Electronic transitions in doped and undoped copper germanate

We report an analysis of the absorption edge anisotropy in CuGeO₃ based on a comparison of polarized optical data with electronic structure calculations in the framework of the extended Hückel tight binding model. Taking the z axis as parallel to the CuO₄ chains, we ascribe the absorption edge in the magnetic chain direction to O 2p_x, 2p_y → singly filled Cu 3d transitions and the edge in the transverse direction to O 2p_z → singly filled Cu 3d excitations. The dual slope in the transverse direction is a direct consequence of the double maximum in the O 2p_z density of states. The influence of Zn and Si doping on the electronic spectra of CuGeO₃ is analyzed as well. Whereas neither impurity affects the phonon-assisted d-d band, Si doping smears the charge-transfer gap. This smearing of the gap by interchain impurity substitution is attributed to random distortions of the crystalline lattice giving rise to structurally induced changes in the electronic properties.