The Barocaloric and Structural Properties of Choline-Based Plastic Crystals

The currently ubiquitous vapor-compression method of refrigeration has a significant downside, namely its reliance on volatile refrigerants with large global warming potentials. In order to overcome this challenge, alternative solid-state refrigerants which exhibit similarly large entropy and temperature changes by the application of external stimuli have to be identified. A promising approach to this problem is found in the utilization of the hydrostatic pressure-induced entropy and temperature change, the so-called barocaloric effect, which accompanies structural phase transitions. This approach avoids the need for volatile refrigerants with large global warming potentials and promises a higher thermodynamic efficiency [1].<br/>Thus far only the class of molecular 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 on par with commercial hydrofluorocarbon refrigerants [2]. The exceptionally large barocaloric response found in plastic crystals originates from structural phase transitions from an ordered state to an orientationally disordered state, characterised by a large isothermal entropy change, d<i>S</i>, and large barocaloric coefficient <i>d</i>T/<i>d</i>P. Plastic phases can also be obtained in ionic systems built from weakly interacting highly symmetric globular ions. Additionally, the introduction of transition-metals into this type of system provides an avenue towards designing multifunctional or multicaloric materials [3].<br/>In this work, we explore the expansive chemical parameter space of hybrid ionic plastic crystals and study the effect of anion substitution on the structural phase transitions and barocaloric properties of plastic crystals based on the choline cation. By employing pressure- and temperature-dependent single crystal/powder diffraction and differential scanning calorimetry experiments, this structure-property relationship is studied in detail. Demonstrating the ability to tune the transition temperature, d<i>S</i> and <i>d</i>T/<i>d</i>P via chemical exchange, the results of this work can then be used to inform the design of future solid-state refrigerants .<br/><br/>References:<br/>[1] C. Aprea, A. Greco, A. Maiorino, and C. Masselli, <i>The Use of Barocaloric Effect for Energy Saving in a Domestic Refrigerator with Ethylene-Glycol Based Nanofluids: A Numerical Analysis and a Comparison with a Vapor Compression Cooler</i>, Energy <b>190</b>, 116404 (2020).<br/>[2] B. Li et al., <i>Colossal Barocaloric Effects in Plastic Crystals</i>, Nature <b>567</b>, 506 (2019).<br/>[3] J. Salgado-Beceiro, J. M. Bermúdez-García, A. L. Llamas-Saiz, S. Castro-García, M. A. Señarís-Rodríguez, F. Rivadulla, and M. Sánchez-Andújar, <i>Multifunctional Properties and Multi-Energy Storage in the [(CH₃)₃S][FeCl₄] Plastic Crystal</i>, J Mater Chem C Mater <b>8</b>, 13686 (2020).