Chemistry and Technology of Samarium Monosulfide

Abstract

<p class="Pa10">Samarium monosulfide SmS (Fm3m, а = 5.967 Å, ΔЕ = 0.23 V, n = 10²⁰ cm^–1, <em>σ</em><em> </em>= 500 Ω^–1 cm^–1, <em>α</em><em> </em>= 350 μВ/K) is a thermoelectric material (Z&gt;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: Sm_1+xS_1-x□_2x (<em>x </em>= 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 Н₂ evolution 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 &lt;60-μm fraction.</p>