Date Published: February 01, 2019
Publisher: International Union of Crystallography
Author(s): Thomas Kader, Berthold Stöger, Johannes Fröhlich, Paul Kautny.
The phase transition behaviour and twinning of 4-aminopyridine-based indolocarbazoles are analyzed using the order–disorder theory and group–subgroup relationships.
Symmetry relationships are crucial in understanding and describing phase transitions (Müller, 2013 ▸). In most cases of displacive phase transition (Tolédanoc et al., 2006 ▸), the symmetry of a high-temperature (HT) phase is a strict super group of the symmetry of the low-temperature (LT) phase (disregarding minor variations of cell parameters). Nevertheless, exceptions exist. For example, numerous incommensurate phases feature a lock-in phase transition to a periodic (and therefore higher-symmetry) LT structure on cooling (Cummins, 1990 ▸). In such a case, both phases are derived from a higher-symmetry prototype structure, which may exist at high temperatures or may be purely hypothetical.
From a crystallo-chemical point of view, the polymorphs of 5NICz, 2NICz and 2,5NICz are all closely related. Their structures are determined by non-classical C—H⋯N hydrogen bonding. The molecular orientations in the thus-formed rods differ owing to either N-substitution at different rings or with respect to the rotation of adjacent molecules.