Ferroelectricity and Dielectric Relaxation in NH4Al(SO4)2·12H2O
1988 年 57 巻 11 号 p. 4004-4010
詳細
1988 年 57 巻 11 号 p. 4004-4010
The pyroelectric charge of NH4Al(SO4)2·12H2O was measured after poling the crystal. It was found that the crystal becomes ferroelectric below the Curie temperature of Tc=−197°C on warming and has spontaneous polarization of 0.10 μC/cm2 at −240°C. The complex dielectric constant was measured at various frequencies in a frequency range between 330 Hz and 1×106 Hz. The crystal exhibited dielectric relaxation and showed a critical slowing down process of polarization. The observed temperature and frequency dependences of the complex dielectric constant can be well approximated in terms of single relaxation time τ(T) of the Debye-type. Above the Curie temperature, τ(T) was well described by the relation τ(T)=(h⁄kB)·(T−T0)−1·exp (ΔF⁄kBT) with the activation free energy ΔF=ΔF0[1−α(T−Tc)]. The values of ΔF0 and α were estimated to be 1.49×10−20 and 0.0028 deg−1, respectively.
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