Two-dimensional copper perovskites for barocaloric devices operating circa 100 ºC

Abstract

Current cooling systems and heat pumps use vapor compression cycles with hydrofluorocarbons (HFCs) as refrigerants. These devices are a major source of greenhouse gases due to their limited efficiency and the huge global warming potential (GWP) of HFCs when leaked into the atmosphere. In the search for a more sustainable and efficient future, there is growing interest in materials that exhibit first-order solid-solid phase transitions with significant latent heat. These materials are being explored for barocaloric devices for solid-state cooling and heating applications. Two-dimensional perovskites with the chemical formula [A]2BX4, where A is an organic cation, such as an alkylammonium chain, M is a divalent metal (B = Co, Zn, Cu, Mn), X is a halogen atom (X = Cl, Br), are emerging as excellent material candidates for these applications, as their chemical flexibility allows tuning of the crystal structure and transition properties (e.g., temperature and latent heat). Until now, two-dimensional halide perovskites have been prepared with up to 16 carbons in the alkylammonium chain, which exhibit energetic phase transitions at temperatures relatively close to room temperature for domestic applications. To develop heat pumps for industrial processes, research into materials that exhibit the barocaloric effect at higher temperatures (e.g., 80-120 ºC) has emerged. This work reports on the preparation of [Cn]2CuCl4 and [Cn]2CuBr4 (n =16, 18, 19, 20 and 22) and the characterization of the crystal structure by powder and single-crystal X-ray diffraction. Differential scanning calorimetry analysis of [C20]2CuBr4 and [C22]2CuBr4 show a large latent heat (60.5 J g-1 and 65.9 J g-1) at 98.1 ºC and 103.9 ºC, respectively.