A comprehensive microscopic description of thermally induced distortions in lead halide perovskites is crucial for their realistic applications, yet still unclear. Here, we quantify the effects of thermal activation in CsPbBr 3 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.

Reducible oxides are effective aerobic oxidation catalysts being able to activate molecular oxygen. This ability is generally attributed to the high concentration of oxygen vacancies serving as oxygen activation sites. At the same time, the mechanism of oxygen activation remains unclear since surface oxygen activation sites cannot be easily detected using conventional methods. In this work, we unraveled the mechanism of oxygen activation over iron sites of Pt-FeO x /Al 2 O 3 during carbon monoxide oxidation using a combination of in situ and operando methods. In situ/operando XAS at the Pt L 3 and Fe K-edges, in situ Fourier transform infrared (FTIR) spectroscopy, and carbon monoxide chemisorption showed that carbon monoxide activation takes place at metallic platinum sites and is not affected by the presence of cationic iron species. Operando time-resolved Fe K-edge X-ray absorption spectroscopy (XAS) demonstrated that the Fe 2+ /Fe 3+ redox pair is directly involved in the mechanism of oxygen activation of Pt-FeO x /Al 2 O 3 . The detailed analysis of oxygen cutoff experiments demonstrated that after switching off oxygen, approximately one carbon dioxide molecule was formed for each Fe 3+ ion reduced to produce Fe 2+ . At the same time, the steady-state carbon dioxide formation rate was about twice higher than the initial rate of Fe 2+ formation after cutoff of oxygen from the catalytic feed. These experiments allude to a catalytic cycle involving electrophilic oxygen species adsorbed on iron centers as reaction intermediates. Similar mechanisms might be expected for other catalytic oxidation reactions over cationic iron of both chemical and biological importance.