Abstract: Last minute contribution – please contact the speaker for further details

Organic ferroelectrics hold significant promise for diverse practical applications, including sensors and Fe-RAM in memory devices. Their distinctive features, such as lightweight, flexibility, and non-toxicity, position them as viable alternatives to lead-containing ferroelectric materials. The attention garnered by above-room-temperature ferroelectricity in croconic acid underscores the interest in exploring novel organic ferroelectric materials.
We have synthesized hexamethylene-based co-crystals, HMX, and studied its crystal structure by single crystal XRD confirming a polar space group for the same. We further investigated the structural properties by powder X-ray diffraction (PXRD), Raman spectroscopy, Differential scanning Calorimetry (DSC), etc. Ferroelectric and dielectric measurements were carried out and a detailed structure-property correlation was performed. The compound exhibits a ferroelectric PE hysteresis loop at room temperature. The dielectric spectroscopy of the HMX system shows a dielectric anomaly around 315 K (T1) possibly suggesting its ferroelectric to paraelectric transition. DSC shows an exothermic change around T1. From the Rietveld refinement analysis of the temperature-dependent PXRD data, an anomaly in the lattice parameters is observed around the same temperature where the dielectric anomaly was observed. Temperature-dependent Raman phonon frequencies (FWHM) also show such anomalous behavior at T1, confirming the structural distortion occurring around T1 = 315 K in HMX. In this work, we demonstrate how different structural characterization techniques, probing different physical properties, can be combined to understand the structure-property correlations in organic ferroelectric materials. We believe that the identification of novel room temperature organic ferroelectric materials, coupled with a comprehensive understanding of their structure-property correlation, has the potential to greatly progress the quest for non-toxic, lead-free functional materials.

DabcoH+A- is a lead free hybrid organic-inorganic ferroelectric family of material simply synthetized in aqueous solution from commercial and low cost molecules, namely tetragonal acids (such as HBF₄, HClO₄ or HReO₄) and DABCO (1,4 diazabicyclo[2.2.2]octane). DabcoHReO₄ in particular shows unusual multiaxial polarization and one of the highest remnant polarization among organic-based materials of nearly 17 µC/cm². A number of articles show its worth for piezo/pyro/ferroelectricity applications for flexible electronics and sensors, blended with various polymers, but very few investigate its dielectric and thermodynamic properties in depth. Our study consists in carrying out joint dielectric/thermodynamic analysis on dabcoHReO₄ ferroelectrics using an original home-made instrument combining in operando dielectric spectroscopy and differential scanning calorimetry. This Calorimetry-Dielectric-Spectroscopy (CDS) can explore in same time low-frequency measurements (1 mHz – 1 MHz) and thermodynamics over a wide temperature range (170-390K). It provides good resolution even with slow temperature ramps such as 0.08K/min. With it, we investigate the phase transition already studied at 371K but show that its mechanisms are more complex that previously reported, as substructures correlated between calorimetry and spectroscopy are revealed. We associate this behavior with the anisotropic multiaxial nature of the material and open up a new way of tuning and studying dabco-based materials.
Secondly, in a cryogenic study, we highlight a new hysteresis effect triggered by a low temperature ferroelectric-ferroelectric phase transition (around 190K) that raises both the dielectric constant and losses over a wide temperature range. We associate this non-equilibrium effect with the increase in domain wall density, poorly documented in the hybrid organic materials.

Piezoelectric MEMS (Micro-Electro-Mechanical Systems) exploit piezoelectric properties to act as sensors and actuators, and to collect energy from vibrations, movements, or shocks. Recent publications have shown that some hybrid perovskites, known mainly for their applications in photovoltaics, exhibit excellent piezoelectric properties, which can even exceed those of PZT and BTO, the materials most used today. Due to their ceramic nature, these materials are fragile and the integration of piezoelectric nanoparticles (NPs) into flexible polymer matrices has been the subject of intense studies. In this project, we are studying tetramethylammonium tetrachloroferrate ((TMA)[FeCl₄]), which has recently been identified as a very good piezoelectric with a coefficient of 80pC/N. We were able to confirm the dielectric and pyroelectric properties of the compound in the form of polycrystalline films. Furthermore, an X-ray diffraction study showed strong hysteresis in some of the phase transitions, as well as the appearance of a new phase at low temperatures. We prepared PVA (polyvinyl alcohol) composites, with (TMA)[FeCl₄] at different concentrations, and observed a correlation of the dielectric constant with perovskite concentration. Preliminary syntheses of NPs will be presented, including mechanical methods of cryo-milling or sonication, as well as micellar water/mineral oil syntheses. The particles and NPs obtained via these methods will then be integrated into a polymer matrix and shaped via drop casting or doctor blade coating.