Reversible Colossal Barocaloric Effects at Low Pressures in Disordered Materials

Materials undergoing order-disorder first-order phase transitions are attracting interest for thermal management because they can act as heat sinks or sources due to the energy stored in the disordered phases. Moreover, the application of pressure on these materials may allow an active control of the heat exchange, which leads to conceive barocaloric methods for solid-state cooling and heat pumping [1]. Interestingly, they promise a more environmentally friendly and efficient alternative to harmful vapor compression. Fast progress on materials research has allowed the identification of some organic plastic crystals as outstanding barocaloric agents, with colossal barocaloric effects much above 100 J K^-1 kg^-1. However, typical moderate or large hysteresis and a moderate sensitivity of the transition to pressure make these colossal barocaloric effects be obtained reversibly upon pressure changes above 100 MPa, which limits their potential applicability. Here we present two recently investigated materials that exhibit relatively little-hysteretic phase transitions releasing strong disorder in the high temperature phase and accompanied by large volume changes. These features allow to obtain reversible colossal barocaloric effects below 100 MPa in both compounds, with simultaneously large values for isothermal entropy changes Δ<i>S</i> and adiabatic temperature changes Δ<i>T</i>. One material is a layered hybrid organic-inorganic pseudoperovskite [2] whose disorder is associated with the melting-like process and rotation of the organic chain linked to the inoganic octahedra. It displays reversible Δ<i>S ∼</i> 230 J K⁻¹ kg⁻¹ under low pressure changes of 50 MPa, and reversible Δ<i>T</i> ∼ 10 K under pressure changes of 80 MPa. The other material is a fast-ionic conductor [3] displaying strong molecular orientational disorder of anions coupled to diffusion of cations. In that case, the material undergoes reversible Δ<i>S ∼</i> 170 J K⁻¹ kg⁻¹ and Δ<i>T</i> <i>∼</i> 10 K under pressure changes of 84 MPa. Here, the high transition temperature suggests that it could be also used for solid-state barocaloric waste heat recovery [4].<br/><br/>[1] P. Lloveras and J.-Ll. Tamarit, MRS Ener. Sustain <b>8</b>, 3 (2021).<br/>[2] J. Li et al., Adv. Func. Mater. <b>31</b>, 2105154 (2021).<br/>[3] K. Sau et al., Sci. Rep. <b>11</b>, 11915 (2021).<br/>[4] H. Tokoro et al., Nat. Commun. <b>6</b>, 7037 (2015).