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, |ΔSrev| = 280 (200) J K−1 kg−1 and |ΔTrev| = 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.

Landau theory of barocaloric plastic crystals

“…Our model predicts that the onset of long-range orientational ordering leads to (i) a change ∆α in the coefficient of thermal expansion (equations ( 10) and ( 11)), (ii) a change in volume ∆V t (equations ( 10) and ( 12)), (iii) a change in entropy ∆S t (equations ( 15) and ( 16)), and (iv) a linear temperature-pressure phase boundary (equation ( 9)), which is in agreement with experiments [7][8][9][10][11][12][13][14][15][16][17][18][19]. We note that ∆α, ∆V t , and dT c /dP (equation ( 14)) depend on the strain-orientation coupling.…”

“…) is the value of the entropy density at ambient pressure as T approaches T 0 c from above, and S C ≡ S 0 (T c ) − (1 − KαdT c /dP) αP + ∆S t is the value of the entropy density at finite pressure as T approaches T c from below, see figures 1(a) and (b), and equation (15). For P ⩽ P * , the temperature-independent isothermal changes in ∆S(T, P) result solely from the thermal expansion of the individual plastic (T > T c ) and crystal phases (T < T 0 c ), while the temperature-dependent part in ∆S(T, P) originates from the phase transition (T 0 c < T < T c ), as shown in figure 1(c).…”

“…where T s is the starting temperature. In general, both ∆S(T, P) and ∆T(T s , P) implicitly involve the background entropy density S 0 , see equation (15). However, given that within our approximations S 0 is independent of pressure (section 2.4), only ∆T(T s , P) depends on S 0 .…”

“…These colossal BC effects are comparable to the pressure-driven thermal changes seen in hydrofluorocarbons/hydrocarbons used in vapor-compression refrigeration 4 . PCs exhibiting colossal BC effects include neopentane derivatives [7][8][9][10], adamantane [11], adamantane derivatives [12,13], carboranes [14], and alkali hydrido-closoborates [15]. In addition, orientationally disordered solids which are not as mechanically soft as PCs, can also display large [16,17] and colossal [18] BC effects.…”

“…The discovery of colossal BC effects in PCs has sparked theoretical and computational interest primarily using density functional theory [24], molecular dynamics methods [15,[25][26][27], supercell lattice dynamics calculations [11], and mean-field microscopic modelling [28], aimed at understanding plastic-to-crystal transitions and predicting their associated BC response. However, a macroscopic description based on Landau theory is missing.…”