Unravelling the Ordered Phase of the Quintessential Hybrid Perovskite MAPbI3 – Thermophysics to the Rescue

Hybrid organic-inorganic perovskites continue to attract an enormous amount of attention, yet a robust microscopic picture of their different phases as well as the extent and nature of the disorder present remain elusive. Using specific-heat data along with high-resolution inelastic neutron scattering and ab initio modeling, we address this ongoing challenge for the case of the ordered phase of the quintessential hybrid perovskite MAPbI3. At low-temperatures, the specific heat of MAPbI3 reveals strong deviations from the Debye limit, a common feature of pure hybrid perovskites and their mixtures. We find that these deviations are exquisitely sensitive to the underlying low-energy phonon band structure, thus paving the way for a quantitative and robust assessment of available structural models beyond what has been possible to date. Our thermophysical analysis demonstrates that the (otherwise ordered) structure around the organic moiety is characterized by a substantial lowering of the local symmetry relative to what can be inferred from crystallographic studies. The ab initio phonon calculations also provide a means of exploring in detail the physical origin of the observed thermophysical anomalies, which can now be unequivocally associated with excitations of sub-Terahertz optical phonons responsible for translational and librational distortions of the octahedral units. To the best of our knowledge, this is the first time that such a model-selection protocol using thermophysical properties as key input has been deployed with success in the study of this important class of materials.