Exploring Entropy Contributions in Colossal Barocaloric Plastic Crystals

Barocalorics are materials exhibiting a solid-solid phase transition between high and low entropy phases, which can be cycled by the application and release of pressure, analogous to traditional vapour-compression refrigeration, but without the risks associated with leaking environmentally harmful fluids [1]. This technological potential for "green" cooling and heating drives research to not just discover new barocalorics but also to optimise the relvant properties, such as phase tranistion temperature, adiabatic temperature change and thermal conductivity, which requires a more quantitative understanding of the entropy change.<br/><br/>Orientationally disordered "plastic" crystals, in which the near spherical molecules freeze into position at low temperatures but are free to rotate at high temperatures, can generate extremely strong barocaloric effects associated with the unlocking of the rotational degrees of freedom. This is clearly demonstrated in quinuclidinium salts, in which the near-spherical quinculidinium cation packs with inorganic counteranions into structures analagous to the alkali halides. The 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 the crystallography of seven quinculidinium salts (with counterions Cl, Br, I, NO₃, BF₃, PF₆and IO₄) and measured the phase transitions using 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]. The dynamics of the Quin ion have been investigated using Quasi-Elastic Neutron Scattering as a function of pressure, while Inelastic Neutron Scattering has alowed an investigation of the low-frequency librational phonon modes and how the phonon density of states varies across the phase transition relating to the disruption of the hydrogen bonding network. Insights from the data are obtained through classical and DFT molecular dynamics simulations.<br/><br/>[1] P. Lloveras & J.-L. Tamarit, <i>MRS Energy Sustain.</i>, <b>8</b>, 3-15 (2021)<br/>[2] R.J.C. Dixey <i>et al.</i>, in preparation