We present the results of room- and low-temperature measurements of second-order Raman scattering for perfect GaN and AlN crystals as well as the Raman-scattering data for strongly disordered samples. A complete group-theory analysis of phonon symmetry throughout the Brillouin zone and symmetry behavior of phonon branches, including the analysis of critical points, has been performed. The combined treatment of these results and the lattice dynamical calculations based on the phenomenological interatomic potential model allowed us to obtain the reliable data on the phonon dispersion curves and phonon density-of-states functions in bulk GaN and AlN.

The phonon symmetry of a series of Aurivillius compounds ${\left[{\mathrm{Bi}}_2{\mathrm{O}}_2\right]}^{2+}$ ${\left[A_{n-1}{B}_n{\mathrm{O}}_{3n+1}\right]}^{2-}$ is studied by a site symmetry analysis of bulk crystals followed by the consecutive analysis of their common structural subunits, i.e., the metal-oxygen octahedra $B{\mathrm{O}}_6$ and individual layers. The division of the bulk crystals into rigid units $\left(B{\mathrm{O}}_6\right)$ allowed to distinguish in the bulk phonon spectra the rigid unit modes involving the rotations of $B{\mathrm{O}}_6$ octahedra, which may be responsible for phase transitions from the tetragonal to intermediate phases. The symmetries of the rigid unit modes were found to vary in different Aurivillius compounds leading to different transition schemes within the Aurivillius family. The division of crystals into separate layers revealed the existence of Davydov doublets and quartets in the bulk spectra originating from the the same layer modes that considerably facilitates their symmetry assignment in Raman and infrared spectra. The group-theory predictions are in a good agreement with the ab initio calculations and experimental data that supports the validity of the single layer approach.

We have investigated the optical spectra of different halogen-fullerene compounds: ${\mathrm{C}}_{60}{\mathrm{I}}_{4-x},$ ${\mathrm{C}}_{70}{\mathrm{I}}_2,$ ${\mathrm{C}}_{60}{\mathrm{Br}}_{24},$ ${\mathrm{C}}_{60}{\mathrm{Cl}}_{24},$ and ${\mathrm{C}}_{70}{\mathrm{Cl}}_{17}.$ Two types of carbon-halogen bonding have been established: (a) ${\mathrm{C}}_{60}{\mathrm{I}}_{4-x}$ and ${\mathrm{C}}_{70}{\mathrm{I}}_2$ compounds are formed by a ${\mathrm{C}}_{60}$ or ${\mathrm{C}}_{70}$ molecule sublattice and an ${\mathrm{I}}_2$ molecule sublattice that weakly interact via van der Waals forces; (b) ${\mathrm{C}}_{60}{\mathrm{Br}}_{24},$ ${\mathrm{C}}_{60}{\mathrm{Cl}}_{24},$ and ${\mathrm{C}}_{70}{\mathrm{Cl}}_{17}$ compounds are characterized by covalent bonds between C and Br/Cl atoms. We have studied in detail the resonance effects in ${\mathrm{C}}_{60}{\mathrm{Cl}}_{24}$ using the methods of Raman scattering, infrared absorption, and absorption in the visible region. The effect originates from the interactions between the phonon subsystem and the electron band at 2.33 eV and manifests itself in a resonant enhancement of the Raman line intensities and in the repetition of the phonon and the luminescence spectra shifted by the frequency of Raman-active phonon at $1508{\mathrm{cm}}^{\mathrm{-}1}.$ The group-theory analysis of phonon symmetries in rigid and nonrigid ${\mathrm{C}}_{60}{\mathrm{Br}}_{24}$ and ${\mathrm{C}}_{60}{\mathrm{Cl}}_{24}$ crystals has been performed.