NQR Study of the Pinning and Depinning of the Incommensurate Modulation Wave in the Presence of Impurities *

High temperature-resolution N Q R measurements in an ul t ra pure Rb 2 ZnCl 4 crystal and a mixed [Rb[ _ x ( N H 4 ) J 2 Z n C l 4 crystal w i th a control led amount of impurit ies (x = 0.01) verify the existence of a temperature region where the characteristic incommensurate line split t ing is averaged out. The results show that for the system wi th high pur i ty the averaging of the incommensurate splitt ing is due to large thermal fluctuations. In contrast, for the mixed crystal the added impurit ies seem to play the predominant role.