The Role of Hydrogen Bond Competition in the Colossal Barocaloric Response of Choline-Based Plastic Crystals

Solid-state refrigerants, known as caloric materials, exhibit temperature changes upon the application of external fields such as hydrostatic pressure and hold the potential to replace the environmentally harmful refrigerants with large global warming potentials currently ubiquitously used in cooling processes. Thus far only the class of plastic crystals, compounds which exhibit solid phases characterized by large degrees of orientational disorder and plastic-like mechanical properties, have been shown to exhibit colossal barocaloric effects (△S > 100 J kg^-1 K^-1) on par with commercial hydrofluorocarbon refrigerants [1]. The exceptionally large barocaloric response found in plastic crystals originates from structural phase transitions from an ordered state to an orientationally disordered state and is typically accompanied by large isothermal entropy changes, △S, and large barocaloric coefficients, dT/dP.

In this work, we explore the structural and barocaloric properties of the [choline]₂MX₄ family of compounds, where M = Co, Zn and X = Cl, Br, I. Through a comprehensive set of pressure- and temperature-variable single crystal/powder diffraction [2,3] and differential scanning calorimetry experiments, performed in tandem with ab initio Molecular Dynamics simulations, we are able to study the structure-property relationship which drives the colossal barocaloric response observed in this system systematically. A first-order phase transition with a small thermal hysteresis of ~2 K is observed around room temperature for the chlorinated samples. Taking into account the change in heat capacity and elastic heating contribution, a maximum isothermal entropy change of △S_it^max = 145 J kg^-1 K^-1 and adiabatic temperature change of △T_ad^max = 11.5 K are obtained for a change in pressure of △P = 100 MPa. Due to the small thermal hysteresis, a maximum reversible isothermal entropy change of △S_it,rev^max = 139 J kg^-1 K^-1 and maximum reversible adiabatic temperature change of △T_ad,rev^max = 6.1 K are obtained for △P = 100 MPa. Additionally, a maximum barocaloric strength of △S_it,rev^max/△P = 1.73 J kg^-1 K^-1 MPa^-1 is determined for △P = 60 MPa, standing among the highest values for the barocaloric strength reported in the literature up to now [4].

We demonstrate that through halide substitution the transition temperatures, △S and dT/dP change significantly and can be tuned. We find that hydrogen bonding competition plays a central role in the stability of the observed crystallographic phases and can be used to rationalize the evolution of the barocaloric properties observed with halide substitution. The results of this work illustrate the potential of chemical exchange for tuning barocaloric properties and can be used to inform the design of future solid-state refrigerants.

References:
[1] B. Li et al., Colossal barocaloric effects in plastic crystals, Nature 567, 506 (2019).
[2] C. J. McMonagle, D. R. Allan, M. R. Warren, K. V. Kamenev, G. F. Turner, and S. A. Moggach, High-pressure sapphire capillary cell for synchrotron single-crystal X-ray diffraction measurements to 1500 bar, J. Appl. Crystallogr. 53, 1519 (2020).
[3] N. J. Brooks, B. L. L. E. Gauthe, N. J. Terrill, S. E. Rogers, R. H. Templer, O. Ces, and J. M. Seddon, Automated high pressure cell for pressure jump x-ray diffraction, Review of Scientific Instruments 81, 064103 (2010).
[4] Q. Ren et al., Ultrasensitive barocaloric material for room-temperature solid-state refrigeration, Nat. Commun. 13, 2293 (2022).