OPTICAL-ENERGY GAP AND CRYSTAL-FIELD EXCITATIONS IN CUGEO3

The optical transmission of CuGeO3 single crystals was measured at dif ferent temperatures in tile 250 to 900 nm spectral range. Two spectral regions were analyzed corresponding to the optical absorption edge an d crystal field excitation. The energy gap values at room temperature of 3.18 and 3.54 eV as well as the energy gap temperature coefficients of 0.3 and 0.1 meV/K were determined along and perpendicular to the G eO4 chains, respectively. In the crystal field spectral region. loweri ng of temperature leads to the appearance of three absorption maxima f rom the broad spectral feature centered at 1.76 eV at room temperature . We assign these low-energy excitations to the crystal field splittin g of 3d(9) energy levels of Cu2+ ions in CuO6 octahedra.