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ISSN 2522-1841 (Online) AZERBAIJAN CHEMICAL JOURNAL № 1
ISSN 0005-2531 (Print)
UDC 541.123/123.8/9:546.568186/23
PHASE EQUILIBRIA IN THE Cu2Se-SnSE-Sb2Se3 SYSTEM
E.N.Ismayilova, L.F.Mashadiyeva, I.B.Bakhtiyarly, M.B.Babanly
M.Nagiyev Institute of Catalysis and Inorganic Chemistry, NAS of Azerbaijan
Received 11.11.2021 Accepted 24.12.2021
Phase equilibria in the Cu2Se-SnSe-Sb2Se3 quasi-ternary system has been studied by differential thermal analysis and powder X-ray diffraction. Some polythermal sections, isothermal sections at 300 K of the phase diagram and the projection of the liquidus surface are constructed. The regions of primary crystallization of phases, the nature and temperatures of non- and monovariant equilibria have been determined. The phase diagram of the SnSe-CuSbSe2 quasi-binary section has been refined. It was found that a quaternary compound CuSnSbSe3 is formed in the system by the peritectic reaction. This compound exists in a narrow range ~(650-723 K) temperatures. In addition, in the studied Cu2Se-SnSe-Sb2Se3 system, limited regions of solid solutions based on the SnSe, Sb2Se3, CuSbSe2 and SnSb2Se4 compounds have also been revealed.
Keywords: phase diagram, liquidus surface, copper-antimony-tin selenides, solid solutions.
doi.org/10.32737/0005-2531-2022-1-73-82
Introduction
Complex copper chalcogenides attract attention due to their photoelectric, thermoelectric, nonlinear optical and other properties [16]. In particular, Cu-Sb-Sn-X (X = S, Se) systems are of great interest for the development of new environmentally friendly thermoelectric materials. Recent studies have shown that some phases formed in these systems, which are synthetic analogs of copper-based chalcogenide minerals, demonstrate high values of the thermoelectric figure of merit (ZT = 1) in the mid-temperature range (600^800 K), which is comparable to or exceeds the values ZT for classical commercial thermoelectrics based on lead and bismuth tellurides [7-15]. In recent years, interest in these compounds has increased due to the possibility of increasing their thermoelectric figure of merit [12-15]. One of the ways to increase the thermoelectric figure of merit of these materials is to obtain solid solutions on their basis. For this, it is advisable to investigate phase equilibria in the corresponding systems [16-18]. Earlier, we carried out similar complex studies of complex systems based on copper and silver chalcogenides, in which new phases of variable composition were revealed [19-24].
This paper presents new experimental data on phase equilibria in a quasi-ternary Cu2Se-SnSe-Sb2Se3 system.
The initial compounds (Cu2Se, SnSe, Sb2Se3) and phase diagrams of the boundary quasi-binary systems (Cu2Se-Sb2Se3, Cu2Se-SnSe, SnSe-Sb2Se3) of the title system have been studied in detail.
Cu2Se undergoes a polymorphous transformation at 396 K and melts congruently at 1403 K [25]. This compound has a region of homogeneity towards excess selenium, which is maximum at 800 K (33.3-36.6 at.% Se). The high-temperature modification of Cu2Se crystallizes in a cubic lattice (Space group Fm-3m) with the parameter a = 5.859 (1) A, and the low-temperature one has a monoclinic lattice (Space group C2/c) with parameters a = 7.1379(4) A, b = 12.3823(7) A, c = 27.3904 (9) A, p = 94.308° [26]. The SnSe compound melts congruently at 1153 K [25] and crystallizes in an orthorhombic lattice (Space group Pcmn) with the parameters: a = 4.44175 (7), b = 4.15096 (5), c = 11.49417 (12) A [27]. Antimony selenide Sb2Se3 melts at 863 K on distectic reaction [25], and crystallizes in the orthorhombic lattice: (Space group Pnma) a = 11.7938 (9) , b = 3.9858 (6), c = 11.6478 (7) A, z = 4 [28].
The Cu2Se-SnSe system has a eutectic-type phase diagram with eutectic equilibrium coordinates of 813 K and 46 mol. % Cu2Se [29].
