Critical slowing down of low-frequency dielectric relaxation in ferroelectric (C3N2H5)5Bi2Cl11

Dielectric dispersion studies in the frequency range between 220 Hz and 1 MHz have been performed on pentakis(imidazolium) undecachlorodibismuthate(III), (C 3 N 2 H 5 ) 5 Bi 2 Cl 11 , single crystals. The measurements have been made over the temperature range between 300 and 140 K. It is found that the dielectric dispersion, observed along the b -axis of the monoclinic symmetry ( P 2 1 / c ), is monodispersive between 166.5 and 168 K, and polydispersive in the close vicinity of T c = 166 K, for T < T c + 0.2 K. The observed dielectric dispersion is analysed using a single relaxation mode of the Debye type. Over the paraelectric phase the process of the dielectric critical slowing down is observed. This is characteristic of the order–disorder ferroelectric phase transition.