Anomalous dielectric aftereffect in ferroelectricKH2PO4

Abstract

Dielectric permittivity, $\ensuremath{\epsilon}={\ensuremath{\epsilon}}^{\ensuremath{'}}\ensuremath{-}i{\ensuremath{\epsilon}}^{\ensuremath{''}},$ of ${\mathrm{KH}}_{2}{\mathrm{PO}}_{4}$ pressed powders was measured between $T=1.4$ and 25 K in the presence of dc electric bias fields. Usually if the bias was switched at time $t=0, {\ensuremath{\epsilon}}^{\ensuremath{'}}$ and ${\ensuremath{\epsilon}}^{\ensuremath{''}}$ jumped to new values, then decreased approximately as $\mathrm{log}t(6<t<2000 \mathrm{s}).$ This well-known effect, that is also found with single crystals, is attributed to switchable microdomains that are present accidentally in crystals but are systematically more numerous in powders. A very different aftereffect was observed in a narrow T range around 7--8 K. ${\ensuremath{\epsilon}}^{\ensuremath{'}}$ jumped to a lower value, then increased with t according to a stretched exponential with a T-dependent time constant. This lay near the extrapolation of the Arrhenius law of a known but unassigned weak-field dispersion that is a property of polydomain single crystals as well as powders. The weak-field dispersion is attributed to the elementary movement of a jog on a lateral step displacement of a domain wall, consisting of a single H-bond reversal. The anomalous aftereffect results from the interaction between these point defects and the microdomain system.