The Dielectric constants at higher harmonic responses $ε^'_n$ and $ε^{''}_n$ (n=1,3,5) has been measured as a function of temperature for $KH_2PO_4$ (KDP) and $CsH_2PO_4$ (CDP) Crystals at 250 Hz, using the lock-in amplifier systems.
The results of the experimental measurements are explained by considering the activation type interactions between domain walls and defects.
It is shown that the dielectric loss dependence on temperature may be explained as due to the increase of the effective viscosity for the motion of the domain wall encountering the pinning effects of the defects, and the nonlinear dielectric responses may be originating from the nonlinearity of the interdomain interaction.
The dielectric constants of $ε^'_n$ and $ε^{''}_n$ of $CsH_2PO_4$ seems to show another domain-state freezing at around 140K in addition to the low temperature domain wall freezing.
