Aperiodicity and Ferroelectricity in Organic Crystals- Precise (3d+n)-Crystal Structure Studies at P24

by Leila Noohinejad (DESY)




I will present the study of mechanism of ferroelectricity and modulation of co-crystals of phenazine with anilic acids (namely: chloranilic acid and bromanilic acid ) by single crystal X-ray diffraction. [1-4]
Ferroelectric crystals are pyroelectric crystals that exhibit reversible polarisation. Most applications of ferroelectric materials involve inorganic compounds, such as lead zirconate titanate Pb(Zr,Ti)O3 (PZT), barium titanate BaTiO3 (BTO), layered perovskites. Molecular materials for electronic applications come with wonderful properties like low specific weight, easy synthesizing, flexibility, potentially cheap to produce, and low environmental impact. Although the discovery of molecular ferroelctrics in single crystals of Rochelle salt, dates back to 1921 [5], the field of molecular ferroelectrics has started to gain interests only recently. Organic hydrogen bonded supramolecular chains with a polar space group form one class of ferroelectric materials [6]. The H-bond is a bridge between functional groups of organic molecule. H-bonded co-crystals of phenazine with anilic acids belongs to this group of ferroelectrics. Overlapping the p-orbitals between adjacent conjugated organic molecules eases charge transfer. The study of phase transitions is at the core of condensed-matter physics. The material undergoes a change of symmetry at most phase transitions. The ferroelectric phase is defined by the possessing a spontaneous polarization (Ps). Paraelectric to ferroelectric phase transition are results from structural changes in crystal. In a series of ferroelectrics, the transition into a polar phase is preceded by an incommensurate intermediate phase [7]. Today it is understood, in order to understand a material’s properties, one has to understand its intrinsic characteristics at both the macroscopic as well as the microscopic level. Most significant for crystalline materials is an accurate and complete description of its chemical and crystallographic structure. The first and last steps in any ferroelectric studies are connected to crystallography from determination of symmetry of crystal to the interpretation of ferroelectric mechanism that are supported by the precise 3d and/ or (3d+n) crystal structures. This will provide information for interpretation of ferroelectricity in three dimensions.

[1] Noohinejad, L, Tolkehin, M., van Smaalen, S. (2020). Joint Polish-German Crystallographic Meeting (DGK and PCrA), Wrocław / Poland.
[2] Noohinejad, L., S. van Smaalen, V. Petricek, A. Schoenleber (2017) Acta. Cryst. B, 73, 836.
[3] Noohinejad, L. (2016). Aperiodic Molecular Ferroelectrics, University of Bayreuth.
[4] Noohinejad, L., Mondal, S., Woelfel, A., Ali, S. I., Schoenleber, A. and van Smaalen, S. (2014). J. Chem. Crystallogr. 44(8): 387.
[5] Valasek, J. (1920). PhysicalReview 15: 537.
[6] Horiuchi, S. and Tokura, Y. (2008). Nature Mater. 7(5): 357.
[7] van Smaalen, S. (2012). Incommensurate Crystallography, Oxford University Press.