Crystal Structure, Raman Spectroscopy and Dielectric Properties of New Semiorganic Crystals Based on 2-Methylbenzimidazole

E. V. Balashova,A. N. Smirnov,V. Yu. Davydov,G. A. Pankova,A. A. Levin,P. N. Brunkov,A. V. Redkov,A. A. Zolotarev,B. B. Krichevtsov,F. B. Svinarev

doi:10.3390/cryst9110573

Abstract

New single crystals, based on 2-methylbenzimidazole (MBI), of MBI-phosphite (C16H24N4O7P2), MBI-phosphate-1 (C16H24N4O9P2), and MBI-phosphate-2 (C8H16N2O9P2) were obtained by slow evaporation method from a mixture of alcohol solution of MBI crystals and water solution of phosphorous or phosphoric acids. Crystal structures and chemical compositions were determined by single crystal X-ray diffraction (XRD) analysis and confirmed by XRD of powders and elemental analysis. Raman spectroscopy of new crystals evidences the presence in crystals of MBI-, H3PO3-, or H3PO4- and water molecules. Dielectric properties of crystals reveal strong increase and low frequency dispersion of dielectric constant and losses at heating, indicating the appearance of proton conductivity. At low temperatures in MBI-phosphate-2, an increase of dielectric constant analogous to quantum paraelectric state is observed.

Highlights

The mechanisms responsible for the appearance of spontaneous polarization, piezoelectricity, and phase transformations in organic and semiorganic materials are of fundamental interest. This is caused by specific features of organic molecules possessing an effect of tautomerism [8], and specific crystal structure, which often contains the chains of hydrogen bonds giving rise to polarization due to freezing of protons, or permits manifestation of charge transfer-induced polarization [9]
The last time, a number of organic and semiorganic ferroelectrics were discovered, which revealed a high value of spontaneous polarization PS up to ~ 20–30 μC/cm2 at room temperature, comparable with that in PbZrx Ti1−x O3 (PZT), Pb1-y Lay Zr1−x Tix O3 (PLZT)
Coordinates of atoms and their equivalent isotropic/isotropic temperature factors Ueq /Uiso, atomic displacemence parameters, bond lengths, bond angles, and geometry of hydrogen bonds according to the results of the single crystal refinements are presented in Tables S1–S15 of the SM

Summary

Introduction

The search for and study of new organic and semiorganic materials possessing ferroelectric, ferroelastic, antiferroelectric, or piezoelectric properties is at present a subject of intensive research [1,2,3]. The mechanisms responsible for the appearance of spontaneous polarization, piezoelectricity, and phase transformations in organic and semiorganic materials are of fundamental interest This is caused by specific features of organic molecules possessing an effect of tautomerism [8], and specific crystal structure, which often contains the chains of hydrogen bonds giving rise to polarization due to freezing of protons, or permits manifestation of charge transfer-induced polarization [9]. Crystals 2019, 9, 573 noting that organic and semiorganic crystals reveal a wide spectrum of phases including ferroelectric, antiferroelectric, ferroelastic phase [1,10,11,12,13], and quantum paraelectric states [14,15,16].

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