Barocaloric Effects in Dialkylammonium Halide Salts
Jinyoung Seo, Rahil Ukani, Juanjuan Zheng, Jason D. Braun, Sicheng Wang, Faith E. Chen, Hong Ki Kim, Selena Zhang, Catherine Thai, Ryan D. McGillicuddy, Hao Yan, Joost J. Vlassak, Jarad A. Mason
Abstract
Barocaloric effects─solid-state thermal changes induced by the application and removal of hydrostatic pressure─offer the potential for energy-efficient heating and cooling without relying on volatile refrigerants. Here, we report that dialkylammonium halides─organic salts featuring bilayers of alkyl chains templated through hydrogen bonds to halide anions─display large, reversible, and tunable barocaloric effects near ambient temperature. The conformational flexibility and soft nature of the weakly confined hydrocarbons give rise to order–disorder phase transitions in the solid state that are associated with substantial entropy changes (>200 J kg –1 K –1 ) and high sensitivity to pressure (>24 K kbar –1 ), the combination of which drives strong barocaloric effects at relatively low pressures. Through high-pressure calorimetry, X-ray diffraction, and Raman spectroscopy, we investigate the structural factors that influence pressure-induced phase transitions of select dialkylammonium halides and evaluate the magnitude and reversibility of their barocaloric effects. Furthermore, we characterize the cyclability of thin-film samples under aggressive conditions (heating rate of 3500 K s –1 and over 11,000 cycles) using nanocalorimetry. Taken together, these results establish dialkylammonium halides as a promising class of pressure-responsive thermal materials.