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AZ9RBAYCAN KÍMYA JURNALI № 1 2017
UDC 544.344.015.3: 546.56/57'23
PHASE EQUILIBRIA IN THE Cu8GeSe6-Ag8GeSe6 SYSTEM V.A.Abbasova, I.J.Alverdiyev, L.F.Mashadiyeva*, Y.A.Yusibov, M.B.Babanly*
Ganja State University *Nagiev Institute of Catalysis and Inorganic Chemistry, NAS of Azerbaijan
[email protected] Received 25.02.2016
Phase equilibria in the Cu8GeSe6-Ag8GeSe6 system were studied by means of differential thermal analysis and powder X-ray diffraction technique as well as electromotive force measurements of concentration chains with the solid electrolyte. The T-x phase diagram and corresponding "composition-property" diagrams were constructed. It was established that the system is characterized by continuous series of solid solutions between high-temperature modifications of starting compounds. Formation of solid solutions decreases the temperature of polymorphic transitions of both compounds.
Keywords: copper-germanium selenides, silver-germanium selenides, phase equilibria, solid solutions, polymorphic transformation.
Introduction
Copper and silver based ternary chalcoge-nides with p2-elements are promising functional materials thanks to their semiconductor, photovoltaic and thermoelectric properties [1-3]. In particular, the argyrodite family compounds with common formula A^X (A1 - Cu, Ag; BIV -Ge, Sn; X - S, Se) possess ionic conductivity and widely used for photoelectrodes, electrochemical converters of solar energy, ionselective sensors, photoelectrochemical imaging etc. [4-7].
Understanding the phase relationships in the corresponding systems is always helpful for the development of novel materials. Binary and more complex systems consisting of isoformula compounds represent certain interest for obtaining wide areas of solid solutions.
Earlier we presented the results of investigations of the systems with the anionic (S^Se) CugGeS6-CugGeSe6 [8], AggGeS6-AggGeSe6 [9], AggSnS6-AggSnSe6 [10], and cationic (Ge^Sn) Ag8GeS6-Ag8SnS6 [11], AggGeSe6-AggSnSe6 [12] substitutions, in which the broad areas of solid solutions were identified.
The aim of the present investigation is study phase relations in the Cu8GeSe6-Ag8GeSe6 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 Cu8GeSe6 compound melts peritec-tically with decomposition at 1083 K and has a phase transformation at 333 K or 328 K. Low-temperature (LT) modification of Cu8GeSe6 crys-
tallizes in the hexagonal system (Sp.gr. P63mcm, a = 1.26438, c =1.17570 nm) [2, 13, 14]. The crystal structure of the high-temperature (HT) modification of this compound was described as face-centered cubic (a = 1.1020 nm) [15].
The Ag8GeSe6 melts peritectically with decomposition at 1175 K and has a phase transformation at 321 K. HT-modification of this compound crystallizes in cubic structure (a = 1.099 nm [16, 17]), and the LT-modification has orthorombic crystal structure (Sp.gr. Pmn21y a = 0.7823, b = 0.7712, c = 1.0885 nm [16]).
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 Cu8GeSe6-Ag8GeSe6 occur.
Materials and methods
The Cu8GeSe6 and Ag8GeSe6 compounds were synthesized from high-purity elements (at least 99.999 wt.% purity). The stoichiometric compositions of elements were placed into quartz ampoules (15 cm in length and 1.5 cm in diameter), being evacuated to ~10-2 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 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 selenium and returns it to the interaction zone. As a
АЗЕРБАЙДЖАНСКИЙ ХИМИЧЕСКИЙ ЖУРНАЛ № 1 2017
result of the reaction in the cold part the mass of the selenium 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 incongruent melting of the Cu8GeSe6, after synthesis, it was annealed at 1000 K for 300 h.
DTA data were employed for identification of the synthesized compounds. Three thermic effects (335, 1080, 1115 K) were observed on the heating curves of Cu8GeSe6 and two thermic peaks (320, 1175 K) for Ag8GeSe6 what near to the results of [2, 14, 16].
All synthesized compounds were also checked by X-ray powder diffraction. All peaks can be indexed as pure phases of Cu8GeSe6 and Ag8GeSe6, with the cell parameters close to data of [2, 13-17].
All investigated samples of the Cu8GeSe6-Ag8GeSe6 system were prepared by melting the stoichiometric quantities of the pre-synthesized compounds in sealed silica ampoules under vacuum. Alloys were heated up to 1200 K and held at this temperature for about 1 h and then were annealed at 900 K for about 500 h. For each composition were prepared two different set of alloys: the first series after annealing were slowly cooled down to room temperature, and the second series were quenched in cold water after annealing.
DTA, XRD, and EMF technique were employed to analyze the samples. The XRD data were collected at room temperature using a Bruker D8 ADVANCE diffractometer (with CuKai-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 reversible cells of the following type were assembled: (-) Cu (solid) | Cu4RbCl3l2 (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-1cm-1 at room temperature [7].
The assembly of an electrochemical cell and measurements are described in detail elsewhere [19]. EMF was measured by the compensation method in the 330-430 K temperature range with an accuracy of ±0.1 mV, using the high-resistance universal B7-34A digital voltmeter.
Results and discussion
The phase diagram of the Cu8GeSe6-Ag8GeSe6 system was constructed based on DTA and XRD data, as well as EMF measurements (Table, Figure 1).
