Tuning Phase Transitions in Two-Dimensional Perovskites for Thermal Energy Storage

Two-dimensional metal halide perovskites absorb and release thermal energy as they undergo structural phase transitions, making them promising materials for applications such as barocaloric cooling and next-generation phase change materials. The general structure of these Ruddlesden-Popper (RP) perovskites consists of alternating organic and inorganic layers following the chemical formula A2BX4, where A is a monoammonium organic spacer cation, B is a metal cation, and X is a halide. These RP perovskites undergo solid-solid phase transitions, where the organic cation spacer layer “melts,” while the outer inorganic metal halide layers remain solid. Typically, the thermodynamic properties of these materials depend on the identity of the organic spacer cation, limiting the tunability of their phase transition temperature. However, we show that we can precisely tune the phase transition temperature by alloying the organic cation chains, the metal cations, and the halide anions in Mn- and Cu-based alkyl bromide and chloride perovskites. Using differential scanning calorimetry (DSC), powder X-ray diffraction (PXRD), and temperature-dependent grazing incidence wide angle X-ray scattering (GIWAXS), we interrogate the structural and thermal changes related to the blending and exchanging of the individual components of lead-free RP perovskites. We find that blending different species typically decreases the phase transition temperature compared to the pure phases with the largest depression occurring when blending decylammonium and dodecylammonium copper bromide. Overall, we demonstrate a powerful strategy for precisely tuning the phase transition temperatures of hybrid materials.