Interaction of an incommensurate modulation with mobile and fixed defects in barium sodium niobate

Abstract The interaction of the incommensurate modulation with mobile point defects provokes in barium sodium niobate phenomena of large amp1itude:a slow time-relaxation of the intensity and width of the satellites and of macroscopic quantities such as the optical birefringence, a thermal hysteresis with a strong but reproducible dependence on the conditions of the measurements, and an extreme sensitiveness on these conditions of the characteristics of the lock-in transition. Qualitative features of the modulation-induced motion of the defects are discussed. We also point out the existence of other anomalies of the behaviour which may be due to an interaction of the modulation with fixed defects.