IMPURITY AGAINST NONLOCALITY IN THE INCOMMENSURATE PHASE OF RB2ZNCL4 - A CL-35 NQR STUDY

The influence of impurities on the phason and amplitudon dynamics in t he incommensurate phase of (Rb1-xCsx2ZnCl4 (for x = 0.005 and 0.01) is investigated using Cl-35 nuclear quadrupole resonance frequency (nu) and spin lattice relaxation time (T1) measurements as a function of te mperature. While the paraelectric-to-incommensurate transition tempera ture (T(I)) is suppressed by about 8 K, the lock-in transition tempera ture (observed at 192 K in the pure compound) is not observed down to 77 K for x = 0.01. The observation of an additional singularity in the NQR spectrum of the x = 0.01 compound is explained as being due to th e manifestation of non-locality of the modulation wave with respect to the region of influence of the nuclear spin probe. The phason and amp litudon contributions to the spin lattice relaxation, T1phi and T1A re spectively, are measured by selective excitation of the broadened NQR spectra. While T1A is insensitive to impurities, T1phi is strongly dep endent on temperature and impurity concentration. The temperature depe ndence of T1phi is explained as being due to the underdamping of phono n modes in the presence of impurities.