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The effect of pressure on the low temperature tetragonal (I4mm) to orthorhombic (Pmn21) phase transition of a potential hydrogen storage compound ammonia borane (NH3BH3) was investigated in diamond anvil cell using Raman spectroscopy. With applied pressure, the transition occurs at higher temperature, which indicates that pressure enhances the ordering of the structure. The positive Clapeyron slope of the transition was determined to be dP/dT = ∼25.7 MPa/K, indicating the transformation is of exothermic. Appearance of some of the characteristic Raman modes of orthorhombic phase requires undercooling of around ∼15 K below the transition, indicating possible existence of an intermediate phase.

High pressure behavior of ammonia borane after thermal decomposition was studied by Raman spectroscopy at high pressure up to 10 GPa using diamond anvil cell (DAC). The ammonia borane was decomposed at around 140 degree Celsius under the pressure at ~0.7 GPa. Raman spectra show the hydrogen was desorbed within 1 hour. The hydrogen was sealed in DAC well and cooled down near to room temperature. Applying higher pressure up to ~10 GPa indicates interactions between the products and loss of dihydrogen bonding. No rehydrogenation was detected in the pressure range investigated.

One of the major obstacles to the use of hydrogen as an energy carrier is the lack of proper hydrogen storage material. Lithium amidoborane has attracted significant attention as hydrogen storage material. It releases ∼10.9 wt% hydrogen, which is beyond the Department of Energy target, at remarkably low temperature (∼90°C) without borazine emission. It is essential to study the bonding behavior of this potential material to improve its dehydrogenation behavior further and also to make rehydrogenation possible. We have studied the high-pressure behavior of lithium amidoborane in a diamond anvil cell using in situ Raman spectroscopy. We have discovered that there is no dihydrogen bonding in this material, as the N-H stretching modes do not show redshift with pressure. The absence of the dihydrogen bonding in this material is an interesting phenomenon, as the dihydrogen bonding is the dominant bonding feature in its parent compound ammonia borane. This observation may provide guidance to the improvement of the hydrogen storage properties of this potential material and to design new material for hydrogen storage application. Also two phase transitions were found at high pressure at 3.9 and 12.7 GPa, which are characterized by sequential changes of Raman modes.H ydrogen economy has been considered as potentially efficient and environmental friendly alternative energy solution (1). However, one of the most important scientific and technical challenges facing the "hydrogen economy" is the development of safe and economically viable on-board hydrogen storage for fuel cell applications, especially to the transportation sector. Ammonia borane (BH 3 NH 3 ), a solid state hydrogen storage material, possesses exceptionally high hydrogen content (19.6 wt%) and in particular, it contains a unique combination of protonic and hydridic hydrogen, and on this basis, offers new opportunities for developing a practical source for generating molecular dihydrogen (2-5). Stepwise release of H 2 takes place through thermolysis of ammonia borane, yielding one-third of its total hydrogen content (6.5 wt%) in each heating step, along with emission of toxic borazine (6-8). Recently, research interests are focusing on how to improve discharge of H 2 from ammonia borane, including lowering the dehydrogenation temperature and enhancing hydrogen release rate using different techniques, e.g., nanoscaffolds (9), ionic liquids (10), acid catalysis (11), base metal catalyst (12), or transition metal catalysts (13, 14). More recently, significant attention is given to chemical modification of ammonia borane through substitution of one of the protonic hydrogen atoms with an alkali or alkaline-earth element (15-21). Lithium amidoborane (LiNH 2 BH 3 ) has been successfully synthesized by ball milling LiH with NH 3 BH 3 (15-18). One of the driving forces suggested for the formation of LiNH 2 BH 3 is the chemical potential of the protonic H δ+ in NH 3 and the hydridic H δ− in alkali metal hydrides making them tend to combine, producing H 2 + LiNH 2 BH 3 ....