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

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 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 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 dense and porous coordination polymers.