Crystallography and the liquid crystal phase. A new approach to structural studies on a thermotropic smectic Schiff base

PIRO, OSCAR E.; ECHEVERRIA, GUSTAVO A; CUKIERNIK, FABIO D.

Crystallography Reviews

Taylor & Francis

Año: 2018 vol. 24 p. 3 - 21

0889-311X

In spite of the apparent contradiction between ?liquid crystals? (LC, materials exhibiting some degree of disorder) and ?crystallography? (a paradigmatic ordered kingdom), X-ray diffraction (XRD) studies make a substantial contribution to the field of LC. Focussing this review on smectic (Sm, lamellar) LC, we first describe the way some very deep and extremely careful XRD studies performed on mono-domain samples in the LC phase helped to elucidate the intimate nature of ordered Sm phases. Then we present selected examples in which single-crystal XRD on the solid state phase of the mesogens provided information about their supra-molecular organization in the Sm phase. Finally, we present a different approach to this problem in the case of a thermotropic Schiff base (SB) which undergoes crystalliquid crystal isotropic liquid phase transformations. By combined single-crystal and variable-temperature powder XRD we show that the SB liquid crystal is a hexatic smectic B phase that derives from the crystal phase by relatively small topological changes promoted by the set in of thermal rotational disorder around the long SB molecular axis.