Investigation of the far-IR reflection spectra of SmS single crystals and polycrystals in the homogeneity range

The far- and mid-IR reflection spectra of Sm$_{1+x}$S ($x$ = 0–0.17) samples are recorded and analyzed, as well as their electrical and structural parameters at a temperature of $T$ = 300 K. The bond ionicity in SmS is shown to fall with a decrease in the area of the X-ray coherent scattering region and an increase in the concentration of donor impurities and, consequently, conduction electron concentration. The electrical conductivity of stoichiometric SmS single crystals and polycrystals can be determined with an error of 10% from the IR reflection spectra. Due to the low structural quality of the samples, the electrical conductivity cannot be determined in the case of deviation from stoichiometry.