In the Cu2Se-Sb2Se3 system, two ternary compounds CuSbSe2 and Cu3SbSe3 are formed [2, 30]. The CuSbSe2 compound melts congru-
ently at 753 K [31]. This compound has the diamond-like structure with Cu atoms occupied in the center of Se-formed tetrahedrons and crystallizes in the orthorhombic system, with space group Pnma and lattice parameters at room temperature: a = 6.467 A, b = 4.045 A and c =
15.048 A; Z = 4 [32]. The compound Cu3SbSe3 melts incongruently at 808 K [2, 30] (according to [33] at 800 K) and crystallizes in an ortho-rhombic system (Space group Pnma), with lattice parameters a = 7.9865(8), b = 10.6138(9), c = 6.8372(7) A, Z=4 [34]. In the system eutectic melt with a composition of 90 mol. % CuSbSe2 crystallizes at 748 K [5, 30].
The results of works [35-38] on the study of phase equilibria in the SnSe-Sb2Se3 boundary system differ significantly. According to [35], the SnSe-Sb2Se3 system is characterized by the formation of one congruently melting ternary compound Sn2Sb6Sen. The authors of [36] assert that in this system two compounds with the compositions Sn2Sb6Sen and Sn2Sb2Se5 are formed. In [37], the formation of one ternary SnSb2Se4 compound in the SnSe-Sb2Se3 system was shown. The authors of [38] assume, that the compounds Sn2Sb6Se11 and SnSb2Se4 lie in the homogeneity region of one phase. Our recent studies on the refinement of the phase diagram of the SnSe-Sb2Se3 system led to the same conclusion [39]. We found that in the SnSe-Sb2Se3 system, a ternary compound Sn2Sb2Se5 and an intermediate y-phase with a homogeneity range of 48-60 mol.% Sb2Se3 are formed. This region includes the stoichiometric compositions of ternary compounds SnSb2Se4 and Sn2Sb6Sen, previously indicated in [35-38].
The SnSb2Se4 compound crystallizes in an orthorhombic structure (Sp.Gr. Pnnm) with lattice parameters a = 26.610 A, b = 21.066 A and ^ = 4.0423 A [40]. According to [41], the Sn2Sb2Se5 compound also has an orthorhombic structure (Sp.Gr. Pnnm, a = 35.16 A, b = 25.96 A, c = 4.14 A). These crystallographic parameters for the SnSb2Se4 and Sn2Sb2Se5 compounds were confirmed by us in [39]: Sn2Sb2Se5 : Sp.Gr. Pbnm a = 35,08(28) A, b = 25,87(22) A, c = 4,09 (6) A. SnSb2Se4 : Sp.Gr.Pnnm a = 26,605 (25) A, b =
21.049 (20) A, c = 4.0385 (5) A.
Experimental part
For the experiments, we used simple substances from the company EVOCHEM ADVANCED MATERIALS GMBH (Germany) of high purity: copper in granules (Cu-00029; 99.9999%), antimony in granules (Sb-00002; 99.999%), tin in granules (Sn-00005; 99.999%), selenium in granules (Se-00002; 99.999%). Binary and ternary compounds for studying phase equilibria in the Cu2Se-SnSe-Sb2Se3 system were synthesized by fusion of simple substances in stoichiometric ratios in evacuated to ~10- Pa and sealed quartz ampoules at temperatures 500C higher than the melting temperatures of the synthesized compounds. The CuSbSe2 and Cu3SbSe3 compounds were synthesized by heating the ampoules with weighed portions at 850 K, and their melts were kept at this temperature for 3-4 h. After the synthesis, the ampoule with CuSbSe2 was cooled in the switched-off furnace to 300 K. Due to the incongruent melting of Cu3SbSe3 and, according to the recommendation [42], after synthesis, the ampoule of this compound was subjected to rapid cooling from the melt, followed by annealing at 600-673 K. The synthesis of Cu2Se and SnSe compounds was carried out in a two-zone inclined furnace. The temperature of the lower "hot" zone for Cu2Se and SnSe was 1420 and 1200 K, respectively, and the upper "cold" zone was 900 K, which is slightly below the boiling point of selenium (958 K [43]). To obtain a homogeneous stoichiometric Cu2Se composition, after synthesis, quenching from a temperature of 1300 K into cold water was carried out [44].
