Giant room temperature elastocaloric effect in metal-free thin-film perovskites

Solid-state refrigeration is gaining traction due to its environmental friendliness and potential in small-scale cooling. Several physical effects, including magnetocaloric (MC), electrocaloric (EC), and mechanocaloric (mC), can be utilized. However, MC and EC effects in typical ferromagnetic/ferroelectric materials exhibit relatively small strengths, requiring strong magnetic/electric fields, limiting practical applications. The mC effect, employing mechanical force for cooling, encompasses elastocaloric (eC) and barocaloric (BC) effects, driven by uniaxial stress and hydrostatic pressure, respectively. An eC refrigeration cycle involves adiabatic stress application (temperature rise), heat transfer to surroundings (temperature drop at constant stress), gradual stress reduction (further temperature drop), and finally, heat absorption from surroundings. Materials with high eC strength are crucial for maximizing temperature change and energy efficiency. While previous research predicted high eC strength in ferroelectric oxide heterostructures like BaTiO₃, their low compressibility limits performance. Recently, extraordinarily large EC strengths were predicted for the metal-free perovskite ferroelectric [MDABCO](NH₄)₁₃, exceeding those of prototype perovskites such as BaTiO₃ and PbTiO₃. Its lower elastic stiffness compared to oxide ferroelectrics suggests potential for a significant mC effect. The study leverages thermodynamic calculations to investigate the polarization properties and phase transitions of (111)-oriented [MDABCO](NH₄)₁₃ thin films under various misfit strains and out-of-plane stresses, calculating eC properties. Thin films offer advantages in miniaturization and provide an additional degree of freedom (substrate strain) for device design.

The authors review existing literature on solid-state refrigeration, highlighting the limitations of magnetocaloric and electrocaloric effects due to the need for strong magnetic or electric fields. They discuss previous work on elastocaloric effects, noting the limitations of ferroelectric oxide materials due to their low compressibility and the promise of organic ferroelectric materials. They specifically reference the prediction of extraordinarily large electrocaloric strength in [MDABCO](NH₄)₁₃ by Wang et al. (2020) and the colossal barocaloric effects observed in plastic crystals, which are characterized by extensive disorder and giant compressibility. The authors mention the predicted large piezoelectric response in [MDABCO](NH₄)₁₃ from first-principles calculations and its advantages over other organic ferroelectric systems, such as high polarization, mechanical flexibility, low weight, and low processing temperatures. Prior studies on elastocaloric effects in ferroelectric oxides (e.g., BaTiO₃, PbTiO₃) are discussed, highlighting their limited eC strength at room temperature and the high temperatures at which significant effects are observed.

The study uses thermodynamic calculations based on the Landau-Devonshire theory to model the (111)-oriented [MDABCO](NH₄)₁₃ thin film. The total thermodynamic free energy density is calculated considering four contributions: polarization energy, thermal energy, mechanical energy, and electric energy. The Landau coefficients, elastic modulus tensor, and other material properties are obtained from previous reports. The model considers in-plane misfit strains and out-of-plane stresses, using a mixed mechanical boundary condition (σx = σy = σxy = 0 and εx = εy = εm, εz = 0). Phase transitions are determined by minimizing the free energy with respect to polarization components. The phase diagram is established by analyzing the equilibrium polarization under different combinations of misfit strain and out-of-plane stress. The elastocaloric effect is calculated from the entropy change due to polarization configuration change (ΔSP) and stress-induced volume change (ΔSε). The adiabatic temperature change (ΔTeC) is derived from the conservation of entropy under reversible adiabatic conditions. The coefficient of performance (COP) is evaluated using the absorbed heat (∫TdS) and specific mechanical work (∫σdε). The authors use this model to predict the elastocaloric properties of the [MDABCO](NH₄)₁₃ thin film and compare them with other ferroelectric materials. They investigate the relationship between elastocaloric properties and material parameters such as thermal expansion and elastic compliance, aiming to identify design principles for high-performance elastocaloric materials.

The study reveals a giant room-temperature elastocaloric effect in (111)-oriented [MDABCO](NH₄)₁₃ thin films. A detailed stress-misfit strain phase diagram is generated, showing four stable phases (rhombohedral, monoclinic, orthorhombic, and cubic) depending on the applied stress and strain. The isothermal elastocaloric entropy change (ΔSec/Δσ) and adiabatic elastocaloric temperature change (ΔTec/Δσ) reach remarkably high values: -60.0 J K⁻¹ kg⁻¹ GPa⁻¹ and 17.9 K GPa⁻¹, respectively, at 300 K, significantly surpassing those of conventional ferroelectric oxides. The maximum adiabatic temperature change (ΔTec) reaches approximately 18.7 K under a compressive misfit strain of -0.01 and an applied stress of 1.28 GPa. With a tensile misfit strain of 0.01, the maximum adiabatic temperature change is 22.9 K. Analysis reveals that the low Young's modulus (one percent of conventional oxides) is a contributing factor to the giant eC effect. The refrigerant capacity (RC) for [MDABCO](NH₄)₁₃ is calculated to be 1760 J kg⁻¹, significantly higher than other ferroelectrics. The study also reveals a dominant role of the thermal expansion coefficient in determining the eC effect; increasing this coefficient dramatically increases ΔTec. The results suggest the possibility of further improving the eC properties by chemical modifications to enhance the thermal expansion coefficient of [MDABCO](NH₄)₁₃.

The findings demonstrate a significant advancement in elastocaloric materials, offering a potential alternative to traditional refrigeration technologies. The exceptionally high eC strength of [MDABCO](NH₄)₁₃ at room temperature is a key contribution, surpassing the performance of previously studied ferroelectric materials. The identification of the thermal expansion coefficient as a dominant factor influencing the eC effect provides valuable insights for the design and optimization of future elastocaloric materials. The work highlights the potential of metal-free organic ferroelectrics as a promising class of materials for solid-state refrigeration applications, opening avenues for the development of efficient and environmentally benign cooling devices. The authors suggest that chemical modification of [MDABCO](NH₄)₁₃ to enhance thermal expansion could lead to further improvements in elastocaloric performance.

This study successfully predicted a giant room-temperature elastocaloric effect in metal-free perovskite [MDABCO](NH₄)₁₃ thin films using a thermodynamic model. The exceptionally high eC strength, significantly exceeding that of traditional ferroelectrics, stems from the material's low Young's modulus and high thermal expansion coefficient. The findings suggest a promising path for developing efficient and environmentally friendly solid-state refrigeration technologies, with potential for further enhancement through chemical modifications. Future research could focus on experimental validation of these predictions and further exploration of chemical modifications to optimize the material's elastocaloric properties.

The study relies on theoretical calculations based on a thermodynamic model. Experimental verification of the predicted elastocaloric properties is necessary to confirm the findings. The model assumes specific material parameters obtained from previous reports; variations in these parameters could influence the results. The study focuses on (111)-oriented thin films; investigating other orientations could reveal different eC behaviors. Furthermore, the practical implementation of these materials in a refrigeration device requires further research into material synthesis, device design, and integration.