Four Directional Twinning Deformation of an Anisotropic Molecular Single Crystal Based on Three Different Modes of Mechanical Twinning

Introduction of mechanical twinnability is one of the most useful strategies to enhance deformability of crystalline materials by giving them an ability of reversible deformation over the elastic limit in maintaining the crystal integrity. The enhancement is especially remarkable in highly symmetric crystals with multiple crystallographically equivalent faces and directions, which result in multidirectional mechanical twinnability. Mechanical twinning in organic single crystals, on the other hand, is generally limited in a specific direction due to strongly anisotropic molecular and crystal structures. Here we demonstrate four directional mechanical twinning in an organic single crystal of 1,3-bis(4-methoxyphenyl)urea. The multidirectionality in an anisotropic single crystal is based on coexistence of three different modes of mechanical twinning attributable to the molecular structure: intrinsic 2-fold and mirror symmetry elements and linearity, and its crystal structure: inversion and pseudo-2-fold symmetry elements. Our findings will contribute to enhancing deformability of anisotropic crystalline materials.