Physicochemical analysis of the SmS – Sm2O2S system

ABSTRACT

Samarium monosulfide SmS (Fm3m, a=5,967 Ǻ, T_mt = 2473 K, ∆Е = 0,23 В, n = 10²⁰sm^-1, ϭ = 500 Ом ^-1 sm^-1, α =350 mkV/K). Samarium monosulfide is a daltonide phase with a solid solution, the length of which is mainly in the range of cation vacancies: Sm_1+x S_1-x*2x (x = 0 – 0,035; 1750K) [1].  Sulfate-sulfide compounds located in the compact part of the concentration triangle, and oxide-sulfide bonds of the Sm₂O₃-Sm₂S₃ section. The nature of the phase equilibrium of the Sm-S-O system is largely determined by the form of the Sm₂O₃-Sm₂S₃ phase diagram. The annealing temperature of 1873-1973 K provides for the production of ceramics for sintering and the required mechanical properties (not less than 1.2-1.5 MPa, bending strength not less than 35-40 MPa). As a result, it is close to the process, with a composition of 55 mol.% SmS. For clarity, 55 and 60 mol%.

АННОТАЦИЯ.

Моносульфид самария SmS (Fm3m, a=5,967 Ǻ, T_пл = 2200^оС, ∆Е = 0,23 В, n = 10²⁰см^-1, ϭ = 500 Ом ^-1 см^-1, α =350 мкВ/К). Моносульфид самария – это дальтонид фаза с твердым раствором, протяженность которого в основном находится в диапазоне катионных вакансий: Sm_1+x S_1-x*2x (x = 0 – 0,035; 1750K) [7].   Сульфатно-сульфидные соединения, находящиеся в компактной части концентрационного треугольника, и оксидосульфидные связи разреза Sm₂O₃-Sm₂S₃. Характер фазового равновесия системы Sm-S-O во многом определяется видом фазовой диаграммы Sm₂O₃-Sm₂S₃. Температура отжига 1873-1973 К обеспечивает получение керамики для спекания с необходимыми механическими свойствами (не менее 1,2-1,5 МПа, прочность на изгиб не менее 35-40 МПа). В результате он близок к технологическому, с составом 55 мол.% SmS. Для наглядности 55 и 60% мол.

Keywords: REE, X-ray diffraction patterns, Van Laar equation, diffractometer, kinetic properties, oxysulfide, double eutectic, phases, phase equilibria, polycrystalline.

Ключевые слова: РЗЭ, РФА – рентгнофазовый анализ, Уравнение Ван Лаара, дифрактометр, кинетические свойства, оксисульфид, двойная эвтектика, фазы, фазовые равновесия, поликристалл.

Introduction.

The preparation process is divided into two main groups depending on the phase composition of the polycrystalline reaction product: the formation of Ln₂O₂S as the only polycrystalline phase and the preparation of several polycrystalline Ln₂O₂S phases [8].        When SmS is synthesized from chemical elements in a 1Sm: 1S ratio by the ampoule method, the resulting mixture is heterogeneous and contains only up to 80 mol% SmS. When sulfiding rare earth oxides by the combined action of hydrogen sulfide and carbon disulfide in the Russian Scientific Center, the method of synthesis of 1.5 sulfides was widely used [3]. Ammonium thiocyanate NH₄SCN is widely used as a starting material for the preparation of mixtures of H₂S and CS₂ gases. If the decomposition of ammonium thiocyanate occurs at a temperature of 470 to 500 K, the reaction can be represented by the following scheme (R is a water-insoluble precipitate of an organic melamine compound) [12]:

NH₄SCN → CS₂ + H₂S + NH₃ + R .

Sulfation with carbon disulfide is more energy efficient than hydrogen sulphide and allows the reaction temperature to be reduced by 200–300 K [2].

Under the influence of an inert carrier gas (argon), sulphide gases enter the sulphide synthesis reactor. Argon flow control allows the rate required to ensure a continuous supply of sulphide gas to the reaction zone. In the synthesis, horizontal and vertical reactors can be used, but the latter provides a more complete contact between the solid (starting material, intermediate synthetic product) and gas (sulfonated gas), passing the gas through the entire layer of material. Sulfide gases that pass through the charge must be disposed of. In the general case, the equation for the sulfiding of oxides has the form [14]:

Ln₂O₃ + 3H₂S + 3/2CS₂ = Ln₂S₃ + 3H₂O + 3/2 CO₂.

In addition to oxides, nitrates, carbonates, and rare earth oxalates, they are used as starting compounds. In the cold part of the reactor and inside the gas tube, ammonium sulfide crystallizes and forms upon interaction with a gaseous agent:

H₂S + 3NH₃ →(NH₄)₂S.

The synthesis temperature varies from 1000 to 1470K [1], depending on the sulfurous material. The average synthesis time is from 10 to 20 hours, and the synthesis scheme can be represented by the continuous transformation Ln₂O₃ → Ln₂O₂S → Ln₁₀S₁₄O → Ln₂S₃. During the synthesis of La2S3, the sulfidation reaction stops with the formation of Ln₁₀S₁₄O on oxysulfide [15]. There is thermodynamic stability. Then the Ln₁₀S₁₄O phase is exposed to high temperature (T 1600 K) in sulfur vapor. The final synthetic product is γ- Ln₂S₃. In other REEs, there is no stage of Ln₁₀S₁₄O production [4].

Figure 1. Units for the synthesis of sulfides of rare earth elements in a stream of sulfiding gases:

1 - Argon cylinder with inert gas, 2 - NH4SCN melt, 3 - heating mantle, 4 - electric furnace, 5 - processed substance, 6 - thermocouple, 7 - programmable heating controller "Thermolux", 8 - water.

