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Organic crystals are known to be brittle materials, which may limit their application in different fields. Cause and effect relationships are usually reported for structures and properties, while molecular composition is usually taken out of scope. Isostructural crystals of both hexachlorobenzene and hexabromobenzene exhibit plasticity on the (001) crystal face, remaining brittle on the (101̅) face. Hexabromobenzene is less prone to plastic deformation in comparison to chlorine substituted benzene. In this work, we examine differences in molecular behavior from crystallographic, topological, and computational points of view, highlighting the influence of the halogen atom on intermolecular interactions, which determine different plasticities in these two crystals. The obtained results show that halogen–halogen interactions play a crucial role in mechanical properties, preserving the hexabromobenzene structure with a more stiff structure because of stronger interlayer bonds, which should be distorted because of the molecular layer slip. The concept of the modeling slip plane process is introduced to supplement traditional computational techniques. The influence of both absolute and relative intermolecular energies is highlighted within each structure because both systems have not only a bending face but a brittle one, too.
Tretyakova et al. (Mon,) studied this question.