The landscape of mechanical properties of molecular crystals
An analysis of compiled literature nanoindentation hardness (Hc) and elastic modulus (E) values of molecular crystals revealed a wide range of mechanical properties of molecular crystals (0.001 – 1.80 GPa for Hc and 0.27 – 46.8 GPa for E). A global approximately linear relationship between E and Hc, was observed and possible reasons for deviation from the line were discussed. A classification scheme for molecular crystals based on the E and Hc is proposed. In addition, results suggest that the effectiveness of crystal engineering strategies in modifying both E and Hc follows the order of cocrystal/salt > polymorph > anisotropy. A clear understanding of the E and Hc landscape lays a foundation for effectively optimizing mechanical properties of molecular crystals through crystal engineering.