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AZERBAIJAN CHEMICAL JOURNAL № 3 2019
ISSN 2522-1841 (Online) ISSN 0005-2531 (Print)
UDC 544.344.015.3: 546.56/57'23
PHASE EQUILIBRIA ALONG THE Cu2GeSe3-Ag2GeSe3 ISOPLETH SECTION OF THE Cu2Se-Ag2Se-GeSe2 SYSTEM
V.A.Abbasova
Ganja State University
[email protected] Received 03.04.2019
DTA and XRD methods were used to study phase equilibria along the Cu2GeSe3-Ag2GeSe3 isopleth section of the Cu2Se-Ag2Se-GeSe2 system and its phase diagram was constructed. It was established that this section is non-quasibinary. In the composition range 0-55 mol% Cu2GeSe3, solid solutions on the basis of high-temperature modification of Ag8GeSe6 primarily crystallize from the melt, and in the range 55-100 mol% Cu2GeSe3 solid solutions based on Cu2GeSe3. Based of Cu2GeSe3, solid substitution solutions were identified, the length of which at 900 K is 20 mol%, and at room temperature -10 mol%.
Keywords: copper-germanium selenides, silver-germanium selenides, phase diagram, solid solutions. https://doi.org/10.32737/0005-2531-2019-3-52-56
Introduction
Complex chalcogenides of copper and silver with elements of the germanium subgroup received a lot of attention over the years due to many phases based on them exhibit excellent properties that make them a good candidate for fabrication of novel high-performance functional materials [1-3]. In particular, A2BIVX3 and AgBIVX6 (A - Cu, Ag; BIV - Ge, Sn; X - S, Se, Te) compounds and alloys based on them are possess thermoelectric, photoelectric, optical, and other properties [4-12].
In order to develop novel preparative methods and growth of single crystals of this kind of complex materials and in particular, to examine the formation of new intermediate phases it is necessary to investigate phase relationships and to plot phase diagrams of the respective systems [13-16].
This paper presents the results of a study of phase equilibria along a section Cu2GeSe3-Ag2GeSe3 of a quasiternary system Cu2Se-Ag2Se-GeSe2. Earlier, a number of works presented phase diagrams of some similar systems in which continuous (Cu2GeS3-Cu2GeSe3 [17, 18]) or wide (Cu2SnS3-Cu2SnSe3 [19]), Cu2GeS3-Ag2GeS3 [20]) areas of substitutional solid solutions were revealed.
Cu2GeSe3 compound melts at 1038 K [2] and crystallizes in a monoclinic structure: Sp.Gr. Cm, a = 0.6772, b = 0.3956, c = 0.3958 nm; p =
125.830 [21]. In the literature, until recently, there was conflicting information about the existence of the Ag2GeSe3 compound [2, 22]. A recently published work [23] proved the absence of this compound. It was shown that the alloy of composition "Ag2GeSe3" is the Ag2Se+GeSe2 two-phase mixture.
Experimental part
For the investigations, Cu2GeSe3, Ag2Se and GeSe2 were synthesized by the direct fusing of the high purity grade (not less than 99.999%) constituent elements in sealed (10-3 Pa) silica ampoules. The synthesis of the first two compounds was performed in the inclined two-zone furnace because of high vapor pressure of elemental selenium at high temperature. The "hot" zone of the furnace was 30-50 K higher than the melting point of the corresponding synthesized compound, whereas the temperature of the "cold" zone was about 900 K (lower than the boiling point of selenium [24]).
The alloys of the investigated system were prepared by melting the stoichiometric quantities of the pre-synthesized compounds in sealed silica ampoules under vacuum. All alloys were heated up to 1050 K and held at this temperature for about 3-4 h and were then annealed at 800 K for about 500 h. Finally, alloys slowly cooled down to room temperature.
DTA and XRD techniques were employed
to check the purity of the synthesized starting compounds and analyze the samples. Differential thermal analysis of the equilibrated alloys was carried out using a NETZSCH 404 F1 Pegasus system. The measurement was performed between room temperature and 1100 K with a heating and cooling rate of 10 K min-1. Temperatures of thermal effects were taken mainly from the heating curves. XRD analysis was done with the Bruker D8 ADVANCE diffrac-tometer (CuKai-radiation).
DTA and XRD data confirmed that the pre-synthesized compounds were phase-pure. Powder XRD data for synthesized compounds were found to be very close to data of Reference [2, 21].
