Optical indirect transitions of semiconducting BaPb1-xBixO3

We measured reflectance and transmittance spectra of epitaxial BaPb1-xBixO3 (x = 1.0, 0.8, and 0.6) thin films, prepared by pulsed-laser deposition. The measured spectra showed strong absorption in the frequency regions below direct energy gaps. Using the intensity transfer-matrix method, absorption coefficients were evaluated. The absorption coefficients were found to be proportional to (omega - omega(0))(2), which is a typical behavior for an indirect gap transition. The values for the indirect energy gap for x = 1.0, 0.8, and 0.6 are 0.84, 0.38, and 0.14 eV, respectively. The decrease in the indirect energy gap was explained by weakening of the breathing mode distortion of the bismuth oxygen octahedron. This evolution of the indirect gap suggests that the metal-semiconductor transition in the bismuthate be of a band-crossing type.