Super-coercive electric field hysteresis in ferroelectric plastic crystal tetramethylammonium bromotrichloroferrate(iii)

Abstract

Ionic plastic crystals are part of an emerging class of hybrid organic–inorganic ferroelectrics. The combination of low dielectric constants (<20), strong piezoelectric coefficients (>100 pC N⁻¹) and low temperature synthesis make ferroelectric plastic crystals strong candidates for sensing and energy harvesting applications so more research of the electrical properties, such as dielectric loss and super-coercive hysteresis is needed. Tetramethylammonium bromotrichloroferrate(III) was studied as a prototypical material and a non-centrosymmetric orthorhombic Amm2 structure was confirmed by X-ray diffraction at room temperature. The relative permittivity was below 20 at high frequencies (10⁵ Hz) but high dielectric losses at frequencies <10³ Hz caused the permittivity to increase. The bulk material showed super-coercive hysteresis with remanent polarizations of 3.8 μC cm⁻² and peak-to-peak strains of 0.046%. The coercive field and peak-to-peak strain exhibited frequency dependence, reducing from >80 to 30 kV cm⁻¹ and 0.046 to 0.017% respectively between 100 and 0.1 Hz. At 100 Hz the material exhibited a strong asymmetric hysteresis that was reversible depending on the electric field direction. The observed behavior was consistent with a defect pinning model of ferroelectric switching behavior and thus some likely point defect species were proposed and thought to result from solution non-stoichiometry or residual moisture and solvent.