Nonlinear dielectric response in the mixed K$_{0.91}$(NH$_4$)$_{0.09}$H$_2$PO$_4$ crystal

The influence of bias and variable electric fields on the dielectric response of the K$_{0.91}$(NH$_4$)$_{0.09}$H$_2$PO$_4$ single crystal has been studied in the vicinity of the ferroelectric phase transition temperature $T_C$. Below $T_C$, the nonlinear response is caused mainly by the domain mechanism. This is confirmed, in particular, by observations of chaotic oscillations in a series $RLC$-circuit containing the studied sample as a capacitor $C$ and excited by a sinusoidal voltage. Peculiarities of the behavior of the dielectric nonlinearity are found near the Curie temperature, which are explained by the emergence of an intermediate heterophase state.