Dielectric Study on the Phase Transitions in Ferroelectric (CH₃NH₃)₂AlBr₅·6H₂O

Abstract

The dielectric properties have been measured for single crystals of ferroelectric (CH 3 NH 3 ) 2 AlBr 5 ·6H 2 O. The dielectric constant along the a direction ε a shows a sharp λ-type peak at the Curie temperature of 76.6 K below which the crystal shows ferroelectricity along the a direction. The dielectric constant along the a direction ε a obeys a Curie Weiss law with the Curie constant of 3.3×10 2 K in a temperature range about 30 K above the Curie temperature. The spontaneous polarization approaches the saturation value of 6.5×10 -3 C/m 2 at low temperatures. Beside the ferroelectric Curie temperature, there is a first order phase transition at about 250 K where the dielectric constant shows a slight discontinuous change. The results are compared with those of the isomorphic ferroelectric crystal of (CH 3 NH 3 ) 2 AlCl 5 ·6H 2 O, and discussed.