Phonon fingerprints of CsPbBr.

CsPbBr is a stable, water-resistant, material derived from CsPbBr perovskite and featuring two-dimensional Pb-Br framework separated by Cs layers. Both compounds can coexist at nanolength scale, which often produces conflicting optical spectroscopy results. We present a complete set of polarized Raman spectra of nonluminescent CsPbBr single crystals that reveals the symmetry and frequency of nondegenerate Raman active phonons accessible from the basal (0 0 1) plane. The experimental results are in good agreement with density functional perturbation theory simulations, which suggests that the calculated frequencies of yet unobserved double degenerate Raman and infrared phonons are also reliable. Unlike CsPbBr, the lattice dynamics of CsPbBr is stable as evidenced by the calculated phonon dispersion. The sharp Raman lines and lack of a dynamic-disorder-induced central peak in the spectra at room temperature indicate that the coupling of Cs anharmonic motion to Br atoms, known to cause the dynamic disorder in CsPbBr, is absent in CsPbBr.