Significance of considering pellet molding pressure and dielectric experimental conditions in the [dielectric constant—temperature] response of switchable dielectric molecular materials: illustration on dabcoHBF4 polycrystals

Abstract

In this study, we investigate the temperature-dependent dielectric response of dabcoHBF4 molecular switchable dielectric materials. This compound made of polycrystals has been shaped into pellets under different controlled applied pressures [0.2-1 GPa]. The dielectric analysis carried out on these samples showed that the mechanical stresses induced by this shaping pressure had a significant effect on the value of the dielectric constant in both the ferroelectric and the paraelectric phase. However, the impact of this pressure remains secondary in the AC electrical conductivity, determined for the first time in these compounds. Thus, conductivity values around 2x10 -7 S/cm in the ferroelectric phase and 2x10 -6 S/cm in the paraelectric phase were found. The behavior of these compounds over wide frequency ranges [1-1 MHz] and temperature ramps [0.5-10 K/min] revealed that the frequency had no effect on the transition temperature at the difference of temperature rate as expected in most ferroelectric materials. When decreasing the frequency, the dielectric constant gains in value and can reach values higher than 50 in the ferroelectric phase and 1000 in the paraelectric phase with a frequency solicitation at 10 Hz.At lower frequencies, the Curie-Weiss law is lost in the paraelectric regime and an enhanced rise of the dielectric constant with increasing temperature probably reflects an interfacial polarization mechanism. Finally, this dielectric analysis in frequency, temperature (323-403 K) and along temperature ramps allowed to extract analytical laws of behavior associating some of these parameters. For example, one of these equations will be useful in the choice of the temperature ramp to be applied to optimize the switching time between a ferroelectric and a paraelectric state according to a condition on the value of the desired dielectric constant at the low and high states in the implementation of memories.