Extensive Polymorphism in the Molecular Ferroelectric 18-Crown‑6 Oxonium Tetrachloro-Gallium(III)

The materials property
of ferroelectricity is intimately
linked
with symmetry-changing phase transitions. Characterizing such transitions
is therefore essential for understanding molecular ferroelectrics.
In this paper, we explore the temperature and thermal history dependence
of polymorphic phase transitions in the multiaxial molecular ferroelectric
18-crown-6 oxonium tetrachloro-gallium­(III). We have solved the structures
of two previously suggested polymorphs (D and Y) ab initio from high-temperature powder diffraction data. We also report the
structure of a new polymorph (X) using low-temperature powder diffraction
data and identify a fifth (W) that can form on cooling. These polymorphs
can be related using two distinct group–subgroup trees. Structure
types A–C observed in this and related compounds can be derived
from high-temperature polymorph D by group–subgroup relationships.
The X and Y polymorphs can be described as child structures of a hypothetical
polymorph Z using a molecular rotational distortion mode description.
The ferroelectric properties of the various polymorphs can be rationalized
based on our structural findings.