Ab initio insight into the Raman scattering peculiarities of Bi₁₂SiO₂₀ crystal

The proper interpretation of the experimental vibrational spectrum of Bi₁₂SiO₂₀ crystal is hindered by two particular factors. First, the crystal exhibits a high degree of optical activity. Second, the Raman scattering of cubic crystals is characterized by a peculiarity whereby vibrations of the A and E irreducible representations are essentially indistinguishable. We have applied a special experimental Raman scattering technique in order to separate the phonon modes of A and E types. The validity of our experimental results is confirmed by ab initio calculation of lattice dynamics and rigorous eigenvector analysis of vibrational modes. The phonon dispersion relations calculated along the symmetry directions of the Brillouin zone prove the dynamic stability of crystal lattice. The calculated density of phonon states allowed us to calculate the heat capacity, whose temperature dependence agrees with experimental data. Polarized Raman spectra calculated within the ab initio approach for various experimental geometries demonstrate reasonable agreement with their experimental counterparts. This finding validates the reliability of our theoretical treatment.