We measured the temperature dependence of dielectric constant of $K_{1-x}Na_xH_2PO_4(X\leqq0.014)$ mixed crystals to study $Na^+$ impurity effect on KDP phase transition.
The transition temperature with increasing $Na^+$ concentration was observed to decrease at the rate of $\triangle{T_c}\simeq-146\triangle{X}$, which was interpreted as $Na^+$ ion played a hard defect role to weaken pseudo-spin interaction J.
