Acknowledgement: this work was funded by the Max Planck-EPFL-Center for Molecular Nanoscience and Technology, the Max Planck Society, the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (Grant Agreement Number ERC-2015-CoG-682843), and partially by the Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 182087777 - SFB 951. H.S. acknowledges support by the Swiss National Science Foundation under Grant No. P2SKP2_184100. V.C.A.T. acknowledges financial support from the Alexander von Humboldt Foundation. M.I.B. thanks for the support from the Swiss National Science Foundation (grant number 200021_192308, project Q-Light). M. I. B. and M.V.K acknowledge support by the Research and Innovation Foundation of Cyprus, under the "New Strategic Infrastructure Units-Young Scientists" Program, grant agreement number "INFRASTRUCTURES/1216/0004", Acronym "NANOSONICS". M. I. B. and M.V.K are grateful for the use of the Empa Electron Microscopy Center facilities. Y.W.W acknowledges funding from the DFG within the Emmy Noether program under Grant No. RE 3977/1, as well as the TRR 227 (Project A10).

Femtosecond electron diffraction data of CsPbBr3 nanocrystals, acquired at the Fritz Haber Institute in Berlin. All measurements are performed at room temperature.