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Description
CsPb₂Br₅ is a low-dimensional metal halide perovskite whose optoelectronic performance is strongly influenced by its lattice dynamics. To probe its vibrational behavior beyond the limits of optical spectroscopy, we performed inelastic neutron scattering (INS) measurements using the MERLIN time-of-flight spectrometer at the ISIS Neutron and Muon Source. At low temperatures (10–100 K), CsPb₂Br₅ exhibits two intense and well-resolved high-energy excitations near 36 and 42 meV. These features do not correspond to any zone-center modes reported in Raman or infrared spectroscopy and are not predicted by existing density functional theory (DFT) phonon calculations. Their presence suggests the involvement of collective or anharmonic excitations that are suppressed or inaccessible in optical measurements, highlighting the unique capability of neutron scattering to reveal hidden vibrational dynamics. Ongoing first-principles simulations (VASP, CASTEP) will refine mode assignment and assess the accuracy of current computational models. This work contributes to the ISIS-BR Collaboration in advancing perovskite lattice-dynamics research.