Chemistry and technology of samarium monosulfide

Abstract

Samarium monosulfide SmS (Fm3m, а = 5.967 Å, ΔЕ = 0.23 V, n = 1020 cm-1, σ = 500 Ω-1 cm-1, α = 350 µВ/K) is a thermoelectric material (Z>1) and, at the same time, a pressure-sensitive material (K≥40−50). Samarium monosulfide is a daltonide phase with a solid solution whose extent is mostly in the range of cationic vacancies: Sm1+x S1-x 2x (x = 0−0.035; 1750 K). The congruent melting temperature of SmS is 2475 K. In the Sm-S system, Sm3S4 crystallizes from melt without change in composition. Samarium monosulfide thermally dissociates to Sm3S4 and Sm. Large-scale SmS lots are produced from samarium and sulfur. Synthesis is carried out in sealed-off silica glass ampoules at 500−1350 K followed by heat treatment in tantalum crucibles at 1500−2400 K. The reaction of metal samarium with sulfur results in the formation of sulfide phases that coat the samarium surface in the following order: SmS, Sm3S4, Sm2S3, and SmS2. Subsequent annealing at 1500-1800 K provides SmS yields up to 96−97 mol %. Equilibrium minor phases for SmS are Sm3S4, Sm2О2S, and Sm. X-ray amorphous SmS was prepared by reacting organic samarium compounds with sulfur or H2S. The samarium (+2) oxidation state determines the chemical specifics of SmS. 90−120 µm SmS powders are thermally hydrolyzed starting at 600 K with Н2evolution and oxidize starting at 520 K to yield Sm3S4 and then Sm2О2S phases. A 90−120 µm SmS fraction for film deposition by flash evaporation is prepared by milling annealed SmS samples. Tablets 75 mm in diameter for use in magnetron sputtering are pressed from a <60-µm fraction.