Ferroelectricity and Dielectric Relaxation in NH₄Al(SO₄)₂·12H₂O

Abstract

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 T_c=−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⁄k_B)·(T−T₀)⁻¹·exp (ΔF⁄k_BT) with the activation free energy ΔF=ΔF₀[1−α(T−T_c)]. The values of ΔF₀ and α were estimated to be 1.49×10⁻²⁰ and 0.0028 deg⁻¹, respectively.