The fundamental optical absorption of Cs 4 PbBr 6 crystals, which are built up of nearly regular Pb 2+ (Br − ) 6 octahedra mutually bound by Cs + ions, exhibits novel features: despite the crystalline entity of Cs 4 PbBr 6 , it shows oscillatorlike absorption peaks and a wide window just above the first peak. The Cs + ions prevent the Pb 2+ 6s and 6p states from taking part in the construction of extended states. These states form a set of localized states confined to within respective octahedra, similar to the case of isolated Pb 2+ ions doped in face-centred cubic alkali halide crystals.

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The fundamental optical absorption spectra of CsPbI3 and Cs4PbI6 have been measured for the first time. There are distinct differences in the absorption characteristic between the two compounds: Cs4PbI6 exhibits strong, oscillator-like absorption and a prominent, blue-shifted excitonic peak, compared to the case of CsPbI3. The differences are explainable in terms of whether the Pb2+(I-)6 octahedra are weakly (CsPbI3) or strongly (Cs4PbI6) mutually isolated in the respective compounds

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