Temperature dependence of εa was studied in detail in the x-cut KDP crystal with ADP impurities near the ferroelectric transition temperature. It was found that $\varepsilon$a showed the antiferroelectric characteristics like ADP crystal. The shifts of Tc to low temperature were found to be proportional to ADP impurity concentrations. But cusp-like singularity near Tc was not changed significantly. This result supports the importance of cooperative mechanism of proton-tunnelling motion and lattice vibration even in the doped KDP.
The small field response currents of the transversal polarization fluctuations $\Delta$Px(t) were examined for samples of varying ADP concentration. Resonance type fluctuations were observed near Tc. As the ADP impurity concentration increases, ADP impurities had the significant influence on the frequency softening near Tc. These results are interpreted by considering the macroscopic coupling between the yz shear strain and the spontaneous xy shear strain.
