Normal modes in the vicinity of the brillouin zone center of tightly bound units in Bi12GeO20 and Bi12SiO20

From Raman data and a group-theoretical analysis, we deduce that in Bi₁₂GeO₂₀ and Bi₁₂SiO₂₀ there are four weakly bound oxygen atoms and a tightly bound unit MO₄, where M = Ge or Si. Some normal modes of the tightly bound units are identified in Raman and in infrared reflection data. The kinds of interatomic binding forces in these crystals are determined. The results are compared with structure data determined by X-ray scattering.