Inelastic Neutron Scattering Study of the Optically Excited State of MAPbBr₃

Halide perovskites (HPs) have emerged as technologically appealing materials for a vast array of optoelectronic applications. However, fundamental questions surrounding the local structure and vibrational dynamics remain to be elucidated for these materials, especially regarding how they respond to the formation of photoexcited polarons. Here, in an inelastic neutron scattering (INS) study of the prototypical HP MAPbBr₃ (MA = methylammonium, ${\mathrm{CH}}_3{\mathrm{NH}}_3^{+}$ ), we show that the formation of photoexcited polarons upon LED (light-emitting diode) illumination at 540 nm leads to changes in the INS spectrum. A comparison to INS spectra measured at different temperatures shows that the spectral change upon LED illumination originates from a change in the local coordination of the MA cations, rather than being only thermal in nature. This is primarily manifested as a change in the shape and intensity of the three strongest vibrational bands, located at ∼93, 110, and 295 cm^-1, which are related to different MA vibrations coupled to vibrational modes of the PbBr₆ octahedra. This new insight, together with the unique sensitivity of INS to hydrogen and its absence of vibrational selection rules, motivates further work utilizing in situ light illumination techniques in INS experiments as a route to develop a better understanding of the local structure and dynamics in HPs under operationally relevant conditions.