The individuality of all synthesized compounds was monitored by DTA and XRD methods. The obtained values of the melting temperatures and the parameters of the crystal lattices of all synthesized compounds within the error limit (±3 K and ±0.0003 A) were close to the above literature data.
More than 50 alloys along the SnSe-CuSbSe2, 0.2Sn2Sb6Seu-CuSbSe2, C^Se-H Sn2Sb2Se5 and Cu2Se-SnSb2Se4, sections, as well as a number of additional alloys outside of them were prepared by alloying the initial com-
pounds under vacuum conditions for experiments. According to DTA data for cast non-homogenized alloys, it was shown that their crystallization from melts is completed at 685 K. Therefore, to achieve an equilibrium state, cast alloys are obtained by rapid cooling of the melts were annealed at 650 K for 500 hours. Visual observation of the annealed samples with compositions 90-95 mol.% Cu2Se along the Cu2Se-V3 Sn2Sb2Se5 and Cu2Se-SnSb2Se4 sections revealed traces of metallic copper, which is associated with the deviation of copper semiselenide from stoichiometry.
DTA was carried out from room temperature to 1400 K range with a heating rate of 10 Kmin-1 on a 404 F1 PEGASUS SYSTEM differential scanning calorimeter (Netzsch). The measurement results were processed using the Netzsch Proteus Software. The temperature measurement accuracy was within ±2 K.
X-ray phase analysis was performed at room temperature on a D8 ADVANCE diffrac-tometer (Bruker) with CuK-1 radiation. The radiographs were indexed using the Topas V3.0 Software program (Bruker).
Results and discussion
The title quasi-ternary system Cu2Se-SnSe-Sb2Se3 was previously studied by us in the Cu2Se-SnSe-CuSbSe2 composition area by differential thermal and X-ray phase analyses [4547]. A number of polythermal sections and an isothermal section at 300 K of the phase diagram, as well as a projection of the liquidus surface, have been constructed. Note that during the study of the SnSe-CuSbSe2 quasi-binary section [45], thermal effects at 720 K for alloys with a composition of 30-50 mol.% CuSbSe2 were recorded, the nature of which we could not establish. At the same time, there are literature data on the existence of the CuSnSbSe3 compound [48, 50]. Therefore, we re-examined this section.
Quasi-binary section SnSe-CuSbSe2. Based on the DTA data, we plotted a new version of the T-x diagram of this section (Figure 1). According to this diagram, thermal effects at 723 K refer to the peritectic reaction of decomposition of the CuSnSbSe3. Limited solubility based on the starting compounds is observed in
the system. The solubility based on SnSe (P-phase) and CuSbSe2 (5-phase) is maximum at the eutectic temperature and is -25 and 5 mol. %, respectively. A eutectic equilibrium is established at -63 mol. % CuSbSe2 and 700 K.
111-CuSnSbSe,
\ L+ß ^ L
ß \ L+m \ \ _ 723 \P-,| y' L+S
700 / 6
/ ß+i" ' i ei 1 III+5 i
SnSe 20 40 60 BO CuSbSc,
mol%
Fig. 1. Phase diagram of the SnSe-CuSbSe2 system.
Solid-phase equilibria in the Cu2Se-SnSe-Sb2Se3 system at the 300 K
Figure 2 shows a diagram of solid-phase equilibria in the Cu2Se-SnSe-Sb2Se3 system, which clearly shows the location of the phase regions at room temperature. The phase compositions of alloys in the SnSe-Sb2Se3-CuSbSe2 subsystem were confirmed by XRD. As can be seen from Figure 3, both alloys consist of three-phase mixtures Sn2Sb2Se5+ SnSb2Se4+CuSbSe2 and SnSb2Se4+CuSbSe2+SnSe. Thus, the existence of the quaternary compound CuSnSbSe3 has not been confirmed at 300 K. At the same time, an analysis of the DTA data indicates the existence of this quaternary compound at high temperatures. Solid solutions based on the compound CuSbSe2 (5-phase) and Sb2Se3 (y-phase), which are in a connode bond with each other and the s-phase are observed in the system at 300 K. The P-phase also forms connode bonding with the Cu3SbSe3 and Sn2Sb2Se5 compounds, as well as with the low-temperature modification of Cu2Se. As a result, the concentration triangle is divided into 7 two-phase and 5 three-phase regions.
