Tetragonal to orthorhombic phase transition of ammonia borane at low temperature and high pressure.

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. [ABSTRACT FROM AUTHOR]