NB-93 NMR IN THE INCOMMENSURATE AND QUASICOMMENSURATE PHASES OF BARIUM SODIUM NIOBATE

NMR spectra have been measured for the quadrupole perturbed 1/2 --> -1 /2 transition of Nb-93 in barium sodium niobate (BSN) between 300 and 650 K. The spectra are inhomogeneously broadened in the incommensurate (I) phase between T(I) = 580 K and T(c) = 540 K as well as in the qua sicommensurate (QC) phase below T(c). However, the observed line shape s do not exhibit the characteristics of the lines for other 1q and 2q modulated incommensurate materials. The temperature dependence of the observed line shape can be explained using a model in which phase fluc tuations induced by random, weak pinning of the modulation wave by def ects produce Gaussian line broadening. Random pinning of the modulatio n wave also creates a large phason gap which makes spin-lattice relaxa tion via phasons inefficient. These results suggest that the presence of defects due to nonstoichiometry causes the I and QC phases of BSN t o be ''chaotic.''