Ferroelectric MartensiticActuation in Single Crystalthrough Molecular Geometry Engineering

Martensitic transformation with remarkable mechanical responses is fascinating for its potential applications as high-precision microscale actuators, while ferroelectrics with electromechanical responses hold promise for transducers, nonvolatile memory, and energy harvesters. However, integrating ferroelectricity with martensitic actuation in a single-phase molecular material remains a significant challenge, owing to the distinct mechanisms of orientational order-disorder transitions and large ionic displacements. Here, we propose and implement a molecular geometry strategy for the rational design of diffusionless martensitic transformation behaviors. By modifying the geometric shape of organic cations, we present an organic-inorganic hybrid ferroelectric crystal, (DMAiB)PbI3 (DMAiB = dimethylallylisobutylaminium), showing intriguingly reversible actuating behaviors with fast response and giant anisotropic shape change. The dynamic disorder of organic cations exerts internal pressure on the stacked structure to bring anomalously significant displacements of adjacent [PbI3]n- chains, manifested as a giant macroscopic thermal expansion of up to 14.84%. This structural arrangement also endows (DMAiB)PbI3 with crystal polarity, exhibiting robust ferroelectricity with the highest spontaneous polarization of 9.8 μC/cm2 among the reported one-dimensional hybrid lead-based ferroelectrics to date. This work provides a feasible molecular design strategy for developing martensitic actuating materials and offers new insights into multifunctional martensitic ferroelectrics with great potential in versatile microelectro-mechanical systems.