Slow relaxation of the line shape of the incommensurate satellites in barium sodium niobate

Abstract

A time dependence of the line shape of incommensurate satellites is disclosed for the first time by accurate x-ray goniometry in barium sodium niobate. It consists in a narrowing of the line and in an increase of the intensity, with characteristic times of several hours. The origin of this slow extension of the long-range order of the incommensurate modulation is assigned to the interaction of the modulation with mobile point defects, as suggested by the interpretation of the macrosocpic thermal hysteresis of this material. The manifestation of a "memory effect" pointed out recently in several incommensurate systems is clarified by our measurements.