Atomic-Level Description of Thermal Fluctuations in Inorganic Lead Halide Perovskites
Oliviero Cannelli, Julia Wiktor, Nicola Colonna, Ludmila Leroy, Michele Puppin, Camila Bacellar, Ilia I. Sadykov, Franziska Krieg, Grigory Smolentsev, Maksym V. Kovalenko, Alfredo Pasquarello, Majed Chergui, Giulia F. Mancini
Abstract
nanocrystals across length scales with atomic-level precision, and we provide a framework for the description of phase transitions therein, beyond the simplistic picture of unit-cell symmetry increase upon heating. The temperature increase significantly enhances the short-range structural distortions of the lead halide framework as a consequence of the phonon anharmonicity, which causes the excess free energy surface to change as a function of temperature. As a result, phase transitions can be rationalized via the soft-mode model, which also describes displacive thermal phase transitions in oxide perovskites. Our findings allow to reconcile temperature-dependent modifications of physical properties, such as changes in the optical band gap, that are incompatible with the perovskite time- and space-average structures.