High resolution 15N NMR of the 225 K phase transition of ammonia borane (NH3BH3):: Mixed order-disorder and displacive behavior

We report high resolution N-15 NMR probing of the solid-solid phase transition of N-15-labeled ammonia borane (NH3BH3) at 225 K. Both the N-15 isotropic chemical shift (delta(iso)) and the spin-lattice relaxation rate (T-1(-1)) exhibited strong anomalies around 225 K. The analysis of T-1(-1) using the Bloembergen, Purcell, and Pound model showed that the motional correlation time, tau, increased from about 1 to 100 ps and the corresponding Arrhenius activation energy increased from 6 to 14.5 kJ/mol on going through the transition toward lower temperatures. The temperature dependence of delta(iso) was interpreted by an extension of the Bayer model. The time scale of the underlying motion was found to be in a reasonable agreement with the T-1(-1) data. These results imply that the NH3 rotor motion plays a pivotal role in the transition mechanism and that the transition is of both order-disorder and displacive type.