In Situ High-Pressure and Low-Temperature Study of Ammonia Borane by Raman Spectroscopy

As a promising hydrogen storage material, ammonia borane was investigated at simultaneous high pressures (up to 15 GPa) in a diamond anvil cell and at low temperatures (down to 80 K) using a cryostat by in situ Raman spectroscopy. When ammonia borane was cooled from room temperature to 220 K at near ambient pressure, an expected phase transformation from I4mm to Pmn2(1) was observed. Then the sample was compressed to 15 GPa isothermally at 180 K. Three pressure induced structural transformations were observed as evidenced by the change in the Raman profile as well as the pressure dependence of the major Raman modes. The decompression and warming-up experiments suggest that these pressure-temperature (P-T)-induced transformations are reversible. These observations, together with factor group analysis, allowed us to examine the possible structures of the new high-pressure phases and the nature of phase transitions. Raman measurements from multiple runs covering various P-T paths, when combined with previously established room-temperature and high-pressure data, enabled the updating of the P-T phase diagram of ammonia borane in the pressure region of 0-15 GPa and the temperature region of 80-350 K.