39K NMR-STUDY OF THE ANTIFERROELECTRIC PHASE-TRANSITION IN POTASSIUM THIOCYANATE

The K-39 electric field gradient (EFG) and chemical-shift tensors have been determined in KSCN above and below T(c). Whereas all the data below T(c) are compatible with the site symmetry of the K+ ions given by the space group D2h11 (Pbcm) of the low-temperature orthorhombic phase, this is not the case for the EFG data above T(c). The symmetry of the paraelectric phase is broken on the quadrupole-coupling time scale but not on the chemical-shift-tensor time scale, allowing for an interesting insight into the dynamics of the SCN- head-tail disordering process. The transition is of the order-disorder type for the SCN- groups and of a displacive type for the K+ ions.