20 (I)""" 40 60 80 SbjSe,
Fig. 2. Diagram of solid-phase equilibria in the Cu2Se-SnSe-Sb2Se3 system at the 300 K.
Fig. 3. Powder diffraction patterns of alloys 1 and 2 in Figure 2. AZERBAIJAN CHEMICAL JOURNAL № 1
SnSe 20 40p. 60 P, e, «0 Sb,Se,
Fig. 4. Projection of the liquidus surface of the Cu2Se-SnSe-Sb2Se3 system.
Liquidus surface. The projection of the liquidus surface of the Cu2Se-SnSe-Sb2Se3 system onto the concentration plane is shown in Figure 4.
Primary crystallization fields: 1 - a; 2 -P(P/); 3 - y; 4 - Cu3SbSe3; 5 - S; 6 - Sn2Sb2Se5; 7 - s; 8 - CuSnSbSe3. Dotted line-quasi-binary section SnSe-CuSbSe2.
As can be seen, the liquidus surface consists of fields of primary crystallization of eight phases. The fields of primary crystallization of a-and P'-phases based on high-temperature modifications of binary Cu2Se and SnSe compounds, as
well as y-phase based on Sb2Se3 (Figure 4, fields 1-3) have the greatest extent. Fields 4-7 refer to ternary compounds formed on boundary quasi-binary systems or solid solutions based on them. The crystallization surface of the quaternary compound CuSnSbSe3 (region 8) has a small area and is bordered by the fields of primary crystallization of five adjacent phases. There are 17 invariant equilibria in the system, including the boundary quasi-binary. The types and coordinates of these equilibria are given in Table 1, and the types and temperature ranges of monovariant equilibria are shown in Table 2.
Table. 1. Invariant equilibria in the Cu2Se-SnSe-Sb2Se3 system
Point in Figure 4 Equilibrium Composition, mol% T, K
Cu2Se Sb2Se3
Pi P2 P3 P4 L^CuSbSe2(S) 50 50 765
L+p^Sn2Sb2Se5 - 41 871
L+Sn2Sb2Se5^ e - 66 833
L+ao-Cu3SbSe3 65 35 808
L+p^CuSnSbSe3 30 30 723
Ui L+p^Sn2Sb2Ses+ CuSnSbSe3 25 34 715
U2 L+ Sn2Sb2Se5 ^eCuSnSbSe3 26 42 687
U3 U4 U5 L+y^S+e L+a^p+ Cu3SbSe3 L+ p ^ Cu3SbSe3+ CuSnSbSe3 36 40 34 48 22 27 735 727 705
ei L o- y+e - 72 818
e2 L^a+p 47 - 815
e3 L^Cu3SbSe3+S 55 45 748
e4 e5 L^y+S L^S+CuSnSb Se3 46 32 54 32 750 700
Ei L^CuSnSb Se3+S+a 30 40 675
E2 L^Cu3SbSe3+CuSnSbSe3+S 35 30 680
Table 2. Monovariant equilibria in the Cu2Se-SnSe-Sb2Se3 system
Curve in Figure 4
Equilibrium Temperature range, K
L+ß^Sn2Sb2Se5 871-715
L+ßoCuSnSbSe3 723-715
L^Sn2Sb2Se5+CuSnSbSe3 715-687
L+Sn2Sb2Se5^e 833-687
LoCuSnSbSe3+e 687-675
Loy+e 818-735
Loy+S 750-735
Lo5+e 735-675
LoCuSnSb Se3+S 700-675
Loa+ß 815-727
L+aoCu3SbSe3 808-727
Loß+Cu3SbSe3 727-705
L+ßoCuSnSbSe3 723-705 705-680
LoCu3SbSe3+CuSnSbSe3 LoCu3SbSe3+S
748-680 700-680
LoCSnSbSe3+S
PiUj P4U1 U1U2 P2U2 U2E1 eiU3
e4U3
U3E1 e5E1 e2U4 P3U4 U4U5 P4U5 U5E2 e3E2
e5E2
Some polythermal sections of the phase diagram. Below are shown and described poly-thermal sections SnSe-CuSbSe2, Sn2Sb6Se11-CuSbSe2, Cu2Se-Sn2Sb2Se5, Cu2Se-SnSb2Se4 in context with Figure 3 and 4.
