Halogen Bonded Network Modulating the Mechanical Property Elastic and Plastic Bending in Nonconventional Molecular Solid Solutions

Anisotropic mechanical response of a material to an applied external stress results in bending of organic crystals in particular directions. This phenomenon is essentially dictated by intermolecular interactions. In this work, we have tactically designed solid solutions of two non-isostructural molecular crystalline phases, 4-bromo-3-chlorophenol (4BR, I41/a) and 3-bromo-4-chlorophenol (3BR, P21/c)─an exception to the Kitaigorodsky rule. Single crystals of 4BR show elastic bending, whereas 3BR crystals are brittle in nature. The solid solutions of 4BR and 3BR in 1:1 and 1:2 stoichiometric ratios attain a unique solid solution crystal structure (P21/c, Z′ = 2). In response to mechanical stimuli, we observed elastic bending in 1:1 solid solution crystals and plastic bending in 1:2 solid soltion crystals. A detailed investigation combining laboratory single crystal X-ray diffraction, powder X-ray diffraction, thermal analysis, and periodic DFT calculations reveals intriguing structural features of these solid solutions and explores the role of halogen bonding in modulating the mechanical property from elastic (1:1) to plastic (1:2) with stoichiometric variation.

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