Bonding in boranes and their interaction with molecular hydrogen at extreme conditions

Abstract

The effects of high pressure and temperature on the bonding in ammonia borane (AB), NH(3)BH(3) and decaborane (DB), B(10)H(14) and their interactions with molecular hydrogen (H(2)) were investigated using Raman spectroscopy in a diamond anvil cell. At 0.7 GPa, AB becomes amorphous between 120 and 127 degrees C, indicating a positive Clapeyron slope. Heated to 140 degrees C, AB begins to undergo decomposition to polyaminoborane. The amorphous and decomposed AB does not recrystallize back to AB during slow cooling to room temperature or upon application of high pressure up to 3 GPa, underscoring the challenge of rehydrogenation of decomposed AB. The molecular Raman modes broaden in the reacted phase, and the NH(3) modes show no pressure dependence. DB was studied at room temperature up to 11 GPa. The observed frequency dependence with pressure (dnu/dP) and mode Gruneisen parameters varied for different spectral groups, and a new transition was identified at approximately 3 GPa. In both DB and heated AB, we found that they could store additional H(2) with the application of pressure. We estimate that we can store approximately 3 wt % H(2) in heated AB at 3 GPa and 1 wt % H(2) in DB at 4.5 GPa.