Section 0.2Sn2Sb6Sen-CuSbSe2 (Figure 5). One of the initial phases of this section is the s-phase with the Sn2Sb6Se11 composition of the SnSe-Sb2Se3 boundary system. Despite its stability below the solidus, it is, on the whole, non-quasi-binary due to the incongruent melting of the s-phase. The process of crystallization of melts in the range of compositions 0-20 mol% CuSbSe2 is especially difficult.
T,K
850
800
750
700
1 L+E \ re+T 8/
-1 L+y+s 735 \
/ 5+8 1 1 1 L+7+8 8' 1
0.2Sn,Sb,Se„ 20
765
80
CuSbSc,
40 60
mol%
Fig. 5. Phase diagram of the 0.2Sn2Se6Se11-CuSbSe2 system.
Comparing Figure 5 with Figure 4 and Tables 1 and 2, it is easy to show that in this region the Sn2Sb2Se5 compound initially crys-
tallizes from the melt, then a monovariant pe-ritectic reaction L+Sn2Sb2Se5o-s(P2U2) proceeds and a three-phase L+s+Sn2Sb2Se5 region is formed in a result. This reaction ends with the complete interaction of Sn2Sb2Se5, and the system transforms into the two-phase state L+s, and the s-phase first crystallizes from the remaining melt, and then the mixture y+s follows the mono-variant eutectic reaction. Crystallization is completed by the transition reaction U3, and a two-phase mixture S+s is formed. The latter reaction takes place in all samples located along this section, confirms its stability in the subsolidus.
Section Cu2Se—/3Sn2Sb2Se5 (Figure 6). The liquidus of this section consists of two curves of primary crystallization of the a-phase based on the high-temperature modification of Cu2Se and the P-phase based on SnSe. Below the liquidus the crystallization of the a-phase continues according to the monovariant eutectic reaction e2U4, which leads to the formation of a three-phase region L+a+p. Then the invariant transient reaction U4 proceeds. The completion of this reaction with an excess of the a-phase leads to the formation of the three-phase region a+P+Cu3SbSe3 in Figure 6, and with an excess of the liquid phase, the region L+P+Cu3SbSe3 is formed.
Below the liquidus of the P-phase, mono-variant peritectic reactions P1U1 (0-20 mol%
Cu2Se), P4U1 (20-25 mol % Cu2Se), P1U5 (2530 mol% Cu2Se) and eutectic reactions U4U5 (30-35 mol % Cu2Se), e2U4 (35-40 mol % Cu2Se) proceed. In the range of compositions 0-25 mol% Cu2Se, crystallization is completed by the transition reaction U1, and in the range of 25-45 mol% Cu2Se - by the transition reaction U5, as a result of which the phase regions Sn2Sb2Se5+CuSnSbSe3 and p+Cu3SbSe3+ CuSnSbSe3 are formed, respectively.
Section Cu2Se-SnSb2Se4 (Figure 7). The liquidus curve consists of five branches corresponding to the a, Cu3SbSe3, 5, CuSnS-bSe3 h Sn2Sb2Se5 phases (see Figure 4). Below the liquidus, crystallization continues according (from left to right) to monovariant peritectic (P3U4), and eutectic U4U5, e3E2, e5E2, e5E1, U2E1 and U1U2 reactions.
