Lattice Vibrations and Interlayer Interactions in Crystalline ${\mathrm{As}}_2$${\mathrm{S}}_3$ and ${\mathrm{As}}_2$${\mathrm{Se}}_3$

Zone-center optical phonons in the layer crystals ${\mathrm{As}}_2$ ${\mathrm{S}}_3$ and ${\mathrm{As}}_2$ ${\mathrm{Se}}_3$ have been investigated in far-infrared reflectivity and transmission measurements (for both polarizations in the layer plane), and in Raman scattering. About a dozen infrared-active fundamentals were found for each crystal, along with a like number of Raman-active modes. Polariton dispersion curves, as well as dielectric dispersion properties and TO and LO frequencies, have been derived from oscillator analyses of the complex reststrahlen spectra. Grouptheoretical analysis of the diperiodic symmetry of the individual layer, as well as of the triperiodic symmetry of the crystal, reveals that the two symmetries predict experimentally distinguishable lattice optical properties. Our experiments demonstrate the dominance of the layer symmetry. By means of the combination of Raman and infrared results, we have been able to observe effects of the weak interlayer interactions; vibrational Davydov splittings have been resolved. A surprisingly simple scaling relation is found to connect the lattice spectra of the two crystals, and these spectra have also been compared to those of the amorphous forms.