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.

The fundamental absorption spectrum of Cs4PbCl6 has been observed for the first time. The spectrum exhibits novel structures. They are explained in terms of Pb2+-ion excitation of the octahedral Pb2+(Cl-)6 quasi-complexes, like the case of isolated Pb2+ ions, doped in fcc alkali halides. The individual quasi-complexes behave like quantum dots. The Cs4PbCl6 crystal provides the regular array of the quantum dots

We have investigated annealing behaviours of quench-deposited films of the binary CsI-PbI 2 system by in situ optical absorption spectroscopy. Various films composed of multiple crystalline phases of CsPbI 3 and/or Cs 4 PbI 6 as well as of CsI and/or PbI 2 are obtained, depending on the mixing ratio of the CsI and PbI 2 . It is difficult to prepare films purely composed of a single CsPbI 3 or Cs 4 PbI 6 phase alone. However, it is possible to obtain films where crystallites of either CsPbI 3 or Cs 4 PbI 6 coexist with the CsI phase. Using such films, we measure the fundamental optical absorption spectrum of CsPbI 3 and Cs 4 PbI 6 for the first time. Cs 4 PbI 6 exhibits stronger oscillator-like absorption compared to CsPbI 3 , due to the localized nature of both the Pb 6s and 6p states.

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