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

The anelastic spectrum of ammonia borane (NH(3)BH(3)) 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 similar to 1 kHz, which is attributed to the dynamics of the rotations and torsions of the NH(3)-BH(3) 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.