The intrinsic ⅛ lock-in improper ferroelectric phase in thiourea: dielectric susceptibility and hysteresis loop analysis

Abstract

The complex dielectric constant of thiourea has been measured in the temperature range from 80 K to 300 K with different measuring-field levels and frequencies, with and without d.c. biasing fields. The temperature interval between the two peaks in the ε*_a(T) curve near 178 K was found to extrapolate to 0.7 K at zero measuring and biasing fields, which indicates that the ⅛ ferroelectric lock-in phase is intrinsic in a non-zero temperature range. The E-T phase diagram near the ⅛ lock-in region was explored by investigating the effects of biasing fields. D-E hysteresis loops were investigated in the vicinity of the ⅛ ferroelectric phase with measuring frequencies from 0.5 Hz to 5 mHz. The different D-E loops observed clearly demonstrate the difference between the intrinsic and “field-induced” lock-in mechanisms. The Landau free energy was analyzed and shown to produce self-consistent explanations of the experimental results and of the successive phase transitions.

Key Words:

• thiourea
• incommensurate
• hysteresis