Infrared vibrational spectrum of crystallineAs2S3

The lattice vibrational spectrum of the layered crystal ${\mathrm{As}}_2$ ${\mathrm{S}}_3$ has been measured with the ir electric field polarized along the three principal axes ($\overrightarrow{\mathrm{E}}\parallel \overrightarrow{\mathrm{a}},\overrightarrow{\mathrm{b}},\overrightarrow{\mathrm{c}}$). The out-of-plane absorption ($\overrightarrow{\mathrm{E}}\parallel \overrightarrow{\mathrm{b}}$) is substantially weaker than that observed in-plane ($\overrightarrow{\mathrm{E}}\parallel \overrightarrow{\mathrm{a}},\overrightarrow{\mathrm{c}}$). The TO- and LO-phonon frequencies are derived from an oscillator fit to the data. Values of the low- and high-frequency dielectric constants are in substantial agreement with independent measurements.