Dielectric relaxation of 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 0.10μ C/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 observed temperature and frequency dependences of the complex dielectric constant can be approximated in terms of single relaxation time τ(T) of the Debye-type. Above the Curie temperature, τ(T) is described well by the relation τ(T) =(h/k_B)·(T-T_o)^-1·exp(ΔF/k_bT) with the activation free energy ΔF =ΔF_o[1 -α (T - T_c)]. The values of ΔF_o and a are estimated to be 1.49 × 10^-20 J and 0.0028 deg^-1, respectively.

Key Words:

• ferroelectricity