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(C 5 H 5 )(C 5 H 4 CHO)]. 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.