Dielectric constants of mixed crystal system of $(NH_4)_2SO_4-K_2SO_4$ were measured as functions of temperature and concentration of $K^+$ ion and the temperature dependence of D-E hysteresis loop of $(NH_4)_2SO_4$ was observed.
And the temperature dependence of the spontaneous polarization of $(NH_4)_2SO_4$ was studied from the D-E hysteresis loop.
With the increase in the concentration of $K^+$ ion, the following changes were observed to; (1) The phase transition changes from the first order to the second order-like character. (2) The phase transition temperature decreases linearly. (3) The Curie-Weiss Constant increases.
These results and discussed in terms of a two nonequivalent sublattice model.
The triple-like D-E hysteresis loops was observed in $(NH_4)_2SO_4$ below the transition temperature. It was also shown that the spontaneous polarization of $(NH_4)_2SO_4$ tended to decrease when the temperature was lowered below T $\simeq$ -65℃.
