Reversible structural phase transition, ferroelectric and switchable dielectric properties of an adduct molecule of hexamethylenetetramine ferrocene carboxylic acid

Abstract

Metallo-organic molecules showing reversible structural phase transitions accompanied with ferroelectric and dielectric properties have seldom been reported. By making use of the “driving/controlling force” from the rotating motion of cyclopentadienyl rings for a reversible structural transition and then carrying out structural modifications on their local parts to create remnant polarization through this molecular system, we designed and constructed a new ferroelectric, which is the adduct of hexamethylenetetramine ferrocene carboxylic acid, (C₆H₁₂N₄)[Fe(C₅H₅)(C₆H₅O₂)] (1). Compound 1 undergoes a reversible structural phase transition at around 187 K, which is confirmed by remarkable dielectric and heat anomalies. The synergistic interactions of the twisting motions of the adduct molecules and the inner cyclopentadienyl rings lead to the reversible phase transition. A dipole moment from amine group to carboxylic acid at low temperature produces a net spontaneous polarization, which results in the ferroelectricity of 1-LTP. On further investigation, it is observed that compound 1 has a perfect ferroelectric hysteresis loop with a spontaneous polarization of 3.95 μC cm⁻².