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CHEMICAL PROBLEMS 2020 no. 2 (18) ISSN 2221-8688
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UDC 546.814.86.23
REFINEMENT OF THE PHASE DIAGRAM OF THE SnSe - Sb2Se3 SYSTEM E.N. Ismailova1*, I.B. Bakhtiyarly1, M.B. Babanly12
1Acad. M. Nagiev Institute of Catalysis and Inorganic Chemistry, National Academy of Sciences of Azerbaijan, 113, H. Javid ave., AZ1143 Baku, Azerbaijan
2 Baku State University, Z. Khalilov 23, Baku, AZ 1148, Azerbaijan e-mail: * [email protected]
Received 15.01.2020 Accepted 18.03.2020
Abstract: Considering the inconsistency of the available literature data on phase equilibria in the SnSe -Sb2Se3 system, the issue was re-examined by differential thermal analysis and X-ray diffraction technique. A new, refined version of the phase diagram was constructed. It was found that the ternary compound Sn2Sb2Se5 and the intermediate y -phase with a homogeneity region of48-60 mol% Sb2Se3 are formed in the system, which melting with decomposition by peritectic reactions at 598°C (Sn2Sb2Se5) and 560°C (y-phase). This area includes stoichiometric compositions of ternary compounds SnSb2Se4 and Sn^bcSieu. A comparative analysis of the results obtained with literature data was carried out. Keywords: SnSe - Sb2Se3 system, phase diagram, eutectic, tin and antimony selenides. DOI: 10.32737/2221-8688-2020-2-250-256
Compounds formed in the AlvX - Bv2X3 (Alv- Sn, Pb; Bv-Sb, Bi; X-Se, Te ) quasi-binary systems are promising functional materials. In particular, compounds of the AIVBV2Te4, AIVBV4Te7, AIVBV6Teio types with tetradimite-like layered structure had higher thermoelectric parameters as compared to AivTe and B2Te3 binary compounds [1-5]. In addition, recent studies show that most of these compounds are three-dimensional topological insulators and can be used in spintronics [6-9]. Alloys of the Sn -Sb - Se system are of interest in thermoelectric materials in both crystalline and glass formed states [10-12].
The development of methods for synthesis of new complex phases was based on phase equilibriia data in the corresponding systems [13-16].
The development of methods for synthesis of new complex phases is based on data on phase equilibria in the corresponding systems [13-16An analysis of the literature data [17-19] on the phase equilibria in the SnSe -Sb2Se3 quasi-binary system shows that they are of controversial nature. According to [17], it was characterized by the formation of one
congruently melting ternary compound SmSb6Sen (563°C). Two eutectics were detected in the system which crystallized at 553° C and 550° C and had compositions of 54.5 and 61.5 mol % Sb2 Se3, respectively. According to [18], two compounds were formed in the system: SmSb6Seii with congruent melting at 561°C, and SmSb2Se5 - with incongruent melting at 563°C. Solid solutions based on both crystal modifications of SnSe were detected. The authors of [19] showed a formation of a single ternary compound, SnSb2Se4 in the SnSe - Sb2Se3 system. This compound crystallized in an orthorhombic structure (Sp.Gr. Pnnm) with lattice parameters a = 26.610 Â, b = 21.066 Â and s = 4.0423 Â [20]. According to [21], the SmSb2Se5 compound also had an orthorhombic structure (Sp.Gr. Pnnm, a = 35.16 Â, b = 25.96 Â, c = 4.14 Â). We have not found data on the crystal structure of a compound with the composition Sn2Sb6Se11.
In this paper, we present a new refined variant of the T - x diagram of the SnSe - Sb2Se3 quasi-binary system and the comparative analysis with literature data attached.
CHEMICAL PROBLEMS 2020 no. 2 (18)
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The starting compounds of the reviewed system were explored in detail. The SnSe compound melts congruently at 880°C [22] and crystallizes in an orthorhombic structure (Sp.Gr. Pcmn) with lattice parameters a = 4.44175, b =
4.15096, c = 11.49417 [23]. The Sb2Se3 compound also melts congruently at 600°C [22] and crystallizes in an orthorhombic structure (Sp.Gr. Pnma, a = 11.7808 A, b = 3.9767 A and c = 11.6311 A [24].
