Hydrogen Dynamics and Characterization of the Tetragonal-to-Orthorhombic Phase Transformation in Ammonia Borane

Abstract

The anelastic spectrum of ammonia borane (NH3BH3) was measured in the temperature range between 1.3 and 300 K. The structural phase transition near 220 K is accompanied by a substantial variation of the elastic modulus (25−30%) and a spike of the elastic energy dissipation. The order−disorder phase transformation that occurs with a minute hysteresis, 0.4 K between heating and cooling, is indicative of a first-order phase transformation with a narrow region of coexistence between the two phases. The kinetics of the transition was investigated in detail. In addition, a thermally activated relaxation peak is revealed around 100 K for a sample vibration frequency of ∼1 kHz, which is attributed to the dynamics of the rotations and torsions of the NH3−BH3 groups. Finally, the time-dependent energy loss on heating after aging the sample below the structural phase-transformation temperature was interpreted in terms of annealing of dislocations.