93Nb NMR in the imcommensurate and quasi-commensurate phases of barium sodium niobate

NMR spectra have been measured for the quadrupole perturbed 1/2 -1/2 transition of 9 3Nb in barium sodium niobate (BSN) between 300 and 650 K. The spectra are inhomogeneously broadened in the incommensurate (/) phase between T7 = 580 K and r c = 540 K as well as in the quasicommensurate (QC) phase below Tc. However, the observed line shapes do not exhibit the characteristics of the lines for other iq and 2q modulated incommensurate materials. The temperature dependence of the observed line shape can be explained using a model in which phase fluctuations induced by random, weak pinning of the modulation wave by defects produce Gaussian line broadening. Random pinning of the modulation wave also creates a large phason gap which makes spin-lattice relaxation via phasons inefficient. These results suggest that the presence of defects due to nonstoichiometry causes the J and QC phases of BSN to be "chaotic.