Domain wall freezing in KDP-type ferroelectrics

Abstract

Hysteresis loops in KH2PO4 (KDP) and its ferroelectric (FE) isomorphs disappear some 60 K below Tc. This disappearance may result from an order–disorder transition of the domain wall. The lowest energy wall consists of a single layer of nonpolar H2PO4 groups of Slater energy ε0. Including only the Slater/Takagi interactions predicts that a domain wall can become wider 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, this dipolar energy is minimized for a zero-entropy smooth domain wall with a particular ordered H-bond arrangement. Accordingly, there could be an order–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 applying this model to predict domain wall mobility temperature dependence, and simultaneously measuring FE hysteresis in KDP-ferroelectrics to determine the nature and sharpness of this proposed domain wall transition.