Spectral analysis of optical absorption near the fundamental edge in amorphous lead halides

Abstract

Absorption spectra of amorphous lead halides are analyzed in the range near the fundamental edge. Amorphous PbI2 is shown to be a typical material obeying the Tauc law. In contrast, amorphous PbX2 (X = Br, Cl, and F) are characterized by a prominent first absorption band whose shape is well fitted by a skewed Gaussian. Based on the obtained fitting parameters, an “ion-glass” model is introduced, in which each Pb2+ ion is surrounded by nine neighboring X− ions without any well-defined site symmetry around it. By assuming an effective Pb2+X− distance d d for the superionic assembly [PbX9]7−, its fluctuation Δd in the disordered network is roughly estimated, resulting in the full width at half maximum about 0.05 for Δd/d.