Colossal Reversible Barocaloric Effects in a Plastic Crystal Mediated by Lattice Vibrations and Ion Diffusion

Abstract

Solid-state methods for cooling and heating promise a sustainable alternative to current compression cycles of greenhouse gases and inefficient fuel-burning heaters. Barocaloric effects (BCE) driven by hydrostatic pressure (p) are especially encouraging in terms of large adiabatic temperature changes (|ΔT| ≈ 10 K) and isothermal entropy changes (|ΔS| ≈ 100 J K^-1 kg^-1). However, BCE typically require large pressure shifts due to irreversibility issues, and sizeable |ΔT| and |ΔS| seldom are realized in a same material. Here, the existence of colossal and reversible BCE in LiCB₁₁H₁₂ is demonstrated near its order-disorder phase transition at ≈380 K. Specifically, for Δp ≈ 0.23 (0.10) GPa, |ΔS_rev| = 280 (200) J K^-1 kg^-1 and |ΔT_rev| = 32 (10) K are measured, which individually rival with state-of-the-art BCE figures. Furthermore, pressure shifts of the order of 0.1 GPa yield huge reversible barocaloric strengths of ≈2 J K^-1 kg^-1 MPa^-1. Molecular dynamics simulations are performed to quantify the role of lattice vibrations, molecular reorientations, and ion diffusion on the disclosed BCE. Interestingly, lattice vibrations are found to contribute the most to |ΔS| while the diffusion of lithium ions, despite adding up only slightly to the entropy change, is crucial in enabling the molecular order-disorder phase transition.

Keywords: barocaloric effects; lithium diffusion; molecular dynamics simulations; orientational order–disorder phase transition; solid‐state refrigeration.