Barocaloric Hybrid Organic-Inorganic Salts for Green Refrigeration

Barocalorics are materials that undergo a solid-solid phase transition between high and low entropy phases, which can be driven by pressure to form the basis of a refrigeration cycle. As solids they are less prone to leaking and causing environmental damage than traditional haloalkane vapour-compression refrigerants.<br/><br/>We recently reported the molecular mechanism underlying the entropy change in the canonical plastic crystal adamantane, showing in the process that this material is an excellent low-temperature barocaloric [1]. We are now extending this approach to orientationally disordered crystals more widely; an obvious extension is to salts, where Coulomb as well as van der Waals interactions are significant.<br/><br/>Based on this approach, we have discovered a new family of hybrid organic-inorganic barocalorics: quinuclidinium salts, in which the near-spherical quinculidinium cation packs with inorganic counteranions into structures analagous to the alkali halides. The quinuclidinium ion is an excellent component for high-entropy phases since it strikes a fine balance between spherical geometry and anisotropy: its quasi-spherical shape promotes vibrational entropy arising from molecular libration, while the deviation from perfect spherical symmetry generates configurational entropy.<br/><br/>We have studied seven quinculidinium salts (with counterions Cl, Br, I, NO₃, BF₄, PF₆ and IO₄) by crystallography and high-pressure differential scanning calorimetry. These materials show solid-solid phase transitions between 290 and 340 K, entropy changes of up to 164 J/K.kg, and barocaloric coefficients dT/dP of up to 60 K/kbar, making them "colossal" barocalorics [2]. We show that chemically and physically simple changes in the inorganic counterion lead, for instance by changes in the hydrogen-bonding network, to subtle differences in the resulting materials' barocaloric properties.<br/><br/>[1] B.E. Meijer <i>et al</i>., <i>Phys. Chem. Chem. Phys.</i> <b>25</b>, 9282 (2023)<br/>[2] R.J.D.Dixey <i>et al</i>., In preparation