Results and discussion
Based on powder X-ray diffraction patterns of the homogenized samples, it was shown that the solubility of Cu2GeSe3 at room temperature about 10 mol% (y-phase). A sample has a diffraction pattern identical to Cu2GeSe3 (Figure 1). Solutions containing 30-80 mol% Cu2GeSe3 were three-phase, while 0-20 mol% Cu2GeSe3 were two-phase. Here P-GeSe2, 5 are solid solutions based on high-temperature cubic modification of Ag8GeSe6. Figure 2 presents the X-ray powder diffraction patterns of Ag8GeSe6, GeSe2 compounds and an alloy of this system
with a composition of 10 mol% Cu2GeSe3. As can be seen, the diffraction pattern of this alloy consists of a set of reflection lines of Ag8GeSe6 and GeSe2 compounds. Figure 3 shows the T-x diagram of the Cu2GeSe3-"Ag2GeSe3" section. As can be seen, this section is non-quasibinary. Liquidus consists of two curves. From left to right, they correspond to the primary crystallization of y-(~55-100 mol% Cu2GeSe3) and 5-phases. Since the region of homogeneity of the y-phase is located in the plane of the section, the tie-lines in the two-phase region L+y are practically in the plane of the section. The region of homogeneity of the y-phase is maximum at 900 K and about 20 mol%. Below liquidus, crystallization takes place according to monovariant eutectic reactions: L^y+5 (30-80 mol% Cu2GeSe3) and L^p+5 (0-30 mol% Cu2GeSe3). In the region of 0-25 mol% Cu2GeSe3, crystallization is completed by the second scheme and a two-phase field P+5 is formed. Near room temperature, the 5 phase decomposes according to the monovariant scheme 5^52, resulting in the formation of P+5+52 three-phase and P+52 two-phase areas (52-solid solutions on the basis of low-temperature modification of Ag8GeSe6). The heating thermogram of a sample with a composition of 10 mol% Cu2GeSe3 (Figure 4a) clearly displays these processes.
2-Theta - Scale
Fig. 1. XRD patterns of the 90 mol% C^GeSes and pure C^GeSes.
2-Theta - Scale
Fig. 2. XRD patterns of the 10 mol% Cu2GeSe3, Ag8GeSe6 and GeSe2.
T, K.
1038 1000
900
800
350 300
L
\ ---'
r-*-•
y L+5
/ L+Y+Ö 820 \e2
L+ß+5
t r P+7+5 ß+5
ß+8+5.
1 1 1 V/W:
843
Cu,GeSe, 80 60 40 20 "Ag.GeSe," mol% Cu,GeSe,
Fig. 3. Polythermal section Cu2GeSe3-"Ag2GeSe3" of the phase diagram of the Cu2Se-Ag2Se-GeSe2.
a)
-y
317
b)
835
820
930
Fig. 4. Heating thermograms of samples of the Cu2GeSe3-"Ag2GeSe3" polythermal section: a -10 mol% Cu2GeSe3, b - 70 mol% Cu2GeSe3.
In the region of 25-85 mol% Cu2GeSe3 crystallization is completed by the formation of a triple eutectic E at 820 K (mixture P+y+S). From the heating thermogram of the sample of 70 mol% Cu2GeSe3 (Figura 4, b), it is seen that upon heating at 820 K initially invariant melting
occurs (eutectic E), then 2 polythermal endo-effects in the temperature range of 820-915 K and 915-970 K take place. These effects reflect the monovariant eutectic process Lo-y+S and the dissolution of the remaining y-phase crystals in the melt, respectively.
Conclusion
The T-x diagram of the Cu2GeSe3-"Ag2GeSe3M isopleth section of the Cu2Se-Ag2Se-GeSe2 quasi-ternary system is constructed. Solid solutions based on Cu2GeSe3 are found in the system, the length of which at 900 K is 20 mol%, and at room temperature - 10 mol%. The obtained phases of variable composition are of interest as potential thermoelectric materials.
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Cu2Se-Ag2Se-GeSe2 SiSTEMiNiN Cu2GeSe3-Ag2GeSe3 POLiTERMiK KOSiYi UZRO FAZA
TARAZLIQLARI
V.A.Abbasova
DTA va RFA usullari ila Cu2Se-Ag2Se-GeSe2 sisteminin Cu2GeSe3-Ag2GeSe3 politermik kasiyi uzra faza tarazliqlan oyranilmiij, faza diagrami qurulmuijdur. Muayyan edilmi§dir ki, bu kasik qeyri-kvazibinardir, 0-55 mol% Cu2GeSe3 tarkib intervalinda arintidan ilkin olaraq Ag8GeSe6 birla§masinin yuksak temperaturlu modifikasiyasi asasinda bark mahlullar, 55-100 mol% Cu2GeSe3 intervalinda isa - Cu2GeSe3 asasinda bark mahlular kristalla§ir. S istemda Cu2GeSe3 asasinda b ark mahlul sahasi 900 К-da 20 mol%, otaq temperaturunda isa 10 mol% ta§kil edir.
Agar sozlar: mis-germanium selenidlari, gumu§-germanium selenidlari, faza diaqrami, bark mahlullar.
ФАЗОВЫЕ РАВНОВЕСИЯ ПО ПОЛИТЕРМИЧЕСКОМУ РАЗРЕЗУ Cu2GeSe3-Ag2GeSe3 СИСТЕМЫ
Cu2Se-Ag2Se-GeSe2
В.А.Аббасова
Методами ДТА и РФА изучены фазовые равновесия по политермическому разрезу Cu2GeSe3-Ag2GeSe3 системы Cu2Se-Ag2Se-GeSe2 и построена ее фазовая диаграмма. Показано, что разрез - неквазибинарный, в области составов 0-55 мол.% Cu2GeSe3 из расплава первично кристаллизуются твердые растворы на основе высокотемпературной модификации соединения Ag8GeSe6, а в области 55-100 мол.% Cu2GeSe3 - твердые растворы на основе Cu2GeSe3. При 900 К области гомогенности твердых растворов Cu2GeSe3 составляют 20 мол.%, а при комнатной температуре - 10 мол%.
Ключевые слова: селениды меди-германия, селениды серебра-германия, фазовая диаграмма, твердые растворы.
