Wydział Chemii

Marcin Moskwa

Emilia Ganczar

Paweł Sobieszczyk

Wojciech Medycki

Piotr Zieliński

Ryszard Jakubas

Grażyna Bator

2020

Journal of Physical Chemistry C

124

18209-18218

10.1021/acs.jpcc.0c05322

Naukowa

Angielski

Artykuł

Molecular–ionic crystals, which are built of organic cations and discrete anionic components with AMX₄ stoichiometry, easily induce structural order–disorder transitions. These phenomena can generate interesting ferroic properties. Herein, we have synthesized and characterized a novel AMX₄-type crystal, (C₈H₂₀N)[BF₄] (1). Thermal analysis, (DSC, TGA) indicates one structural phase transition (PT) at 257/259 K (cooling/heating, I ↔ II). The structure of 1 has been solved at two temperatures, 120 and 298 K, with single-crystal X-ray diffraction. The high-temperature phase I (1-HT) shows a tetragonal structure with the space group I4/mmm, whereas the low-temperature phase II (1-LT) is monoclinic with the space group P2₁/a. The PT belongs, thus, to the improper ferroelastic species 4/mmmF2/m with a 4-fold multiplication of the unit cell. All four possible ferroelastic domains of phase II have been identified in polarized light microscopy. The monoclinic deformation has been shown to give rise to a characteristic parquet-like texture preserving integrity of 1. The molecular mechanism of PT involving changes in the cationic and anionic dynamics has been postulated based on measurements of the ¹H and ¹⁹F NMR spin–lattice relaxation times, T₁, and second-moment, M₂, and of the dielectric responses. The ac and dc conductivity measurements have been used to determine the transport properties of the charge carriers in 1. The intrinsic features of molecular–ionic crystals, such as the dynamics of molecular motions, structural instability with a lattice-symmetry change, formation of domain patterns, etc., provide a crystal with unique ferroelastic properties. This type of crystal may be useful as an functional material.

Crystals, Insulators, Anions, Electrical conductivity, Cations

CC-BY

http://dx.doi.org/10.1021/acs.jpcc.0c05322

https://www.acs.org/content/acs/en.html