Optical absorption and localized electronic states in amorphous CsPbX3, PbX2and TlX (X = Cl, Br)

Systematic comparison of the absorption spectra in the amorphous state has been made between ionic compounds (CsPbX 3 , TlX and PbX 2 with X = Br and Cl) containing 6s 2 ions as a constituent. The spectra exhibit several common features including nearly the same optical energy gap observed for all the compounds with the same X, despite the largely different spectral outline in their crystalline state. The common features are related to strong localization of the one-electron states of the 6s 2 ions. In contrast to the case of covalent semiconductors, the introduction of the long-range order due to crystallization gives rise to a significant change of the density of states related to the 6s 2 ions.