Reversible Colossal Barocaloric Effect of a New Fe II Molecular Complex with Low Hysteretic Spin Crossover Behavior

[EN] Barocaloric cooling, that is, lowering the temperature of a material under pressure action, is an attractive solid-state effect that can potentially compete with volatile gas-based cooling. To observe the barocaloric effect (BCE), it is necessary for materials to have high-entropy, low-hysteretic phase transitions with a large volume change between phases. Here details on a new Fe-II complex [Fe(L)(NCS)(2)], L = N-1,N-3-bis((1-propyl-1H-1,2,3-triazol-4-yl)methylene)-2,2-dimethylpropane-1,3-diamine) possessing spin crossover (SCO) behavior near room temperature with large entropy and volume change are reported, which provides high sensitivity to external pressure. The observed BCE effect, characterized using variable pressure calorimetry, powder X-ray diffraction, UV-vis, IR, and Raman spectroscopy, shows a colossal isothermal entropy change of >100 J kg(-1) K-1 and a reversible adiabatic temperature change of approximate to 16 K at a pressure of 1 kbar, demonstrating a high refrigerant efficiency compared to other solid-state materials. These results stimulate further investigations of SCO materials as barocaloric refrigerants, which depend on the proper design of their constituent organic ligands. M.S. and R.L. contributed equally to this work. M.S. and K.Z. acknowledge the support by the Ministry of Education and Science of Ukraine (Grant nos. 22BF037-03, 22BF037-04, and 24BF037-03). F.J.V.M. acknowledges the support of the European Social Fund (ESF) and Generalitat Valenciana for his postdoctoral fellowship (APOSTD/2021/359). M.C.M. and J.A.R. thank the support given by the Spanish Ministerio de Ciencia e Innovacion (grant PID2019-106147GB-I00 funded by MCIN/AEI/10.13039/501100011033). B.L. and Z.Z. acknowledge the support provided by the Ministry of Science and Technology of China grants (2022YFE0109900) and (2023JH6/100500003) and the Key Research Program of Frontier Sciences of Chinese Academy (Grant no. ZDBS-LY-JSC002). The authors are grateful to Dr. S. Malinkin for help with organic synthesis. Seredyuk, M.; Li, R.; Znovjyak, K.; Zhang, Z.; Valverde-Muñoz, F.; Valverde-Muñoz, FJ.; Li, B... (2024). Reversible Colossal Barocaloric Effect of a New Fe II Molecular Complex with Low Hysteretic Spin Crossover Behavior. Advanced Functional Materials. 34(30). https://doi.org/10.1002/adfm.202315487