Ammonium dihydrogen phosphate (ADP) impurity effects on phase transition and domain-wall freezing in potassium-ammonium dihydrogen phosphate [(KDP)1−x(ADP)x] crystals

Dielectric constants and polarization reversal currents are measured in ${\left(\mathrm{KDP}\right)}_{1-x}{\left(\mathrm{ADP}\right)}_x$ crystals for $x\lesssim 0.015$ to study the roles of ADP impurities. In this low-concentration limit it seems that the ADP impurities form hard defects responsible for lowering both the ferroelectric transition temperature ($T_c$) and the domain-wall freezing temperature ($T_f$). Qualitative discussion is given on the experimental observation of $\left|\frac{d{T}_f\mathrm{}\left(x\right)\mathrm{}}{\mathrm{dx}}\mathrm{}\right|>\mathrm{}\left|\mathrm{}\frac{d{T}_c\mathrm{}\left(x\right)\mathrm{}}{\mathrm{dx}}\right|$, which causes the range of the plateau anomaly between $T_c$ and $T_f$ to widen rather than narrow in the presence of impurities.