Giant and Reversible Barocaloric Effect in Trinuclear Spin‐Crossover Complex Fe3(bntrz)6(tcnset)6

Abstract

A giant barocaloric effect (BCE) in a molecular material Fe₃(bntrz)₆(tcnset)₆ (FBT) is reported, where bntrz = 4‐(benzyl)‐1,2,4‐triazole and tcnset = 1,1,3,3‐tetracyano‐2‐thioethylepropenide. The crystal structure of FBT contains a trinuclear transition metal complex that undergoes an abrupt spin‐state switching between the state in which all three Fe^II centers are in the high‐spin (S = 2) electronic configuration and the state in which all of them are in the low‐spin (S = 0) configuration. Despite the strongly cooperative nature of the spin transition, it proceeds with a negligible hysteresis and a large volumetric change, suggesting that FBT should be a good candidate for producing a large BCE. Powder X‐ray diffraction and calorimetry reveal that the material is highly susceptible to applied pressure, as the transition temperature spans the range from 318 at ambient pressure to 383 K at 2.6 kbar. Despite the large shift in the spin‐transition temperature, its nonhysteretic character is maintained under applied pressure. Such behavior leads to a remarkably large and reversible BCE, characterized by an isothermal entropy change of 120 J kg⁻¹ K⁻¹ and an adiabatic temperature change of 35 K, which are among the highest reversible values reported for any caloric material thus far.