Experimental part
Materials and synthesis
Compounds SnSe and Sb2Se3 were synthesized using high-purity antimony (Sb-00002, 99.999%), tin (Sn-00005, 99.999%), selenium (Se-00002, 99.999%) purchased from Evochem Advanced Materials GMBH (Germany). Stoichiometric mixtures of elementary components were placed in a quartz ampoule which was evacuated to a residual pressure of ~ 10-2 Pa. The synthesis of SnSe was carried out in a dual-zone inclined furnace. The temperature of the lower "hot" zone was 930 °C, and that of the upper "cold" zone was up to 630 °C which was somewhat lower than the boiling point of selenium (685 °C) [25]. Sb2Se3 was synthesized in a one-zone furnace at 650°C.
The identity of both synthesized compounds was controlled by DTA and XRD. The obtained melting points and crystal lattice parameters within deviation (± 2 °C and ± 0.0003 A) were in good agreement with the
literature data.
Alloys of the studied system were prepared by fusion of the starting compounds at various ratios in evacuated quartz ampoules followed by homogenizing annealing at 450 °C for ~ 500 h and quenching into cold water.
Analysis
For the analysis of samples, differential thermal analysis (DTA) and X-ray powder diffraction (XRD) were used. DTA was performed using the NETZSCH 404 F1 Pegasus system with the heating rate reaching 10 °C min-1. Temperatures of thermal effects were taken mainly from heating curves. Accuracy of temperature measurement was ± 2°C.
Powder X-ray diffraction patterns were obtained on Bruker D2 Phaser diffractometer (Cu Ka1 radiation) at a room temperature. The X-ray images were indexed using Topas V3.0 software Bruker.
Results and its discussion
Results of XRD of annealed samples are shown in Fig. 1. As can be seen, samples containing 33.3; 50 and 60 mol. % Sb2Se3 had diffraction patterns that differed from original components. On the other hand, samples containing 50 and 60 mol.% Sb2Se3 had the same qualitatively diffraction patterns. This confirms the existence of individual phases with compositions SmSb2Se5 SnSb2Se4 and Sn2Sb6Seii and shows that the latter two are isostructural to be within the homogeneity region of some intermediate phase of variable
composition.
The diffraction patterns of samples containing 20 and 80 mol.% Sb2Se3 were two-phase mixtures of SnSe + SmSb2Se5 and SmSb6Seii + Sb2Se3, respectively. A comparison of diffraction patterns with published data [18, 20, 21] shows that they were identical to the known SmSb2Se5 and SnSb2Se4 compounds. Types and parameters of crystal lattices of intermediate phases were determined on the basis of the powder diffractograms:
Sn2Sb2Se5 : Sp.Gr. Pbnm a = 35.08(28) Â, b = 25.87(22) Â, c = 4.09 (6) Â. SnSb2Se4 : Sp.Gr..Pnnm a =26.605 (25) Â, b =21.049 (20) Â, c = 4.0385 (5) Â.
2Thwla (Coupled TwoThetefTtieU) VML=1 HMO
Fig. 1. X-ray powder diffraction patterns for alloys of the SnSe - Sb2Se3 system.1- Sb2Se3; 2- 80 mol% Sb2Se3; 3- 60 mol% Sb2Se3; 4- 50 mol% Sb2Se3; 5- 33.3 mol% Sb2Se3; 6- 20mol% Sb2Se3; 7-SnSe
The powder diffraction pattern (Fig. 2) of a sample with a composition of 60 mol% Sb2Se3 (Sn2Sb6Seii) was also completely indexed based on the crystallographic data of SnSb2Se4: a = 26.604 (25), b = 21.068 (25), c = 3.8265 (5).
A comparison of crystal lattice
parameters of phases with the SnSb2Se4 u SmSb6Sencompositions shows that increase of the Sb content led to a significant decrease in the parameter c which was due to the crystallographic radii of Sn2+ (1.02 A) [26] and Sb3 + (0.76 A) [27]
Fig. 2 Powder XRD pattern for the alloy with composition 60 mol% Sb2Se3 (SmSb6Seii). Red lines - data of [20] for the SnSb2Se4 compound.
A phase diagram of the SnSe-Sb2Se3 system was constructed based on the DTA results with due regard for RFA data (Fig. 3). As can be seen from the Fig.3, the system is
quasibinary and has two intermediate phases that decompose according to the peritectic reaction. The compound SmSb2Se5 crystallizes at 598 °C according to the
L + a' ~ SmSb2Se5 reaction. (a' is a solid solution based on a high-temperature modification of SnSe).