Despite the complexity of the picture of phase equilibria, especially in the composition range of 40-80 mol% SnSb2Se4, the correct in-
terpretation of the DTA data is beyond doubt, since the sequence of crystallization processes in the arrangement of heterogeneous regions in Figure 7 is in full agreement with the general T-x-y diagram of the Cu2Se-SnSe-Sb2Se3 system (Figure 4).
Crystallization is completed by the invariant reactions U4 (727K), U5 (705K), E2(680K), and El (675K) with the formation of three-phase mixtures a+P+Cu3SbSe3, P+Cu3SbSe3+ CuSnSbSe3, 5+Cu3SbSe3+CuSnSbSe3 and 5+s+ CuSnSbSe3.
In conclusion, we note that with prolonged annealing at 600 K, we could not obtain a clear diffraction pattern of the CuSnSbSe3 compound. It undergoes decomposition by a solid-phase reaction in the proximity of the El and E2 eutectics. Therefore, in Figures 6 and 7, the phase regions below 650 K are shown in accordance with Figure 2.
Fig. 6. Phase diagram of the Cu2Se-Sn2Sb2Se5 system. Fig. 7. Phase diagram of the Cu2Se-SnSe2Se4 system.
Conclusion
Thus, a complete picture of phase equilibria in the Cu2Se-SnSe-Sb2Se3 system has been obtained. It was found that the liquidus surface of the system consists of fields of primary crystallization of eight phases, including the quaternary compound CuSnSbSe3, which melts by a peritectic reaction at 723 K. However, this compound could not be detected by XRD. We believe that it decomposes in the sub-
solidus in the vicinity of ternary eutectics into solid solutions based on SnSe and CuSbSe2.
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Cu2Se-SnSe-Sb2Se3 SÎSTEMÎNDO FAZA TARAZLIGI
E.N.ismayilova, L.F.Maçadiyeva, LB.Baxtiyarh, M.B.Babanli
Cu2Se-SnSe-Sb2Se3 kvazi-uçlu sisteminda faza tarazligi diferensial termiki va rentgen faza analizi usullari ils tadqiq edilmiçdir. Sistemin 300 K temperaturda bark faza tarazligi, bazi politermik kasiklari va likvidus sathinin proyeksiyasi qurulmuçdur. Fazalarin ilkin kristallaçma sahalari, mono- va non variant tarazliqlann koordinatlari muayyan edilmiçdir. SnSe-CuSbSe2 kvazibinar kasiyinin faza diaqrami daqiqlaçdirilmiçdir Muayyan edilmiçdir ki, peritektik reaksiya naticasinda sistemda CuSnSbSe3 dordlu birlaçma amala galir va ~ 650-723 K temperatur intervalinda movcuddur. Bundan alava, tadqiq olunan Cu2Se-SnSe-Sb2Se3 sisteminda SnSe, Sb2Se3, CuSbSe2 va SnSb2Se4 birlaçmalari asasinda mahdud bark mahlul sahalari da açkar edilmiçdir.
Açar sozlar: faza diaqrami, likvidus sathi, mis-surma-qalay selenidbri, Ьэгк тэЫиПаг.
ФАЗОВЫЕ РАВНОВЕСИЯ В СИСТЕМЕ Cu2Se-SnSe-Sb2Se3.
Э.Н.Исмайлова, Л.Ф.Машадиева, И.Б.Бахтиярли, М.Б.Бабанлы
Методами дифференциального термического анализа и порошковой рентгенографии изучены фазовые равновесия в квазитройной системе Си28е-8п8е-8Ъ28е3. Построены некоторые политермические сечения, изотермическое сечений при 300 К фазовой диаграммы и проекция поверхности ликвидуса. Определены области первичной кристаллизации фаз, характер и температуры нон- и моновариантных равновесий. Уточнена фазовая диаграмма квазибинарного сечения SnSe-CuSbSe2. Установлено, что в системе по перитектический реакции образуется четверное соединение CuSnSbSe3, которое существует в узком интервале (~650-723К) температур. В системе также выявлены ограниченные области твердых растворов на основе
соединений SnSe, Sb2Se3, CuSbSe2 и SnSb2Se4.
Ключевые слова: фазовая диаграмма, поверхность ликвидуса, селениды медь-сурьма-олово, твердые растворы.
