Lattice dynamics of Bi12GeO20 crystal: Polarized Raman scattering and first-principles analysis

Our paper presents a combined experimental and theoretical study of the lattice dynamics of Bi₁₂GeO₂₀ crystal. Polarized Raman spectra were measured in two x-ray oriented single crystals. In a crystal of cubic symmetry, phonon modes of A- and E-types are spectroscopically indistinguishable. Using a special experimental technique, we separated the phonon modes that transform according to A and E irreducible representations. Since the number of E-type lines observed in Raman scattering is higher than theoretically allowed by group theory, we performed a first-principles calculation of the Bi₁₂GeO₂₀ lattice dynamics to accurately identify the symmetry of phonon modes. All experimental lines observed in Raman scattering are classified according to irreducible representations of the cubic I23 group. Using samples of different thicknesses, 10 mm and 0.120 mm, we have demonstrated that the optical activity plays an insignificant role at Raman backscattering in Bi₁₂GeO₂₀ crystal.