The processing of SmS samples at temperatures above 1800 K will lead to the decomposition of SmS into Sm and Sm₃S₄. The impurity phase of Sm₂O₂S always exists in the charge. According to the established chemical properties of the interaction of metal Sm and sulfur in a closed ampoule and phase equilibrium in the Sm–Sm₂S₃–Sm₂O₃ system, it can be determined that the solid solution Sm_1+x S_1-x ([Sm])_1-y 1- exceeds 98.5 mol. %. Synthesis parameters of the mixture y [ ]_x)_2x (х = 0–0,035, y = 0–1), excess Sm. Powdered industrial ceramics were obtained from the charge, consisting of particles formed from crystals with a size of 90-110 microns, and a sintered sheet target with a diameter of 75 mm. In the case of direct contact with the thermal field of a tungsten heater with a temperature of 2570-2670 K, the use of a solid solution in the process of thermal explosive spraying can ensure its preferential evaporation. The gain of the strain gauge SmS statistically increases [9].

Figure 2.  Phase position in triangle Sm-S-O; the legs are installed experimentally

The samples were studied by X-ray (RFA) (diffractometer DRON-6, CuKa radiation Fe filter), visual poly thermal (VPTA), microstructural (MSA) (Microstructural analysis was performed on a metallographic microscope AxioVert .A1MAT manufactured by ZEISS (Germany), resolution 0.5 μm. The image from the microscope was transmitted to a computer through a video camera[11]. Software complex AxioVision SE64) analysis was used for obtaining photos of grain sizing. Cell parameters are calculated using the High Score Plus program (data collection for the Drone-6 set; diffrac.file exchange. V5) with accuracy ± 0.001 and ± 0.0001 nm for rhombic and cubic structures, respectively. Mathematical processing of data of thermal methods of research and graphical constructions are performed in the program Edstate 2D. Evaluation of melting heat was carried out according to Van Laar equation [3].

Results and their discussion.

The initial charge contains an excess of samarium metal (1.2Sm: 1S). Metallic samarium increases the partial vapor pressure of the metal. At temperatures above 1023 К, metallic samarium begins to noticeably evaporate, being adsorbed on the walls of the ampoule, minimizing the contact between the synthesized sample and the walls of the ampoule[17]. At a temperature of 1023-1173 К, the vapor pressure of samarium in the ampoule increases, which leads to a shift in the equilibrium of the reaction product. If a chemical element with a 1Sm: 1S ratio is placed in an ampoule, it is best to obtain a batch containing up to 80 mol % SmS during the synthesis [13]. This charge contains more than 20 mol% of Impurity phases: Sm, Sm₃S₄, Sm₂S₃.

When the initial composition of the filler is 1.2Sm: 1S and the synthesis is carried out under the temperature conditions specified in this patent, the following sulfide phases are 100 mol%: 92.5 mol% SmS, up to 7.5%  Sm₂O₂S, according to X-ray structural analysis. The filler containing the Sm₃S₄ phase was not found in trace amounts. As a result of the compact structure of the sample particles, SmS particles are mainly formed in the form of lumps with a size of 15-25 microns and thermal dissociation of SmS_2-х, small particles with a size of 2-6 microns are formed.

Figure 3. Diffraction pattern of the low-temperature modification of SmS. Shooting conditions: CuKα, Ni-filter. X-ray diffraction pattern of a sample of the SmS- Sm₂O₂S system containing 99.5 mol. % SmS

The values ​​of the melting points required for the calculation were taken from the literature data:

Tm (Sm₂O₂S) = 2370 К, Tm (SmS) = 2475 К.

In this system has a value of 2170 К, so the temperature range 2320-2020 К was used for the calculation.

As a result, the calculation gave a range of concentrations for component A: 23.08% -75.61%; component B: 76.92-24.39%.

The melting point required for the calculation was obtained from the literature: T_m (Sm₂O₂S) = 2370, T_m (SmS) = 2475 К.

In the literature, the melting point of the eutectic point in this system is 2170 К; therefore, the temperature range of 2320-2020 К was used for the calculation.

Based on the calculated concentration range, a sample of the starting material was first measured in an area close to the published data. The result is 53 mol. % Composition of the SmS sample, but its microstructure did not correspond to the eutectic. A concentration range of 50 to 70 mol% SmS was then taken and the samples were weighed at 5% intervals. As a result, the inventors obtained a sample with a microstructure close to the process, with a composition of 55 mol% SmS. For clarity, 55 and 60 mol%. Two more samples were obtained with the composition% SmS. Eutectic is represented by a mixture of small crystals of particles of samarium sulfide and green oxysulfide. X-ray phase analysis (Fig. 4.)

Figure 4. X-ray diffraction pattern of a sample of the SmS - Sm₂O₂S system containing 45 mol% Sm₂O₂S

Conclusions.

During the work, phase equilibria in the SmS- Sm₂O₂S system were studied. The methods described in the literature were used to synthesize starting materials with a main phase content of at least 99%, which were used for the synthesis of samples. The composition of the precursors was confirmed by X-ray phase analysis. After obtaining a sample with a composition calculated on the basis of experimental data, the point of two eutectics was also not found, but it turned out to be close in its microstructure to the desired one. After correcting the composition, based on the assumption about the influence of the position of the eutectic in the Sm₂S₃ – Sm₂O₃ system on the position of the two eutectic in the Sm – S - O triangle, an exact eutectic composition was obtained containing all two components in an equilibrium state, which was confirmed by X-ray phase analysis data.

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