The pressure-induced tetragonal (I4mm) to orthorhombic (Cmc2(1)) 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 similar to 88 degrees C, but at higher pressure, ammonia borane is stable up to 100 degrees C above 0.5 GPa. In the pressure-temperature phase diagram, the boundary between tetragonal (I4mm) and orthorhombic (Cmc2(1)) phase was determined by the characteristic change in Raman spectra during compression and decompression at 3 constant temperatures: 20, 60 and 100 degrees 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. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.