Dielectric and thermal properties of the ferroelectric NH4HSO4 close to phase transitions

This work gives the analysis of the dielectric and thermal properties close to the second order and first order types of ferroelectric-paraelectric phase transitions in ferroelectrics, in particular, NH₄HSO₄. The power-law formula is used by adapting the Kouvel-Fisher (KF) method describing the magnetization ($M$) and magnetic susceptibility (${\chi }_M$) in the case of the spontaneous polarization ($P_S$) and the dielectric constant ($ϵ$), respectively, in ferroelectric systems. Similar treatment is performed to describe the heat capacity ($C_P$) and the thermal expansivity (${\propto }_P$) close to the phase transitions in NH₄HSO₄.

We show that the variations of $P_S$ and $ϵ$ with the temperature near the upper Curie point (T_C1 $\cong$ 270 K) exhibit linearity for the second order transition in NH₄HSO₄. A linear variation is also obtained between the $C_P$ and ${\propto }_P$ with the temperature close to the lower Curie point (T_C2 $\cong$ 160 K) for the first order transition in this crystal. Experimental data are used from the literature for our analysis. Our approach given here to describe dielectric and thermal properties of NH₄HSO₄ close to the Curie points, can be applied to some other ferroelectric materials.