Influence of defects on the incommensurate modulation in irradiated Ba2NaNb5O15.

Abstract

Defect-induced properties of barium sodium niobate (Ba{sub 2}NaNb{sub 5}O{sub 15}) have been studied as a function of electron irradiation dose. Birefringence measurements at low doses indicate that the lock-in transition temperature, originally at about 280 {degree}C, decreases linearly with increasing dose. Complementary irradiations {ital in} {ital situ} in a high-voltage electron microscope show that the incommensurate phase can be stabilized down to room temperature. Simultaneously the satellite diffraction spots broaden and the lock-in transition becomes diffuse. The modification of the hysteresis of the birefringence, observed already at the lowest doses, indicates a progressive extension of the stability range of the 2{ital q} modulated phase to lower temperatures as the defect concentration increases. A similar conclusion can be drawn from the satellite reflection dark-field electron micrographs that show, once the incommensurate phase is stabilized at room temperature, the doubly modulated texture characteristic of the 2{ital q} modulated phase. Low doses of irradiation do not change qualitatively the configuration of the residual discommensurations in the lock-in phase nor the temperature dependence of the incommensurability. Accordingly, already in the as-grown samples the defects dominate the pinning of the incommensurate modulation and the intrinsic properties of this incommensurate system are not clearly observable.