Cooling without gases: molecular design brings solid-state cooling closer to reality

Some solid materials can cool down or heat up when pressure is applied or released. This behaviour enables cooling and heating technologies that do not rely on climate-damaging refrigerant gases. In practice, however, a major obstacle remains: many materials behave differently during heating and cooling, which makes their response difficult to use reliably in real devices.

In this work, the authors investigate a solid material known for its exceptionally large cooling/heating response (thermal response) under pressure and ask a simple question: can this response be made more reliable? They show that a very small change in composition leads to a clear improvement and use neutron experiments to explain why this improvement occurs.

Cooling technologies at a crossroads

Refrigeration is an essential part of modern life, from food preservation and medical storage to climate control and industrial processes. However, most of today’s systems still rely on vapour-compression cycles using refrigerant gases that contribute to global warming and face increasing regulatory constraints. This has triggered research into alternative solutions that are both efficient and sustainable.

One promising route is the use of solid materials that can absorb or release heat when pressure is applied or removed: these are known as barocaloric materials. For practical applications, these materials must:

have nearly identical behaviour on heating and cooling (operate reversibly, with minimal thermal hysteresis)
be efficient under pressures relevant for real-world operation, typically of the order of 1 kbar.

Balancing these requirements without sacrificing their cooling capacity remains challenging.

Among barocaloric materials, plastic crystals have attracted particular interest because they display a strong cooling response close to room temperature. This behaviour arises from a transition between disordered and ordered crystal phases. The associated entropy change reflects how much heat can be absorbed or released during this transition. In many cases, however, only a small fraction of this entropy change can be accessed reversibly under realistic operating conditions, limiting their use in practical cooling cycles.

Engineering molecular interactions to improve reversibility

Researchers from the University of Glasgow, the University of Cambridge, Universitat Politècnica de Catalunya, Diamond Light Source and the ILL examined how the barocaloric behaviour of plastic crystals can be improved through compositional tuning. In their work, they investigated neopentyl glycol (NPG), a well-known barocaloric plastic crystal that exhibits a large caloric response at its order–disorder phase transition, but whose practical use is limited by strong thermal hysteresis.

To bring the transition temperature (T₀ - the temperature around which the barocaloric effect is strongest) closer to room temperature, NPG was first combined with pentaglycerine (PG) to form a binary solid solution. An NPG:PG ratio of 60:40 yields a single-phase solid solution with a T₀ ≈ 302 K (≈ 29 °C) which is about 10 degrees lower than that of pure NPG and 50 degrees lower than that of pure PG.

The key result of the study emerges with the introduction of a third molecular component. Adding only 2 mol % of pentaerythritol (PE), corresponding to an NPG:PG:PE ratio of 60:38:2, leads to a pronounced improvement in reversibility. At pressures around 1 kbar, the ternary material exhibits a reversible entropy change of 13.4 J kg⁻¹ K⁻¹, about seven times larger than that of pure NPG under similar conditions. Importantly, this reversible response extends over a temperature range of 18 K, representing an approximately twenty-fold increase in the usable operating window. When combined, these two effects enhance the reversible refrigeration capacity by more than a factor of seventy.

Crucially, this improvement is not achieved by weakening the cooling effect itself. The material still absorbs and releases a large amount of heat during the transition. Instead, the small amount of PE alters the molecular environment in a way that reduces the thermal hysteresis as the material is compressed and decompressed, making the thermal response more consistent from one cycle to the next.

Molecular structure and phase behaviour of neopentyl plastic crystals.
(a) Tetrahedral neopentyl molecules that differ only in the number of hydroxyl (–OH) groups. The –OH groups are shown in red (oxygen) and white (hydrogen).
(b) Simplified phase diagram showing different solid phases. α and β correspond to different ordered crystal phases, where molecules are fixed in position and orientation. γ corresponds to a disordered (plastic crystal) phase, where molecules can rotate more freely. The green hatched region indicates coexistence of ordered and disordered phases during the transition, and the red dot marks the composition studied in this work.

Neutrons reveal the microscopic origin of improved reversibility

Panoramic view inside IN16B's secondary spectrometer.

To understand why a small compositional change leads to such a large improvement in barocaloric reversibility, it is necessary to go beyond thermodynamic and structural measurements and directly probe molecular dynamics. The team turned to quasielastic neutron scattering (QENS) which is ideally suited to this task. By measuring very small energy transfers, corresponding to molecular motions on picosecond to nanosecond timescales, QENS provides direct access to rotational and translational dynamics in hydrogen-rich molecular solids.

In this study, QENS measurements were performed on the IN16B spectrometer at the ILL. Using a measurement mode known as inelastic fixed-window scans, the instrument allows molecular dynamics to be followed at selected temperatures during heating and cooling. This makes it possible to directly correlate dynamic behaviour with the thermal hysteresis observed in calorimetry and with structural information from diffraction.

The neutron measurements show that adding a small amount of pentaerythritol modifies how molecular motion evolves with temperature. In the ternary NPG–PG–PE material, molecular reorientations develop more gradually across the phase transition, extending over a broader temperature range than in pure neopentyl glycol.

In contrast, pure NPG shows a more abrupt onset of motion and a clearer difference between heating and cooling. In the ternary material, this difference is reduced, indicating more similar behaviour in both directions of the transition. The team explains this effect with large hydrogen bonded structures that stabilize the ordered phase in pure NPG and NPG:PG, but are easily disrupted by adding small amounts of PE.

Taken together, the neutron results show that the enhanced reversibility of the barocaloric effect in the ternary material is rooted in a change in how molecular motion develops across the order–disorder phase transition. By revealing this microscopic origin of hysteresis reduction, neutron scattering provides a crucial link between molecular design and macroscopic cooling performance.

From molecular insight to sustainable cooling technologies

This study shows how subtle molecular engineering can address one of the central challenges in barocaloric materials: achieving large cooling effects under conditions that are compatible with practical use. By combining compositional tuning with a detailed investigation of molecular dynamics, the researchers demonstrate that cooling performance and reliability can be improved at the same time.

More broadly, this work illustrates how understanding materials at the molecular level can guide the development of efficient solid-state cooling and heating technologies for climate control with lower environmental impact than conventional refrigerant-based systems. As the demand for sustainable cooling solutions continues to grow, such insights will be essential for turning promising physical effects into technologies with real societal impact.

References:

Frederic Rendell-Bhatti, Melony Dilshad, Celine Beck, Markus Appel, Alba Prats, Eamonn T. Connolly, Claire Wilson, Lewis Giannelli, Pol Lloveras, Xavier Moya, David Boldrin & Donald A. MacLaren, Enhanced reversible barocaloric effect at low pressure in neopentyl plastic crystal solid solutions. Communications Materials (2026). (https://doi.org/10.1038/s43246-026-01084-2)

ILL instruments: IN16B

ILL Contact Person: Markus Appel

Institutions involved in the research: University of Glasgow, University of Cambridge, Universitat Politecnica de Catalunya, Diamond Light Source