N-14 NMR INVESTIGATION OF KSCN

We have directly determined with N-14 NMR the SCN head-tail flipping r ate in KSCN on going through the order-disorder phase transition at T- c. The results can be explained in terms of an asymmetric double-well potential for the SCN reorientations below T-c and a symmetric double- well potential above T-c. The time scale for the SCN fluctuations is s o slow that low symmetry K-39 1/2 --> -1/2 NMR angular rotation patter ns are observed in the high-temperature high-symmetry phase. KSCN thus seems to be the first known example where a structural phase transiti on can be observed with NMR in the slow-motion and not in the fast-mot ion regime and where a dynamic breaking of the symmetry of the high-te mperature phase takes place.