Critical slowing down of dielectric relaxation in ferroelectric NH4HSO4

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Abstract
Dielectric constant and dielectric loss of NH4HSO4 along the ferroelectric c axis have been measured at 100 kHz, 300, 660, 1145, and 1440 MHz as a function of temperature. It was found that the Curie–Weiss law held both in paraelectric and in ferroelectric phases of the upper Curie point which was T0=−2.3 °C. Dielectric relaxation was observed both in paraelectric and in ferroelectric regions. The former relaxation was of the Debye type with a single relaxation time, which was expressed by a relation: τ=1.70×10−10/(T−T0) sec in the temperature region down to T−T0?0.5 °C. It was found that after the same specimen was repeatedly cooled and heated through the upper Curie point, this relaxation gradually changed into that with distributed relaxation times. The dielectric relaxation in the ferroelectric region was complex; that is, the characteristics of the relaxation differed with every specimen.