Optical energy gap and crystal field excitations in CuGeO3

The optical transmission of CuGeO 3 single crystals was measured at different temperatures in the 250 to 900 nm spectral range. Two spectral regions were analyzed corresponding to the optical absorption edge and crystal field excitation. The energy gap values at room temperature of 3.18 and 3.54 cV as well as the energy gap temperature coefficients of 0.3 and 0.1 meV/K were determined along and perpendicular to the GeO 4 chains, respectively. In the crystal field spectral region, lowering of temperature leads to the appearance of three adsorption maxima from the broad spectral feature centered at 1.76 eV at room temperature. We assign these low-energy excitations to the crystal field splitting of 3d 9 energy levels of Cu 2+ ions in CuO 6 octahedra.