Pressure-Induced Ferroelastic Transition Drives a Large Shape Change in a Ni(II) Complex Single Crystal

Crystals with significant length reduction at an accessible low pressure are highly desirable for piezo-responsive devices. Here, we show a molecular crystal [Ni(en)3](ox) (en = ethylenediamine and ox = oxalate anion) that exhibits an abrupt shape change with a contraction rate of ∼4.7% along its c axis near the phase transition pressure of ∼0.2 GPa. High-pressure single-crystal X-ray diffraction and Raman spectroscopy measurements reveal that this material undergoes a first-order ferroelastic transition from high-symmetry trigonal P3̅1c to low-symmetry monoclinic P21/n at ∼0.2 GPa. The oxalate anions serve as unique components, and their disorder–order transformation and rotation of 90° through cooperative intermolecular hydrogen bonding triggered unconventional anisotropic microsize contraction under compression, which can be appreciated visually. Such a prominent directional deformation at a low pressure driven by molecular motors of oxalate anions provides insights for the design of novel molecular crystal-based piezo-responsive switches and actuators in deep-sea environments.