  The development of next generation perovskite-based optoelectronic devices
relies critically on the understanding of the interaction between charge
carriers and the polar lattice in out-of-equilibrium conditions. While it has
become increasingly evident for CsPbBr3 perovskites that the Pb-Br framework
flexibility plays a key role in their light-activated functionality, the
corresponding local structural rearrangement has not yet been unambiguously
identified. In this work, we demonstrate that the photoinduced lattice changes
in the system are due to a specific polaronic distortion, associated with the
activation of a longitudinal optical phonon mode at 18 meV by electron-phonon
coupling, and we quantify the associated structural changes with atomic-level
precision. Key to this achievement is the combination of time-resolved and
temperature-dependent studies at Br K-edge and Pb L3-edge X-ray absorption with
refined ab-initio simulations, which fully account for the screened core-hole
final state effects on the X-ray absorption spectra. From the temporal
kinetics, we show that carrier recombination reversibly unlocks the structural
deformation at both Br and Pb sites. The comparison with the
temperature-dependent XAS results rules out thermal effects as the primary
source of distortion of the Pb-Br bonding motif during photoexcitation. Our
work provides a comprehensive description of the CsPbBr3 perovskites
photophysics, offering novel insights on the light-induced response of the
system and its exceptional optoelectronic properties.
