We report high resolution 15N NMR probing of the solid−solid phase transition of 15N-labeled ammonia borane (NH3BH3) at 225 K. Both the 15N isotropic chemical shift (iso) and the spin−lattice relaxation rate (T1-1) exhibited strong anomalies around 225 K. The analysis of T1-1using the Bloembergen, Purcell, and Pound model showed that the motional correlation time, , 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 isowas 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 T1-1data. These results imply that the NH3rotor motion plays a pivotal role in the transition mechanism and that the transition is of both order−disorder and displacive type. [ABSTRACT FROM AUTHOR]