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AZ9RBAYCAN KIMYA JURNALI № 2 2017
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UDC 544.344.015.3: 546.56/5723
PHASE RELATIONS IN THE Cu8GeS6-Ag8GeS6 SYSTEM AND SOME PROPERTIES OF SOLID SOLUTIONS
V.A.Abbasova\ I.J.Alverdiyev1, E.Rahimoglu2, R.J.Mirzoyeva2, M.B.Babanly2'3
1Ganja State University 2Baku State University Institute of Catalysis and Inorganic Chemistry, NAS of Azerbaijan
Received 06.01.2017
Phase equilibria in the Cu8GeS6-Ag8GeS6 system were experimentally investigated by means of differential thermal analysis, powder X-ray diffraction techniques, and electromotive force (EMF) measurements of concentration chains with Cu4RbCl3I2 solid electrolyte. The phase diagram and some "composition"-"property" graphs for title system were plotted. We found that continuous series of solid solutions were formed between high-temperature modifications of starting compounds of the system. Formation of solid solutions causes decreasing of the polymorphic transitions' temperature for both compounds.
Keywords: copper-germanium sulfides, silver-germanium sulfides, phase equilibria, solid solutions, polymorphic transformation.
Introduction
In recent years, Cu- or Ag-containing ternary chalcogenides have attracted much attention, because these materials have potential applications for using as semiconductors, nonlinear optical materials, superconductors, and as photovoltaic cell materials [1-7].
Optimization of performance of these materials can be achieved by changing their composition. This is based in turn on the study of phase equilibria in relevant systems. Binary and more complex systems consisting of isoformula compounds represent certain interest for obtaining wide areas of solid solutions.
Our recent studies show that a broad area of solid solutions are formed in the systems with the anionic (S^-Se) Cu8GeS6-Cu8GeSe6 [8], CusGeSe6-AggGeSe6 [9], Ag8SnS6-Ag8SnSe6
[10], and cationic (Ge^Sn) Ag8GeS6-Ag8SnS6
[11], Ag8GeSe6-Ag8SnSe6 [12] substitutions.
The aim of the present investigation is to study phase relations in the Cu8GeS6-Ag8GeS6 system in which the formation of solid solutions with Cu^Ag substitution can be expected.
Both components of the studied system are well known.
The Cu8GeS6 compound melts peritectical-ly with decomposition at 1253 K and has a phase transformation at 328 K. Low-temperature (LT)
modification of Cu8GeS6 crystallizes in the ortho-rhombic system (Sp.gr. Pmn2i, a = 0.70445, b = 0.69661, c = 0.98699 nm) [3, 13]. The crystal structure of the high-temperature (HT) modification of this compound was described as face-centered cubic (Sp.gr. F-43m, a = 0.99567 nm) [14].
The Ag8GeS6 melts congruently at 1218 K and has a phase transformation at 496 K. HT-modification of this compound crystallizes in cubic structure (F-43m, a = 1.070 nm [15, 16]), and the LT-modification has orthorhombic crystal structure (Sp.gr. Pna2u a=1.5149, b=0.7476, c=1.0589 nm [17]).
As seen from the literature data, the formation of continuous solid solutions between the HT-modifications of starting compounds is expected, however, the different chemical interaction between the LT-modifications of components in the system Cu8GeS6-Ag8GeS6 occur.
Materials and methods
Starting compounds Cu8GeS6 and Ag8GeS6 were synthesized from high-purity elements (at least 99.999 wt.% purity). The stoichiometric compositions of elements were placed into quartz ampoules (15 sm in length and 1.5 sm in diameter), being evacuated to ~10 Pa and fused. The ampoules were placed in an inclined two-zone furnace for 2/3 of their length. The "hot zone" was slowly heated from
PHASE RELATIONS IN THE Cu8GeS6-Ag8GeS6 SYSTEM AND SOME
26
room temperature to ~(30-50) K above the melting point of the synthesized compounds [18] and outside part of the ampoule was quenched with water ("cold zone"). The "hot zone" plays a role of the interaction zone and the "cold zone" condenses Sulfur and returns it to the interaction zone. As a result of the reaction in the cold part the mass of the sulfur decreases and within 1-2 hours it is spent almost. Thereafter, the ampoule completely placed in a furnace and kept at the pointed temperature for 1-2 h taking into account the in-congruent melting of the Cu8GeS6, after synthesis, it was annealed at 1000 K for 300 h.
The synthesized compounds were identified using differential thermal analysis (DTA). Three thermic effects (328, 1253, 1280 K) were observed on the heating curves of Cu8GeS6 and two thermic peaks (495, 1220 K ) for Ag8GeS6 what near to the results of [3, 19, 20].
