Brittle or plastic? Revisiting the mechanical behavior of benzoic acid crystals

Abstract

Understanding the mechanical behaviour of organic molecular crystals is essential for their reliable handling, processing, and performance in solid-state applications. Benzoic acid crystals have been reported to be brittle in previous studies. However, in this work, the benzoic acid crystals grown using slow evaporation and sublimation processes exhibited plastic bending upon the application of force on a wider (001) crystallographic face. Single-crystal X-ray diffraction analysis and pairwise interaction energy calculations revealed the presence of a slip plane along the (001) direction, which was formed mainly due to the herringbone packing arrangement of the benzoic acid molecules and a weak edge-to-edge aromatic interaction. A low-energy (001) slip plane with an interlayer separation of approximately 0.896 Å facilitates the plastic deformation of benzoic acid crystals. Nanoindentation analysis of the (001) face yielded elastic modulus (E) and hardness (H) values of 4.26 ± 0.68 GPa and 128.03 ± 38 MPa, respectively, which fall within the range of values reported for plastic organic crystals.