Optical properties in the form of the complex dielectric function spectra (ε = ε1 + iε2) of commercially produced single crystal bismuth germanate (Bi4Ge3O12) have been studied using spectroscopic ellipsometry from 0.033 to 6.478 eV. Transmission measurements have also been performed in order to more accurately obtain low values of the absorption coefficient close to the band edge. Critical point parameters have been determined by analyzing the interband transitions. Infrared extended measurements yield vibrational modes corresponding to chemical bonding environments and the lattice structure. The overall analysis yields ε from 0.033 to 6.478 eV and provides information about the crystal such as the energy of the band gap at 4.16 ± 0.01 eV; interband transitions at 4.49, 4.75, 4.81, 5.08, 5.59, and 6.08 eV; transverse optical phonon modes at 283, 364, 395, 447, 702, 729, and 778 cm−1 and longitudinal optical phonon modes at 292, 386, 445, 458, 710, 779, and 817 cm−1.

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