The landscape of mechanical properties of molecular crystals

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