Optical phonon anisotropies in the layer crystals SnS2and SnSe2

We have measured the far-ir reflectance spectra for the layer crystals Sn${\mathrm{S}}_2$ and Sn${\mathrm{Se}}_2$ for the two principal polarizations, $\overrightarrow{E}\perp \overrightarrow{\mathrm{c}}$ and $\overrightarrow{E}\parallel \overrightarrow{\mathrm{c}}$. We find a large difference in the frequencies of the two ir-active phonons, ${\nu }_{\mathrm{TO}}\left(A_{2u}\right)>\mathrm{}{\nu }_{\mathrm{TO}}\left(E_u\right)$. Analysis of the lattice dynamics indicates that this anisotropy is due to Coulomb forces associated with charge localized on the atomic sites and as such it provides an empirical measure of the bond ionicity. On the other hand, anisotropies in the Raman modes are explained by noncentral intralayer forces, with smaller contributions from interlayer and Coulomb terms.