Colossal Barocaloric Effect in Carboranes as a Performance Tradeoff

The discovery of colossal barocaloric effects in organic plastic crystals has significantly advanced the development of solid-state refrigerant techniques. Adapting to the real application, a tradeoff of various barocaloric performances has to be achieved. Here, it is reported a novel plastic crystal system, that is, carboranes (C2B10H12), including three positional isomers: ortho-carborane, meta-carborane, and para-carborane, which are characterized by C-2v, C-2v, D-5d point groups, respectively. They all undergo an orthorhombic-to-tetragonal phase transition around room temperature. Compared to the previously reported organic plastic crystals, this system exhibits a combination of large pressure-normalized entropy changes, the high-pressure sensitivity of the transition temperature, small thermal hysteresis, and so forth. Their barocaloric performances are positional-isomerism dependent, and the best performances are obtained in para-carborane with maximum entropy changes of about 106.2 J kg(-1) K-1 achieved under pressure changes below 30 MPa. This study not only suggests that carboranes would be a considerably promising working material for barocaloric refrigeration at room temperature but also indicates that delicate tuning of molecular isomerism is an effective strategy to enhance barocaloric performances.