Dynamic symmetry breaking in KSCN: A K-39 and N-14 NMR study

The (39)K1/2 --> -1/2 NMR angular rotation patterns in KSCN show no symmetr y change on going through T-c. We have directly determined with N-14 NMR th e SCN head-tail flipping rate. The results show that the absence of a symme try change in K-39 angular rotation patterns is due to the fact that the ti me scale for the SCN fluctuations is much longer than the characteristic ti me scale of this experiment. The same is true for the diffuse neutron scatt ering data where both above and below T-c the SCN groups appear ordered and static and disorder is restricted to domain boundaries. KSCN thus seems to be the first known example where a structural phase transition can be obse rved in the slow motion and not in the fast motion regime and where a dynam ic breaking of the symmetry of the high temperature phase takes place.