Annealing behaviors of quench-deposited (CsCl)1−x(PbCl2)x films studied by optical absorption spectroscopy

All the films of the (CsCl)1−x(PbCl2)x system prepared by quench deposition exhibit a similar fundamental absorption spectrum over the full mixing range. On annealing the films, drastic changes occur in the spectra showing three characteristic types of spectral features depending on the mixing ratio. Type 1 (x≥0.7) suggests the formation of mixed crystals of a PbCl2 base. Type 2 (0.5≥x≥0.3) exhibits the spectrum of CsPbCl3 crystal. Type 3 (x=0.1, 0.05), the most characteristic, is the spectrum of Cs4PbCl6 crystal, which was observed for the first time. The spectrum of the Cs4PbCl6 crystal exhibits novel structures. Despite the crystalline entity of the compound, it shows oscillator-like fundamental absorption and a wide (≈1 eV) window just above the first absorption peak, suggesting the peculiarity of the associated energy band structure. To a first approximation, the spectrum is explainable in terms of Pb2+-ion excitation of the Pb2+(Cl−)6 quasi-complexes.