Flexible Aliphatic Carboxylates for Colossal Thermal Expansion Engineering: From Local and Extended Structure Analysis to Thermomechanical Devices

Materials with colossal thermal expansion coefficients (CTEs) hold a significant application potential, and the development of new synthetic approaches toward those is a crucial goal in modern chemistry and solid-state science. In particular, the introduction of molecular rotors into the crystal structure is a prospective and versatile strategy to design colossal thermal expansion materials, yet being represented by only a few scattered studies with no direct practical application reported. In the present work, we unveil a huge potential of rotationally flexible aliphatic carboxylates as a synthetic platform for stimuli-responsive materials on the case-study for a 2D-layered praseodymium propionate [Pr2(H2O)2Prop6]∞ (Pr; Prop– = C2H5COO–). As revealed from variable-temperature PXRD data, Pr demonstrates a highly anisotropic thermal expansion with peak positive (+1295­(22) MK–1) and negative (−837­(32) MK–1) linear CTEs near 165 K. Atomic mechanisms of anomalous thermal expansion have been studied by a comprehensive set of experimental (variable-temperature SCXRD, PDF, and calorimetry) and computational (DFT-D) methods. Namely, the disorders of flexible ethyl groups coupled with nonequivalent intra- and interlayer interactions are the driving forces of the colossal thermal expansion. Crystalline samples of Pr have been applied for low-temperature device demonstration on the example of an optothermal actuator and a capacitance temperature sensor. The key idea of using aliphatic carboxylates as molecular rotors is expandable to analogous metal carboxylate-based systems that makes them a wide playground for the tailored design of stimuli-responsive materials.