Plastic behavior of 3-Bromo-5-chlorobenzoic acid: structural, thermal, and plastic deformation analysis

Plastic crystals are a unique class of dynamic crystalline materials that combine long-range order with partial molecular mobility, resulting in remarkable mechanical properties, such as plasticity and flexibility. These crystals have emerged as promising candidates for advanced flexible devices and pharmaceutical applications. In this study, we investigate the plastic behavior of 3-bromo-5-chlorobenzoic acid (3B5CBA), a globular halogenated aromatic compound. Crystals of 3B5CBA grown via a fast evaporation technique exhibited plastic bending when stressed along the (001) plane. Thermal analysis of crystals revealed a sublimation onset at approximately 70 °C and a melting point of 195.3 °C. Single-crystal X-ray diffraction (SCXRD) analysis and energy framework calculation revealed strong 2D sheet-like interactions parallel to the (001) plane (−299.9 kJ/mol) and weaker interlayer C–Br···O interactions (−79.6 kJ/mol) forming low-energy slip planes. Nanoindentation on the (001) face confirmed homogeneous plastic deformation with a moderate hardness of 217.48 ± 19.87 MPa and a Young’s modulus of 2.94  ± 0.35 GPa. Furthermore, shape analysis using Hirshfeld surface parameters showed high globularity (G = 0.808) and low asphericity (Ω = 0.108). Together, these results showed that 3B5CBA being an organic aromatic compound with a nearly globular shape, hydrogen and halogen synthon-forming functional groups on its periphery can show long-range disordered crystal structure, sublimation ability, and irreversible plastic deformation, offering new insights into the design of aromatic plastic molecular solids.