Interplay between various crystalline and hexatic-B phases in 75OBC liquid crystal: X-ray diffraction and calorimetry study

ABSTRACT

X-ray diffraction and high-resolution adiabatic calorimetry have been used to study the polymorphic behaviour of 75OBC liquid crystal. On heating of 75OBC from the room temperature, the crystal phase (Cr-I) melts directly to smectic A (Sm-A) and then to isotropic phase. On cooling from isotropic phase the 75OBC undergoes successive transitions to Sm-A, hexatic B (Hex-B), crystalline (Cr-II) phases, and next to another crystal phase – Cr-III. The latter two crystalline phases were shown to relax slowly to Cr-I phase, which is stable at low temperatures. X-ray data indicate that the Cr-II phase possesses orthorhombic lattice with herringbone in-plane order of the molecules (Cr-E). The Cr-I and Cr-III phases belong to different low symmetry monoclinic lattices. Our results demonstrate a complex polymorphism of 75OBC compound: the thermodynamic pathway that involves monotropic Hex-B and orthorhombic Cr-E phases corresponds to a local thermodynamic minimum, while the other ones showing melting from the Cr-I to Sm-A phase belongs to the global minimum. The similar thermal behaviour was observed for other members of nmOBC homologous series, possessing a Sm-A – Hex-B – Cr-E phase sequence on cooling. The 75OBC polymorphic behaviour is discussed in terms of molecular packing and stability of Cr-E orthorhombic lattice that precedes the transition to the Hex-B phase.

GRAPHICAL ABSTRACT

KEYWORDS:

• Hexatic and smectic liquid crystals
• crystalline phases
• adiabatic calorimetry
• X-ray diffraction
• structure analysis
• complex polymorphic behaviour

Acknowledgments

We thank Wim de Jeu, C.C. Huang and Ewa Gorecka for providing us hexatic materials and for stimulating discussions. B.I.O. appreciates common work on the structure of hexatic LCs and valuable discussions with Ivan Vartanyants, Ruslan Kurta and Ivan Zaluzhnyy. Special thanks are due to Efim Kats, Vladimir Lebedev and Elena Pikina for fruitful discussions on the thermodynamic stability of the hexatic LCs in bulk and in freely suspended films. We are thankful to Alexey Piryazev for assistance in DSC measurements of 75OBC samples. We acknowledge support from the Russian Science Foundation (Grant No. 18-12-00108, high-resolution calorimetry, X-ray data analysis, structure determination). The X-ray experiments at Kurchatov Synchrotron Centre and FSRC “Crystallography and Photonics” RAS were supported by the Ministry of Science and Higher Education within the corresponding State assignments.

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No potential conflict of interest was reported by the authors.

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Funding

This work was supported by the Ministry of Science and Higher Education of Russian Federation within the corresponding State assignments (X-ray experiments at Kurchatov Synchrotron Centre and FSRC “Crystallography and Photonics” RAS) and by Russian Science Foundation [Grant No. 18-12-00108 (high-resolution calorimetry, X-ray data analysis, structure determination)].