Thermal decomposition of ammonia borane at high pressures

The effects of high pressure (up to 9 GPa) on the thermal decomposition of ammonia borane, BH 3 NH 3 , were studied in situ by Raman spectroscopy in a diamond anvil cell. In contrast with the three-step decomposition at ambient pressure, thermolysis under pressure releases almost the entire hydrogen content of the molecule in two distinct steps. The residual of the first decomposition is polymeric aminoborane, ( BH 2 NH 2 ) x , which is also observed at ambient pressure. The residual after the second decomposition is unique to high pressure. Presumably it corresponds to a precursor to hexagonal BN where macromolecular fragments of planar hexagon layers formed by B and N atoms are terminated by H atoms. Increasing pressure increases the temperature of both decomposition steps. Due to the increased first decomposition temperature it becomes possible to observe a new high pressure, high temperature phase of BH 3 NH 3 which may represent melting.