As the demand for sustainable energy solutions increases, solid-state refrigeration is emerging as a promising alternative to traditional systems that rely on the compression of greenhouse gases. Solid-state cooling systems exploit the thermodynamic responses of materials to external fields or mechanical forces — which result in changes in entropy and adiabatic temperature. Among these, the barocaloric effect (BCE), enables materials to reversibly absorb or release heat in response to variations in pressure.

Spin-crossover (SCO) compounds, particularly those based on Fe(II), are promising candidates for barocaloric applications. These materials can switch back and forth between low-spin and high-spin electronic states in response to external stimuli. This process involves abrupt structural and electronic changes, often accompanied by thermal hysteresis — a temperature gap between the forward and reverse transitions. A narrow hysteresis is preferred for practical use, as it enables reversible and efficient operation near a defined working temperature. The combination of pressure sensitivity, large entropy changes, and tunable hysteresis makes SCO materials attractive for solid-state refrigeration without conventional gas compression.

While several SCO systems have been predicted to show strong pressure-driven thermal effects, only a few have been experimentally characterized using high-pressure calorimetry and X-ray diffraction. This study focuses on [Fe(pap-5NO₂)₂], a molecular material that exhibits a sharp and cooperative spin transition near room temperature. Its structural sensitivity to pressure enables effective heat exchange, with additional advantages including cycling stability, and performance comparable to the best inorganic barocaloric materials.

To investigate the material's properties, the researchers combined synchrotron-based X-ray powder diffraction with high-pressure calorimetry. The measurements were carried out at the Materials Science and Powder Diffraction (MSPD) beamline of the ALBA Synchrotron. High-resolution diffraction patterns were collected under variable isothermal pressures, enabling the team to monitor in real time how the crystal structure evolved as the material switched between low-spin and high-spin states.

The results confirm that [Fe(pap-5NO₂)₂] exhibits a giant barocaloric effect near room temperature, which remains stable over at least ten pressure cycles. For a moderate pressure change of just 2.0 kbar, the material shows isothermal entropy changes of up to 79 J·kg⁻¹·K⁻¹ and adiabatic temperature changes reaching 26 K — values among the highest reported for spin-crossover compounds. Under reversible cycling conditions — essential for practical applications — the material maintains strong performance, exhibiting reversible entropy changes of 70 J·kg⁻¹·K⁻¹ and temperature shifts of 14 K. These observations highlight the material’s strong and durable barocaloric response under operational conditions.

This study positions [Fe(pap-5NO₂)₂] as a leading example of a molecular material suitable for near-room-temperature cooling applications. Its combination of large entropy and temperature changes, structural responsiveness, and partial reversibility under moderate pressure makes it a strong candidate for future gas-free refrigeration systems.

Synchrotron X-ray diffraction under high and moderate pressures was essential to understand the structural mechanisms driving the thermal response. While ongoing research aims to further reduce thermal hysteresis and enable large-scale implementation, these findings represent a major advance toward cleaner, more efficient, and sustainable cooling technologies.

Isobaric thermograms of [Fe(pap-5NO2)2] at different pressures, during heating and cooling, upon baseline subtraction. For each pressure, a sharp, dominant peak identifies the transition between the low-spin and high-spin phases.