Entropy-Driven Reversible Melting and Recrystallization of Layered Hybrid Perovskites

Typical layered 2D A₂PbX₄ (A: organic ammonium cation, X: Br, I) perovskites undergo irreversible decomposition at high temperatures. Can they be designed to melt at lower temperatures without decomposition? Which thermodynamic parameter drive the melting of layered perovskites? These questions are addressed by considering the melt of A₂PbX₄ as a mixture of ions (like ionic liquids), and hypothesized that the increase in the structural entropy of fusion (ΔS_fus) will be the driving force to decrease their melting temperature. Then to increase structural ΔS_fus, A-site cations are designed that are rigid in the solid crystal, and become flexible in the molten state. Different tail groups in the A-site cations form hydrogen-, halogen- and even covalent bonding-interactions, making the cation-layer rigid in the solid form. Additionally, the rotation of ─NH₃ ⁺ head group is suppressed by replacing ─H with ─CH₃, further enhancing the rigidity. Six A₂PbX₄ crystals with high ΔS_fus and low melting temperatures are prepared using this approach. For example, [I-(CH₂)₃-NH₂(CH₃)]₂PbI₄ reversibly melts at 388 K (decomposition temperature 500 K), and then recrystallizes back upon cooling. Consequently, melt-pressed films are grown demonstrating the solvent- and vacuum-free perovskite films for future optoelectronic devices.