Optical phonons inSnGeS3

The polarized infrared and Raman spectra of ${\mathrm{SnGeS}}_3$ are reported. Infrared-active phonon frequencies and dielectric constants are obtained by oscillator fitting of reflectivity data. A group-theoretical analysis of the crystal, one of its constituent layers, and the (${\mathrm{GeS}}_2$ ${\mathrm{S}}_{2/2}^{2\mathrm{-}}$ ${)}_{\mathrm{\infty }}$ chain has been performed to identify the symmetries of the observed modes. A force-constant anisotropy of about 50 between the intralayer and interlayer forces is deduced from rigid-layer mode and Davydov splitting observations.