All synthesized compounds also were checked by powder X-ray diffraction (XRD) technique. All peaks can be indexed as pure phases of Cu8GeS6 and Ag8GeS6, with the cell parameters close to the [3, 13-17].
More than twenty alloys of the Cu8GeS6-Ag8GeS6 system were prepared by melting the stoichiometric quantities of the pre-synthesized compounds in sealed silica ampoules under vacuum. All samples were heated up to 1300 K and held at this temperature for about 1 h and then were annealed at 900 K for about 500 h. There were prepared two different set of alloys for each composition the first series after annealing were slowly cooled down to room temperature, and the second series were quenched in cold water after annealing.
All prepared samples were analyzed by means of DTA, XRD, and EMF technique. The XRD data were collected at room temperature using a Bruker D8 ADVANCE diffractometer (with CuXa1-radiation). DTA of the equilibrated alloys was carried out using an NETZSCH 404 F1 Pegasus system device. The heating rate was 10 K/min. For the EMF measurements, the following type reversible cells were assembled: (-) Cu (solid) | Cu4RbCl3I2(solid)|(Cu in alloys)(solid) (+) (1)
In the cell (1), the solid-state superionic
conductor Cu4RbCl3I2 was used as the electrolyte. This compound exhibits a high ion conductivity, a = 0.28 Ohm-1-cm-1 at room temperature [2].
The assembly of an electrochemical cell and measurements are described in detail elsewhere [21]. EMF was measured by the compensation method in the 330-430 K temperature range with an accuracy of ±0.1 mV, using the highresistance universal B7-34A digital voltmeter.
Results and discussion
Obtained data of DTA, XRD and EMF techniques for the Cu8GeS6-Ag8GeS6 system are listed in Table 1. Based on these data the phase diagram of the studied system was plotted (Figure 1a). We found that the Cu8GeS6-Ag8GeS6 system is the quasi-stable section of the corresponding quaternary system. Continuous solid solutions (y-phase) are formed in the system between HT-modifications of starting compounds. However, the system is non-quasi-binary due to the peritectic melting of the Cu8GeS6. This leads to crystallization of the aphase based on HT-modification of the Cu2S in a 0-25 mol.% Ag8GeS6 composition range and formation of L+a and L+a+5 areas. Monovariant peritectic reaction L+ao-5 occur in the studied system and the L+a+5 three-phase area is formed as a result. In the composition area more than 25 mol.% Ag8GeS6 solid solutions based on HT-modifications of starting compounds primary crystallize from the melt. The system becomes single-phase (y) when crystallization ends.
Table 1. The results of DTA and EMF measurements for the Cu8GeS6-Ag8GeS6 alloys_
Composition, mol.% AggGeSe Thermal effects, К Е, mV (350 К)
0 (CugGeSe) 328, 1250, 1280 398.4
10 312, 1235-1245, 1265 -
20 1220-1245 418.8
30 1215-1235 -
40 1210-1228 438.0
50 1205-1220 448.7
60 1200 460.2
70 400, 1195 464.0
80 445, 1200 470.1
90 475,1205-1217 515.4
100 495, 1220 -
E, mV (350K)
500 450 400'
, 10.1 10.2 9.8 1300 1250'
T, K
500
400
328
- © " Y 464 / /
y+8. 8;
____________________________________________
L+a ®
L
L__JVI
_ 1.1 cr+y L+T
y
5, I / S2
/ y+S2 i i
1220
495
CusGeSc 20 40 60 mol% Ag3GeS6
80
Fig. 1. Phase diagram (a), cubic lattice parameters of alloys quenched after annealing at 900 K (b) and concentration dependence of EMF of the chains type (1) at 350 K (c) for the Cu8GeS6-Ag8GeS6 system.
AgsGeS6
I......... I.........I.........I
3
d - Scale
Fig. 2. XRD patterns for some alloys of the Cu8GeS6-Cu8GeS6 system at room temperature.
