Room-temperature barocaloric effect in [Fe(pap5NO 2 ) 2 ] spin-crossover material

Unlocking giant barocaloric effects near room temperature with a spin-crossover material

An international team of researchers led by INMA researchers in collaboration with colleagues from the CNRS-Paris-Saclay University and ALBA synchrotron has unveiled breakthrough findings in the field of solid-state refrigeration. Their study, recently published in The Journal of Materials Chemistry A (DOI: 10.1039/d5ta00033e), introduces the compound [Fe(pap-5NO 2 ) 2 ] as a powerful barocaloric material with exceptional performance near room temperature.

This material undergoes a pressure-sensitive spin-state transition near room temperature and exhibits giant barocaloric effects (BCE) — a thermodynamic response that could be harnessed for green solid state refrigeration. Using high-pressure calorimetry and X-ray diffraction, the authors demonstrate entropy changes as high as 79 J·kg⁻¹·K⁻¹, adiabatic temperature changes up to 26 K under a moderate pressure change (2.0 kbar) and substantial performance even under reversible conditions (∆S ≈ 70 J·kg⁻¹·K⁻¹, ∆T ad ≈ 14 K).

While thermal hysteresis introduces challenges for reversibility, the findings highlight the potential of molecular SCO systems to compete with — or complement — current caloric materials, especially in compact or tunable cooling applications.

This work not only expands the library of barocaloric materials, but also strengthens the bridge between molecular materials chemistry and functional solid-state cooling — an exciting direction for researchers in smart materials, phase transitions, and sustainable technologies.

“Room-temperature barocaloric effect in [Fe(pap5NO 2 ) 2 ] spin-crossover material”

DOI: 10.1039/d5ta00033e

D. Gracia, V. Cuartero, C. Popescu, A. Trapali, T. Mallah, M-L Boillot, J. Blasco, G. Subías and M. Evangelisti

J. Mat. Chem. A (2025)

Accepted 12th May 2025

Abstract:

We examine the pressure dependence of the spin-crossover transition in [Fe(pap-5NO₂)₂] that occurs near room temperature. We employ a combination of high-pressure calorimetry and powder X-ray diffraction measurements, conducted both under variable-pressure and variable-temperature conditions. Both methods indicate that the spin-crossover transition shifts linearly to higher temperatures with increasing pressure, while simultaneously exhibiting an increase in the width of the thermal hysteresis. We report a giant barocaloric effect, revealing isothermal entropy changes in the 70–79 J kg⁻¹ K⁻¹ range and adiabatic temperature changes between 20 and 26 K for a pressure change of 2.0 kbar. Although the effect diminishes under reversible conditions, it remains substantial, with values of 70 J kg⁻¹ K⁻¹ and 14 K, respectively.

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