The second intermediate phase (y) had a wide (48-60 mol.% Sb2Se3) region of homogeneity and crystallized at 560 °C by L + Sn2Sb2Se5 ~ y Compositions of peritectic points pi and p2 were 40 and 65 mol.% Sb2Se3, respectively. An eutectic with coordinates 72 mol.%
Sb2Se3 and 545 °C was found in the system. Based on both initial components, there were limited regions of solid solutions (a', a- and P-phases).
Thermal effects reflecting the phase transition in SnSe-based solid solutions were not found on the DTA curves. Apparently, these effects overlap with more intense peaks of the peritectic reaction (598 °C).
Fig. 3. Phase diagram of the SnSe- Sb2Se3 system
The T - x diagram differed significantly from the results of [17-19]. In contrast to the data of these works, all three compounds previously indicated in the literature are reflected in Fig. 3. We found that phases SnSb2Se4 and SmSb6Sen were isostructural within the homogeneity region of the y phase which melts incongruently. The presence of a distectic point with a composition of 60 mol%
Sb2Se3 (Sn2Sb6Seii) indicated in [17, 18] was not confirmed by us. In addition, according to our data, the temperature of the peritectic reaction of the formation of the Sn2Sb2Se5 compound is 598°C, which is 35°C higher than that indicated in [18]. There are also discrepancies in the composition of the peritectic (pi) and eutectic (e) points.
Conclusion
A new scheme of phase equilibria in the quasi-binary SnSe - Sb2Se3 system was obtained to differ from those previously reported in the literature [17-19]. According to our data, this system is characterized by the formation of two intermediate phases, melting with
decomposition by peritectic reactions at 598°C (Sn2Sb2Se5) and 560°C (y-phase). The latter has a wide homogeneity region (48-60 mol% Sb2Se3), which includes the ternary compounds SnSb2Se4 and SmSb6Sen previously mentioned in the literature.
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SnSe-Sb2Se3 SiSTEMiNiNFAZA DiAQRAMININDdQiQLdgDiRILMdSi
E.N. ismayilova1*, i.B. Baxtiyarli1, M.B. Babanli1,2
'AMEA akademik M. Nagiyev adina Kataliz va Qeyri-üzvi Kimya înstitutu Baki AZ 1143, H.Cavid pr.113 2Baki Dövlat Universiteti, Z.Xalilov 23, AZ 1148 Baki e-mail:* Ismayilova818@,mail.ru
ddabiyyat malumatlarinin ziddiyyatli olmasini nazara alaraq SnSe-Sb2Se3 sisteminda faza tarazligi DTA va RFA üsullari ila takrar tadqiq edilmiçdir. Müayyan edilmiçdir ki, sistemda SmSb2Se5 birla§masi va 48-60 mol% tarkibda homogenlik sahasinda araliq y-faza amala galir va har iki faza peritektik reaksiya üzra parçalanmaqla ariyir. Qvvallar adabiyyatda qeyd olunan SnSb2Se4 va SmSb6Sen ^lü birlaçmalari stexiometrik tarkibda bu sahada kristallaçirlar. Alinan naticalarin adabiyyat malumatlari ila müqayisali tahlili aparilmiçdir.
Acar sözlar: SnSe - Sb2Se3 sistemi, faza diaqrami, evtektika, qalay va sürma selenidlari, bark mahlullar
УТОЧНЕНИЕ ФАЗОВОЙ ДИАГРАММЫ СИСТЕМЫ SnSe - Sb2Se3
Э.Н. Исмайлова1' *, И.Б. Бахтиярлы1, М.Б. Бабанлы1'2
1 Институт Катализа и Неорганической Химии им. акад. М. Нагиева Национальной АН Азербайджана, пр. Г. Джавида, 113, Баку AZ-1143 2 Бакинский Государственный Университет, ул. З.Халилова, 23, AZ-1148 , Баку, e-mail: *[email protected]
Учитывая противоречивость имеющихся литературных данных, фазовые равновесия в системе SnSe - Sb2Se3 повторно изучены методами ДТА и РФА, построен новый вариант Т-х диаграммы. Установлено, что в системе образуются тройное соединение SmSb2Se5 и промежуточная у-фаза с областью гомогенности 48-60 мол% Sb2Se3, плавящихся с разложением по перитектическим реакциям при 598 С (SmSb2Se5) и 560 С (у-фаза). Эта область включает стехиометрические составы ранее указанных в литературе тройных соединений SnSb2Se4 и SmSbeSen. Проведен сравнительный анализ полученных результатов с литературными данными.
Ключевые слова: система SnSe - Sb2Se3, фазовая диаграмма, эвтектика, селениды олова и сурьмы, твердые растворы.
