SUBNANOMETRIC-SCALE MOTION OF THE MODULATION WAVE IN INCOMMENSURATE SOLIDS STUDIED BY 2-DIMENSIONAL EXCHANGE-DIFFERENCE NMR

The two-dimensional (2D) exchange-difference NMR technique is suitable for the observation of slow molecular motions in solid systems with s ingle inhomogeneously broadened NMR spectra. The difference signal ori ginates from those nuclei only, which undergo motion during the mixing period, whereas the signal from the effectively static nuclei is elim inated from the spectrum. In an inhomogeneously broadened 2D exchange difference spectrum the continuous diagonal peaks appear negative wher eas the continuous cross peaks show positive intensity. These two kind s of peaks appear resolved in the difference technique, whereas they r emain unresolved in the standard 2D exchange NMR experiment, which sho ws positive intensity only. From the analysis of the cross peaks it is possible to determine the maximum traveling distance and the frequenc y range of the random thermally induced modulation wave motion in INC systems. The motion in space is traced by the changes of nuclear reson ance frequencies which is possible in view of the existing frequency-s pace relation in solids with modulated structure. Slow motion of the i ncommensurate modulation wave has been studied in substitutionally dis ordered (Rb1-xKx)(2)ZnCl4 mixtures by the Rb-87 2D exchange-difference NMR. The maximum traveling distances of the modulation wave in the th ermally induced motion have been determined for the K+ impurity concen trations x=0.0, 0.02, and 0.06. The motion has been observed on the su bnanometric scare and the traveling distances were found small compare d to the wavelength of the modulation wave. The increase of the K+ con centration results in a decrease of the maximum traveling distance, re flecting the increased pinning of the modulation wave by impurities.