Landau Theory of Barocaloric Plastic Crystals

Abstract

We present a simple Landau phenomenology for plastic-to-crystal phase transitions and use the resulting model to calculate barocaloric effects in plastic crystals that are driven by hydrostatic pressure. The essential ingredients of the model are (i) a multipole-moment order parameter that describes the orientational ordering of the constituent molecules, (ii) coupling between such order parameter and elastic strains, and (iii) the thermal expansion of the solid. The model captures main features of plastic-to-crystal phase transitions, namely large volume and entropy changes at the transition, and strong dependence of the transition temperature with pressure. Using solid C$_{60}$ under $0.60\,$GPa as case example, we show that calculated peak isothermal entropy changes of $\sim 58 \,{\rm J K^{-1} kg^{-1}}$ and peak adiabatic entropy changes of $\sim 23 \,{\rm K}$ agree well with experimental values.Comment: 17 pages, 3 figure