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Solid-state refrigeration using barocaloric materials is environmentally friendly and highly efficient, making it a subject of global interest over the past decade. Here, we report giant barocaloric effects in sodium hexafluorophosphate (NaPF6) and sodium hexafluoroarsenate (NaAsF6) that both undergo a cubic-to-rhombohedral phase transition near room temperature. We have determined that the low-temperature phase structure of NaPF6 is a rhombohedral structure with space group R 3 ¯ by neutron powder diffraction. There are three Raman active vibration modes in NaPF6 and NaAsF6, i.e., F2g, Eg, and A1g. The phase transition temperature varies with pressure at a rate of dTt/dP = 250 and 310 K GPa−1 for NaPF6 and NaAsF6. The pressure-induced entropy changes of NaPF6 and NaAsF6 are determined to be around 45.2 and 35.6 J kg−1 K−1, respectively. The saturation driving pressure is about 40 MPa. The pressure-dependent neutron powder diffraction suggests that the barocaloric effects are related to the pressure-induced cubic-to-rhombohedral phase transitions. [ABSTRACT FROM AUTHOR]

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 C2v, C2v, D5d 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. [ABSTRACT FROM AUTHOR]

One of the greatest obstacles to the real application of solid-state refrigeration is the huge driving fields. Here, we report a giant barocaloric effect in inorganic NH4I with reversible entropy changes of Δ S P 0 → P max ∼71 J K−1 kg−1 around room temperature, associated with a structural phase transition. The phase transition temperature, Tt, varies dramatically with pressure at a rate of dTt/dP ∼0.79 K MPa−1, which leads to a very small saturation driving pressure of ΔP ∼40 MPa, an extremely large barocaloric strength of Δ S P 0 → P max / Δ P ∼1.78 J K−1 kg−1 MPa−1, as well as a broad temperature span of ∼41 K under 80 MPa. Comprehensive characterizations of the crystal structures and atomic dynamics by neutron scattering reveal that a strong reorientation-vibration coupling is responsible for the large pressure sensitivity of Tt. This work is expected to advance the practical application of barocaloric refrigeration. A small driving pressure is desirable for practical application of barocaloric materials. Here, the authors demonstrate a sensitive barocaloric effect in NH4I due to strong reorientation-vibration coupling. [ABSTRACT FROM AUTHOR]