Low-Pressure-Driven Colossal Barocaloric Effect in an Organometallic Sandwich Compound

The past decade has witnessed remarkable progress in barocaloric refrigeration, but it remains challenging to find a suitable working material with sufficiently large entropy changes under the lowest possible driving pressures. In this study, we report a colossal barocaloric effect in an organometallic sandwich compound, formylferrocene [Fe(C₅H₅)(C₅H₄CHO)]. It undergoes a first-order phase transition from orthorhombic to face-centered cubic structure around room temperature, characteristic of a large, pressure-induced entropy change (191 J·kg^–1·K^–1), an ultralow saturation pressure (20 MPa), and a high pressure sensitivity of its transition temperature (0.27 K·MPa^–1). Notably, the pressure-normalized entropy change reaches a record-high value of 9.55 J·kg^–1·K^–1·MPa^–1, surpassing those of all previously reported barocaloric materials. This work highlights the exceptional barocaloric performance of formylferrocene and the potential of organometallic sandwich compounds for emerging barocaloric refrigeration.