Domain wall freezing in KDP-type ferroelectrics

Hysteresis loops in KH2PO4 (KDP) and its ferroelectric (FE) isomorphs disap pear some 60 K below T-c. This disappearance may result from an order-disor der transition of the domain wall. The lowest energy wall consists of a sin gle layer of nonpolar H2PO4 groups of Slater energy epsilon(0). Including o nly the Slater/Takagi interactions predicts that a domain wall can become w ider by having small protrusions that then diffuse along the wall. Reducing temperature would decrease domain wall mobility without causing a freezing transition. However, if one includes the Ishibashi dipolar interaction, th is dipolar energy is minimized for a zero-entropy smooth domain wall with a particular ordered H-bond arrangement. Accordingly, there could be an orde r-disorder transition within the wall, if the bias "field" favoring this H- bond ordering is not great enough to smear out the transition. We are apply ing this model to predict domain wall mobility temperature dependence, and simultaneously measuring FE hysteresis in KDP-ferroelectrics to determine t he nature and sharpness of this proposed domain wall transition. (C) 1999 E lsevier Science Ltd. All rights reserved.