Long-wavelength phonons in mixed-valence semiconductor ${\mathrm{Sn}}^{\mathrm{II}}$${\mathrm{Sn}}^{\mathrm{IV}}$${\mathrm{S}}_3$

The results of Raman scattering and infrared reflectivity studies of the mixed-valence ternary sulphide ${\mathrm{Sn}}^{\mathrm{II}}$ ${\mathrm{Sn}}^{\mathrm{IV}}$ ${\mathrm{S}}_3$ are presented. The infrared-active phonon frequencies and the dielectric constants for all the principal directions have been determined. A group-theoretical analysis of the full space group has been performed to identify the symmetries of the observed modes. Despite the complexity of the crystal structure, the observed spectra contain a relatively small number of modes. Qualitative arguments for this behavior are presented.