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ISSN 2522-1841 (Online) AZERBAIJAN CHEMICAL JOURNAL № 1 2022 ISSN 0005-2531 (Print)
UDC 544.344:546.57'289'28/24
PHASE EQUILIBRIA IN THE Ag8SiTe6-Ag8GeTe6 SYSTEM
G.M.Ashirov
M.Nagiyev Institute of Catalysis and Inorganic Chemistry, NAS of Azerbaijan
Received 19.10.2021 Accepted 25.11.2021
Phase equilibria in the Ag8SiTe6-Ag8GeTe6 system were experimentally investigated by means of differential thermal analysis and powder X-ray diffraction techniques. It was established that continuous series of solid solutions with Si^Ge substitution are formed in the system. T-x-diagram of the title system and concentration dependence of lattice parameters of the obtained solid solutions were plotted.
Keywords: silver-germanium-silicon tellurides, phase equilibria, solid solutions, T-x-diagram.
doi.org/10.32737/0005-2531-2022-1-89-93
Introduction
Binary and more complex chalcogenides of copper and silver are valuable functional materials [1-3]. In particular, copper and silver chalcogenides with heavy p-elements are of interest as thermoelectric materials [4-8]. Several representatives of this class, due to their unique crystal structure, have ionic conductivity for Cu+ or Ag+ cations, which makes them very promising for use in the development of photoelectrode materials, electrochemical solar energy converters, ion-selective sensors, etc. [1, 9-13]. Recent studies [13-22] have shown that synthetic analogs of the argyrodite mineral with the general formula A8BIVX6 (where A - Cu, Ag; BIV - Si, Ge, Sn; X - S, Se, Te) have a number of valuable functional properties that are the subject of many research groups. In addition, these compounds are environmentally friendly, and their constituent components are widely distributed and low-cost [23].
It is known that the development of new multicomponent materials is based on data on the phase equilibria of the corresponding systems and the thermodynamic properties of phases formed in them. The study of phase equilibria in systems composed of synthetic analogues of ar-gyrodite showed that in a number of such systems, for example Cu8GeS6-Cu8GeSe6, Ag8SnS6-Ag8SnSe6, Ag8GeS6-Ag8GeSe6, Ag8GeS6-Ag8SnS6, Ag8GeSe6-Ag8SnSe6 and Cu8GeSe6-Ag8GeSe6, the continuous solid solutions are formed [24-30]. This is probably due to the similarity of the crystal structure and chemical nature of the starting compounds.
In the present paper phase equilibria in the Ag8SiTe6-Ag8GeTe6 system was studied since there is a possibility of the formation of a solid solution. Starting Ag8SiTe6 and Ag8GeTe6 compounds have very low thermal conductivity and high thermoelectric performance [21, 22].
The Ag8SiTe6 compound melts congru-ently at 1143 K [21], and Ag8GeTe6 melts incon-gruently at 918 K [31] (congruently at 916 K according to [1]). Both compounds crystallize in cubic crystal lattice (F.gr. F-43m) with following parameters: a = 1.1523 nm (for Ag8SiTe6) [32] and a = 1.1563 nm (for Ag8GeTe6) [33].
Experimental part
Starting compounds Ag8SiTe6 and Ag8GeTe6 were synthesized using high-purity elements (at least 99.999 wt.% purity) from the EVOCHEM ADVANCED MATERIALS GMBH (Germany). Both compounds were synthesized by fusion of simple substances in stoichiometric ratios in evacuated to ~10-2 Pa and sealed quartz ampoules (15 cm in length and 1.5 cm in diameter) at temperatures 50° higher than the melting points of the synthesized compounds. The ampoules with the obtained melts were kept at these temperatures for 3-4 hours and then cooled in the switched-off furnace to room temperature. After synthesis, both compounds were heat-treated at 800 K for 500 hours.
The individuality of all synthesized compounds was monitored by powder X-ray diffraction (PXRD) technique and differential thermal analysis (DTA).
