Reflectivity of bismuth germanate

We have measured the reflectivity of Bi4Ge3O12 single crystals at 300 and 80 K from 4 to 40 eV. Beyond the first sharp peak at 4.57 eV, assigned to an excitonic transition, several other structures have been found between 5 and 10 eV, followed by a broad reflectivity band with maximum at about 15 eV. These structures have been assigned to transitions from energy bands derived mainly from the O 2p states and the Ge-O bonding states. Further sharp features with the Fano resonance line shape detected around 30 eV have been assigned to core excitons associated with the Bi 5d levels. From our assignments we propose a simple model for the density of states of the valence bands and lowest conduction bands of Bi4Ge3O12.