pressure P21 phase was solved by powder diffraction data and further optimized using DFT calculations. The high pressure phase transitions were found to be reversible upon releasing pressure. The behavior of the N–H···H–B dihydrogen bonding framework, inter- and intramolecular interactions at high pressure was also investigated. The origin of the phase transition at 12.9 GPa is attributed to the reorganization of the dihydrogen bonding network and the change in the rotational dynamics of the NH3 and BH3 groups.