Entropy-Driven Reversible Melting and Recrystallization of Layered Hybrid Perovskites.

Typical layered 2D APbX (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 APbX as a mixture of ions (like ionic liquids), and hypothesized that the increase in the structural entropy of fusion (ΔS) will be the driving force to decrease their melting temperature. Then to increase structural ΔS, A-site cations are designed that are rigid in the solid crystal, and become flexible in the molten state.