2B PHASE RELATIONS IN THE CuBGeS6-AgBGeS6 SYSTEM AND SOME
Table 2. Types and crystal lattice parameters for the alloys of the Cu8GeS6-Ag8GeS6 system
Composition, mol.% AgBGeS6 Room temperature Quenched from 900 K
Syngony, Sp.gr a, Â b, Â с, Â Syngony, Sp.gr. a, Â
G (CuBGeS6) Orthromb. P ? P mn2l 7.G41(1) 6.963(1) 9.5B6 (2) Cubic., F 43m 9.957(2)
2G Cubic, F 43m 1G. 111(3) — — Cubic., F 43m 1G. 113(3)
4G Cubic, F 43m 1G.255(3) — — Cubic, F 43m 1G.25B(3)
5G Cubic, F 43m 1G.32B(3) — — Cubic., F 43m 1G.332(3)
6G Cubic, F 43m 1G.4G7(5) — — Cubic., F 43m 1G.412(4)
BG Orthromb. Pna2j 15.214(7) 7.5G6(4) 1G.624(5) Cubic, F 43m 1G.567(5)
1GG Orthromb. Pna2j 15.129(5) 7.457(2) 1G.553(3) Cubic, F 43m 1G.7G6(5)
The temperature of polymorphic transitions of both starting components formation decreases thanks to formation of HT 5-solid solutions. The temperature of polymorphic transition of Ag8GeS6 for compositions less than 70 mol.% AggGeS6 decreases from 495 K to 400 K and at the even lower composition of Cu8GeS6, this transition is not observed at temperatures higher than at room one. The polymorphic transition of Cu8GeS6 is not observed at >10 mol.% Ag8GeS6. The results show that the formation of substitutional solid solutions extends the area of high-temperature cubic phase and exists at 10-60 mol.% Cu8GeS6 at room temperature.
Based on the EMF measurements of the cell (1) we found out the continuous character of the concentration dependence of EMF within 330-390 K temperature range. The EMF data increase monotonically with decreasing of the copper concentration (Figure 1c).
The plotted phase diagram of the title system was also confirmed by XRD data. The XRD data for the slowly cooled after annealing alloys (Figure 2) show that diffraction pattern of samples containing (10-60) mol.% Cu8GeS6 and HT-modifications of Cu8GeS6 and Ag8GeS6 are qualitatively similar. The powder diffraction patterns of the samples with >70 mol.% Ag8GeS6 are similar to that for the LT-modification of Ag8GeS6.
Topas V3.0 software (Bruker) was used
for indexing the powder XRD patterns for starting compounds and solid solutions (Table 2). From the comparative analysis of the data of the Table we established that in the region of (1060) mol.% Cu8GeS6 the solid solutions have a cubic structure at room temperature. All samples and starting compounds quenched from 900 K have a cubic structure too (Table 2). The concentration dependence of the cubic lattice parameters for HT solid solutions follows the Vegard's rule (Figure 1b).
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Cu8GeS6-Ag8GeS6 SlSTEMlNDO FAZA TARAZLIQLARI УЭ BORK M OHLULLARIN BOZl XASSOLORl
V.A.Abbasova, l.C.Alverdiyev, E.Rahimoglu, RC.Mirzayeva, M.B.Babanli
DTA va RFA usullari ila, hamginin Cu4RbCl3I2 bark elektrolitli qatiliq dovralarinin EHQ-nin olgulmasi ila Cu8GeS^-Ag8GeS6 sisteminda faza tarazliqlari tadqiq edilmi§dir. Sistemin faza diaqrami va muvafiq "tarkib-xassa" diaqramlari qurulmu§dur. G6starilmi§dir ki, sistemda ilkin birla§malarin yuksak temperaturlu kubik qurulu§lu modifikasiyalari arasinda fasilasiz bark mahlullar amala galir. Bark mahlullarin amalagalmasi har iki ilkin birla§manin polimorf gevrilma temperaturunu azaldir.
Agar sozlar: mis-germanium sulfidlari, gumu§-germanium sulfidlari, faza tarazliqlari, bark mahlullar, polimorf gevrilma.
ФАЗОВЫЕ РАВНОВЕСИЯ В СИСТЕМЕ Cu8GeS6-Ag8GeS6 И НЕКОТОРЫЕ СВОЙСТВА ТВЕРДЫХ РАСТВОРОВ
В.А.Аббасова, И.Дж.Алвердиев, Э.Рагимоглы, Р.Дж.Мирзоева, М.Б.Бабанлы
Фазовые равновесия в системе Cu8GeS6-Ag8GeS6 изучены методами ДТА и РФА, а также измерением ЭДС концентрационных цепей с твердым электролитом Cu4RbCl3I2. На основании полученных экспериментальных данных построены фазовая диаграмма и диаграммы "состав-свойство". Показано, что система характеризуется образованием непрерывного ряда твердых растворов между высокотемпературными модификациями исходных соединений. Образование твердых растворов сопровождается понижением температуры полиморфного перехода обоих соединений.
Ключевые слова: сульфиды меди-германия, сульфиды серебра-германия, фазовые равновесия, твердые растворы, полиморфный переход.
