Abstract
A comparative analysis of crystal structure, Raman spectra, and dielectric hysteresis loops was carried out for organic ferroelectric crystals of 2-methylbenzimidazole (MBI) grown from ethanol (MBIet), acetone (MBIac), deuterated acetone (MBId-ac), or prepared by sublimation from gas phase (MBIgas). Raman spectroscopy shows identical frequencies of molecular vibrations in all studied crystals, proving the same molecular structure. At the same time, a detailed analysis of the asymmetry of the powder XRD reflection profiles indicates the presence of nano-scaled regions with the same MBI symmetry and crystal structure but slightly different sizes and unit cell parameters. The formation of the MBI modifications is associated with possible penetration of solvent molecules into the voids of the MBI crystal structure. Dielectric hysteresis loops in MBIet and MBId-ac crystals at room temperature demonstrate significantly different values of coercive fields Ec. Analysis of hysteresis loops within the framework of the Kolmogorov-Avrami-Ishibashi (KAI) model shows that the polarization switching in MBId-ac occurs much faster than in MBIet crystals, which in the KAI model is associated with different values of the characteristic frequency ω0 and the activation field Ea of the domains wall motion.
Highlights
At present, an active search and study of new organic and semi-organic ferroelectrics and related materials is underway
This band is absent in the MBI compounds with inorganic acids [19], in which the MBI molecules are in a protonated form with a single valence carbon–nitrogen bond in the imidazole ring
The presented study showed that MBI crystals grown from different solvents, having almost identical chemical composition and type of crystal structure, can, at the same time, have different microstructure and dielectric properties
Summary
An active search and study of new organic and semi-organic ferroelectrics and related materials is underway. Scientific interest in organic and semi-organic ferroelectrics is mainly associated with a variety of microscopic mechanisms leading to the appearance of ferroelectricity in them. These include, in particular, the phenomena of tautomerism, spontaneous ordering of hydrogen ions in chains formed by hydrogen bonds, cooperative displacement of atomic groups, and charge transfer between molecules in different crystallographic planes. Notable progress in the field of organic ferroelectrics was associated with the discovery of ferroelectricity in polymers polyvinylidene fluoride PVDF and P(VDF-TrFE) [3], organic molecular crystals of croconic acid (CA) [4], which have a high value of spontaneous polarization Ps = 30 μC/cm at room temperature, as well as a number of other high temperature small-molecular hydrogen bonded ferroelectric crystals [2] including 2-methylbenzimidazole (MBI) [5]
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