Halogen and hydrogen bonded 2-X-pyridin-3-ol (X = Cl, Br, I) organic crystals with large shear piezoelectricity

The confluence of crystal engineering with piezoelectric material discovery is allowing researchers to design sustainable molecular piezoelectrics at the nanoscale. Here, we use this bottom-up design approach to crystallize a series of analogous organic molecules, 2-X-pyridin-3-ol (1X; X = Cl, Br, I), with structures sustained by both halogen bonds and hydrogen bonds. Density functional theory calculations predict, quantify, and rationalize the piezoelectric response of the analogous series 1X. Our calculations reveal high shear piezoelectricity in all three crystals, with the highest predicted response of d₁₅ = 99.19 pC/N for 1Cl. Piezoresponse force microscopy experiments confirm effective shear piezoelectric constants of 54–74 pC/N. The space groups allow for unpoled longitudinal piezoelectric responses, with experimental d₃₃ values of 5–10 pC/N. This highlights the ability of halogen substitution to induce and modulate piezoelectricity and adds to the growing number of molecular crystals approaching triple-digit piezoelectric responses to rival conventional perovskite ceramics.