Pressure-freezing of dodecane: exploring the crystal structures, formation kinetics and phase diagrams for colossal barocaloric effects in n-alkanes

Barocaloric (BC) materials provide cheaper and more energy efficient alternatives to traditional refrigerants. Some liquid alkanes were recently shown to exhibit a colossal BC effect, matching the entropy changes in commercial vapour-liquid refrigerants. Dodecane was predicted to have the largest entropy change among the studied alkanes. Using synchrotron powder and single-crystal X-ray diffraction, Raman spectroscopy, and lattice energy calculations, we investigated the BC effects of n-dodecane at high pressures and room temperature. Remarkably, a colossal entropy change |Delta S| of 778 J kg(-1) K-1 at 0.15(3) GPa and 295 K was observed. Spectroscopic studies revealed that this entropy change correlates closely with the conformational transition from mixed gauche to all-trans forms during pressure-induced crystallization. Additionally, the usage of a diamond anvil cell allowed the determination of the crystal structures of in situ crystallized n-un- and dodecane, as well as evaluation of the pressure-dependent crystal growth kinetics. Furthermore, our research suggests that the entropy change (per kilogram) upon compression should be similar for all n-alkanes within the range of 9-18 carbon atoms in the molecule, based on their lattice energies. Even-numbered alkanes are predicted to exhibit superior BC properties compared to their odd-numbered counterparts due to the more symmetric crystal structures and lower propensity to form plastic phases with lower transition entropy.