Infrared studies of lattice vibrations in Bi12GeO20 and Bi12SiO20 crystals

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Abstract
The reflection spectra of Bi12GeO20 and Bi12SiO20 single crystals are studied in the spectral region 50 to 1000 cm−1 at 90 and 300K. Both the single crystals are found to exhibit several reflection bands which are ascribed to the interaction of electromagnetic radiation with normal modes of F‐symmetry in the central region of the Brillouin zone. The frequencies of the longitudinal (LO) and the transverse (TO) component is calculated from the obtained reflection spectra by the Kramers‐Kronig method. The results are compared with Raman data. In order to verify the applicability of the notion of molecular crystal to the compound studied, the frequencies of the normal modes of the groups GeO4 and SiO4 are calculated and the results are compared with the experimental data obtained in this work.