Understanding the Behavior of the Above-Room-Temperature Molecular Ferroelectric 5,6-Dichloro-2-methylbenzimidazole Using Symmetry Adapted Distortion Mode Analysis

The
exploitable properties of many functional materials are intimately
linked with symmetry-changing phase transitions. These include properties
such as ferroelectricity, second harmonic generation, conductivity,
magnetism and many others. We describe a new symmetry-inspired method
for systematic and exhaustive evaluation of the symmetry changes possible
in molecular systems using molecular distortion modes, and how different
models can be automatically tested against diffraction data. The method
produces a quantitative structural landscape from which the most appropriate
structural description of a child phase can be chosen. It can be applied
to any molecular or molecular-fragment containing material where a
(semi) rigid molecule description is appropriate. We exemplify the
method on 5,6-dichloro-2-methylbenzimidazole (DC-MBI), an important
molecular ferroelectric. We show that DC-MBI undergoes an unusual
symmetry-lowering transition on warming from orthorhombic Pca21 (T ≲ 400 K) to
monoclinic Pc. Contrary to expectations, the high
temperature phase of DC-MBI remains polar.