The results of DTA and EMF measurements for the Cu8GeSe^Ag8GeSe6 alloys
Composition, mol% Ag8GeSe6 Thermal effects, K Cubic lattice parameter, nm E, mV (350 K)
0 (Cu8GeSe6) 335; 1080; 1115 *1.10202(6) 281.9
10 105;1110 1.01165(8) -
20 1090-1105 1.02483(10) 295.1
40 1103-1125 1.04502(8) 313.9
60 1120-1143 1.06476(9) 330.7
80 308;1150-1163 *1.08344(10) 367.0
90 315; 1170 *1.09004(10) 407.1
100 320; 1175 *1.09921(8) -
: - alloys quenched from 900 K
40 60 mol.% AgsGeSe6 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 Cu8GeSe6-Ag8GeSe6 system.
As can be seen, the Cu8GeSe6-Ag8GeSe6 system is the quasi-stable section of the corresponding quaternary system and characterized by continuous solid solutions field between HT-modifications of starting compounds. However, the system is nonquasibinary due to the peritectic melting of the Cu8GeSe6. This leads to crystallization of the a-phase based on HT-modification
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V.A.ABBASOVA et al.
of the Cu2Se in a 0-20 mol.% Ag8GeSe6 composition range and formation of L+a and L+a+5 areas. The L+a+5 area is formed in a result of the L+aoô monovariant peritectic reaction. In the composition area more than 20 mol.% Ag8GeSe6 only 5-solid solutions based on HT-modifications of starting compounds crystallize from the melt. The system becomes single-phase (5) when crystallization ends.
The formation of HT ô-solid solutions decreases the temperature of polymorphic transitions of both components. The temperature of polymorphic transition of Ag8GeSe6 for compositions less than 80 mol% Ag8GeSe6 decreases from 320 K to 305 K and at the even lower composition of Cu8GeSe6 this transition is not observed at higher than room temperature. The polymorphic transition of Cu8GeSe6 is not observed even at 10 mol% Ag8GeSe6. The results show that the formation of substitutional solid solutions extends the area of high-temperature cubic phase and exists at 10-70 mol% Cu8GeSe6 at room temperature.
The results of the EMF measurements of the cell (1) show the continuous character of the
concentration dependence of EMF within 330430 K temperature range: the EMF data increase monotonically with decreasing of the copper concentration (Figure 1c).
The XRD data confirm the plotted phase diagram. The XRD data of the alloys slowly cooled after annealing (Figure 2) show that diffraction patterns of samples containing 10^70 mol% Cu8GeSe6 are qualitatively similar to HT-modifications of Cu8GeSe6 and Ag8GeSe6. The diffraction patterns of the samples with >80 mol% Ag8GeSe6 are similar to that for the LT-modification of Ag8GeSe6.
The powder XRD patterns for starting compounds and solid solutions were indexed using Topas V3.0 software (Table). The comparative analysis of the data of the Table showed that in the region of 10-70 mol% Cu8GeSe6 the solid solutions at room temperature have a cubic structure. All samples and starting compounds quenched from 900 K have a cubic structure also (Table). The concentration dependence of the cubic lattice parameters for HT solid solutions follows the Vegard's rule (Figure 1b).
Fig. 2. XRD patterns for some alloys of the Cu8GeS6-Cu8GeSe6 system at room temperature.
АЗЕРБАЙДЖАНСКИМ ХИМИЧЕСКИМ ЖУРНАЛ № 1 2017
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Cu8GeSe6-Ag8GeSe6 SISTEMINDO FAZA TARAZLIQLARI
V.A.Abbasova, i.C.Alverdiyev, L.F.Maçadiyeva, Y.A.Yusibov, M.B.Babanli
DTA, RFA va bark elektrolitli EHQ üsullan ils Cu8GeSe6-Ag8GeSe6 sistemi tadqiq edilmiç, onun T-x faza diaqrami va muvafiq "tarkib-xassa" diaqramlari qurulmuçdur. Müayyan edilmiçdir ki, sistemda ilkin birlaçmalarin yüksak temperaturlu modifikasiyalari arasinda kubik quruluçlu fasilasiz bark mahlullar amala galir. Bark mahlullarin amala galmasi har iki ilkin birlaçmanin polimorf çevrilma temperaturunu azaldir.
Açar sözlar: mis-germanium selenidlari, gümü§-germanium selenidlari, faza tarazliqlari, bark mahlullar, polimorf çevrilma.
ФАЗОВЫЕ РАВНОВЕСИЯ В СИСТЕМЕ Cu8GeSe6-Ag8GeSe6
В.А.Аббасова, И.Дж.Алвердиев, Л.Ф.Машадиева, Ю.А.Юсибов, М.Б.Бабанлы
Методами ДТА, РФА и ЭДС с твердым электролитом изучена система Cu8GeSe6-Ag8GeSe6. Построены Т-х-фазовая диаграмма и диаграммы "состав-свойство". Установлено, что система характеризуется образованием непрерывных рядов твердых растворов между высокотемпературными модификациями исходных соединений. Образование твердых растворов понижает температуру полиморфного перехода обоих соединений.
Ключевые слова: селениды меди-германия, селениды серебра-германия, фазовые равновесия, твердые растворы, полиморфный переход.
