Chemistry and Technology of Samarium Monosulfide

<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>

Pulsed Laser Induced Melt and Phase Transformation of Ni Silicide Layers on Si Substrate

Thermally grown Ni2 Si and NiSi2 layers on <111> Si substrates were irradiated by 40 ns Nd laser pulses in the energy density range 0.3–2.0 J/cm2. The samples were analyzed by time-resolved reflectivity, 2.0 MeV He+ Rutherford backscattering in combination with channeling and by transmission electron microscopy. In the NiSi2/Si system the melt starts at the free surface (1280 K) and propagates towards the inside. Dissolution of substrate silicon atoms occurs when the silicon temperature reaches the liquidus temperature (1400 K). In the Ni2Si/Si samples the melt starts instead at the interface when it reaches the eutectic temperature (1250 K). The subsequent propagation towards the surface is limited by the mass transport of silicon atoms to maintain a composition near that of the eutectic. In some cases the surface may melt also at the congruent melting temperature (1570 K), giving rise after solidification to a quite complex structure. The different behaviour of the two silicides/silicon systems is explained in terms of phase diagram.