Kan-ichi Kamiyoshi

Critical slowing down of dielectric relaxation in ferroelectric NH4HSO4

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.

Brillouin Scattering in Ammonium Sulfate

Six different sound velocities of ammonium sulfate in GHz region have been measured from room temperature to –70°C by Brillouin scattering method utilizing a differential type of Fabry-Perot interferometer. Sound velocities, v 1 , v 4 and v 5 show a softening in the disordered phase near the transition temperature, T c =-49.5^° C . All the sound velocities show an abrupt fall at T c . The change amounts to 70% of the velocity at room temperature in the case of v 1 . A new anomaly is observed at T c - 8^° C on v 1 , and no remarkable effect of depolarization field on v 3 is observed in the ordered phase. It is attempted to interpret the phase transition of ammonium sulfate by the model of an improper ferrielectric type with a non-polar or weak-polar soft mode.