The Sm2S3-X-SmS-Sm2O2S refractory system: thermal analysis, phase diagram, and properties of the phases

Yurev, I.O., Aleksandrovsky, A.S., Kamaev, D.N., (...), Parfenova, M.D., Andreev, O.V.// Journal of Thermal Analysis and Calorimetry//

Samarium monosulfide, a strain gauge and barometric material, exists in equilibrium with Sm3S4 and Sm2O2S in the S-Sm–O system. Therefore, studying phase equilibria in the refractory Sm2S3-X-SmS-Sm2O2S system is a scientifically interesting task. In this system, 49 samples were synthesized and studied by powder XRD, differential scanning calorimetry, visual thermal analysis, and microstructural analysis. Melting points of Sm3S4, SmS, and Sm2O2S compounds were determined. Eutectic diagrams of Sm3S4-Sm2O2S, SmS-Sm2O2S, SmS-Sm3S4 systems were constructed. Temperatures and compositions of the binary eutectic points were determined. Fusion enthalpies for Sm3S4, SmS, and Sm2O2S phases were estimated using the Schröder–Le Chatelier equation. The liquidus lines were calculated using second-degree polynomials and Redlich–Kister model. Coordinates of the ternary eutectic point in the Sm3S4-SmS-Sm2O2S system were calculated using the cutting-plane method and the Scheffé method. The calculated compositions of ternary eutectic points were averaged at one most probable point, in accordance with the data on the samples microstructure. The experimental temperature of the ternary eutectic point coincides with the calculated values within the margin of error. Positions of eutectic valleys and approximate positions of isotherms in the system were established. Thermodynamic parameters of the α-Sm2S3 → γ-Sm2S3 polymorphic transition and the dependence of the Sm2S3-X composition on heat treatment conditions were determined. According to the scanning electron microscopy data, the approximate composition of the crystallized from the melt Sm2S3 sample is Sm2S2.95. The Sm10S14O phase decomposes at 1470 ± 15 °C in the course of a solid-phase reaction. The phase diagram of the Sm2S3-X-Sm2O2S system was revisited. Optical band gaps of Sm10S14O and Sm2O2S phases were determined. The Sm10S14O compound was optically characterized for the first time; its direct and indirect optical bandgaps were found equal to 2.48 and 2.37 eV, respectively. The determined direct and indirect optical bandgaps of Sm2O2S (4.4 eV and 3.95 eV, respectively) agree with the earlier measurements, thus confirming the accuracy of the chosen synthesis procedures.