Temperature dependence of dielectric constant of $K_x(NH_4)_{1-x}H_2PO_4$ System for x=0%, 2.7%, 5.2%, 7.1% respectively was measured to study on the KDP impurity effects in the anti-ferroelectric phase transition of ADP.
Qualitative change of ADP phase transition by impurity effects was not observed, but increase of Curie-Weiss constant and decrease of transition temperature with increasing $K^+$ ion concentration could be observed. These results suggest that $K^+$ ion acts so as to weaken the N-H--O bond and acts as competing centers of ferroelectric interaction against antiferroelectric interaction. Decrease of transition temperature was compared with the theory of Prelovsek and Blinc quantitatively and discussed briefly.
And a small qualitative change in dielectric constant anomaly could be observed when poling ADP along a-axis with 6kV/cm DC electric field, which suggests a new transition to some(ferroelectric?) phase other than the anti-ferroelectric phase may be possible.
