Phase boundary of pressure-induced I4mm to Cmc21 transition in ammonia borane at elevated temperature determined using Raman spectroscopy

The pressure-induced tetragonal (I4mm) to orthorhombic (Cmc21) phase transition in ammonia borane has been studied in a diamond anvil cell (DAC) using in situ Raman spectroscopy. At near ambient pressure in DAC, ammonia borane started decomposing as low as ∼88 °C, but at higher pressure, ammonia borane is stable up to 100 °C above 0.5 GPa. In the pressure-temperature phase diagram, the boundary between tetragonal (I4mm) and orthorhombic (Cmc21) phase was determined by the characteristic change in Raman spectra during compression and decompression at 3 constant temperatures: 20, 60 and 100 °C, respectively. The result shows that the phase transition is reversible with a small hysteresis, while the phase boundary shifts towards higher pressure with increasing temperature and a positive Clapeyron-slope. The positive Clapeyron-slope indicates that the phase transition is exothermic and it is predicted that the rehydrogenation of the high pressure orthorhombic phase would be easier due to its lower enthalpy.