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 I 4 mm to Pmn 2 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.