Hydrogen Bonding versus Entropy: Revealing the Underlying Thermodynamics of the Hybrid Organic–Inorganic Perovskite [CH₃NH₃]PbBr₃

Abstract

The enormous research efforts dedicated to hybrid organic−inorganic perovskites have led to a deep understanding of these materials; however, the role of entropy and its ramifications for the properties of the materials have<br/>been only sparsely explored. In this study, we quantify the phase transition mechanism in the hybrid organic−inorganic perovskite [CH3NH3]PbBr3 by studying low-energy collective phonon modes using a combination of inelastic neutron scattering and ab initio lattice dynamics. We demonstrate that a delicate interplay among hydrogen bonding interactions, lattice vibrational entropy, and configurational disorder determines the thermodynamics and results in the rich<br/>phase evolution of [CH3NH3]PbBr3 as a function of temperature. Our results have important implications for the manipulation of macroscopic properties and provide a blueprint for future studies that will focus on unravelling phase transition mechanisms in hybrid perovskites and related materials such as<br/>dense and porous coordination polymers.