Fig. 1. Cooling DTA curve for the Ag8GeTe6 compound
The powder diffraction patterns are fully indexed in a cubic system and calculated lattice parameters agree well with the data of [32, 33]. On DTA heating curves of compounds, one endo-effect was observed at 1141 K (Ag8SiTe6) and 917 K (Ag8GeTe6), which were close to the above literature data within the error limit (±2 K). Note that, taking into account the inconsistency of the data [1] and [31] on the nature of the melting of the Ag8GeTe6 compound we also obtained the cooling DTA curve for this compound. It turned out that two thermal effects were recorded on the cooling DTA curve (Figure 1). One of them corresponds to the liquidus temperature (931 K), and the other one (917 K) to the peritectic reaction of formation of Ag8GeTe6, which confirms the data of [31].
About 8 alloys of the Ag8SiTe6-Ag8GeTe6 system were prepared in different ratios by melting of starting ternary compounds at ~1200 K. The synthesized samples were gradually cooled down to room temperature inside the furnace and then annealed at 800K for 500 hours for the maximum achievement of a state of equilibrium. The synthesized alloys were investigated by DTA and PXRD methods.
DTA was performed on a 404 F1 PEGASUS SYSTEM differential scanning calorimeter from NETZSCH (heating rate 10 K/min, chromel-alumel thermocouples). The DTA measurement results were processed using the NETZSCH Proteus Software. The temperature measurement accuracy was within ±20.
PXRD was carried out at room temperature on a BRUKER D8 ADVANCE diffrac-
tometer with CuKa1 radiation in the range 20 = 5-650. The X-ray patterns were indexed using the Topas V3.0 Software Bruker.
Results and discussion
Figure 2 shows the powder diffraction patterns of annealed Ag8SiTe6-Ag8GeTe6 alloys. As can be seen, all intermediate alloys have qualitatively the same diffraction picture with starting compounds. This indicates the formation of continuous solid solutions in the Ag8SiTe6-Ag8GeTe6 system over the entire concentration range. Slightly shifting of diffraction lines towards small angles with Si^Ge substitution is observed. This is due to the fact that the ionic radius of germanium is larger compared to silicon.
Lattice parameters of both ternary compounds and solid solutions were calculated using T0PAS3.0 computer program. Results are given in Table 1. As can be seen, lattice parameters increase linearly with increasing Ge content (Figure 3b). I.e., the concentration dependence of the lattice parameters follows Vegard's rule.
The formation of continuous solid solutions in the Ag8SiTe6-Ag8GeTe6 system was also confirmed by the DTA results (Figure 3a). Due to the incongruent melting of the Ag8GeTe6 compound, in the >90% mole Ag8GeTe6 composition area from the liquid, the a-phase based on Ag2Te first crystallizes. The crystallization process ends with the formation of a S-phase. The relevant part of the diagram is theoretically constructed because no thermal effects are observed in this area.
Figure 2. Powder diffraction patterns of the Ag8SiTe6 - Ag8GeTe6 alloys (at room temperature). Table 1. DTA and PXRD results for the Ag8SiTe6-Ag8GeTe6 system
Composition, mol% Ag8GeTe6 Thermal effects, (K) Lattice parameters, (Â)
0 (Ag8SiTe6) 1141 11.5244
10 1115 11.5293
20 1086-1105 11.5331
40 1022-1055 11.5392
60 982-1013 11.5463
80 928-960 11.5531
90 921 11.5585
100 917 11.5612
a, A
11.58 11.54 11.50
T, K
1141
1050 950
Ag85iTe6
20
40
60
(b)
(a)
- L + S L
i i 5 i i L+a L+a+6 i
11.56
931 917
80
Ag8GeTe6
mol % Ag8GeTe6
Fig. 3. Phase diagram (a) and concentration dependence of lattice parameters (b) for the Ag8SiTe6-Ag8GeTe6 system.
Conclusion
Phase equilibria in the Ag8SiTe6-Ag8GeTe6 system were studied using DTA and PXRD techniques. It has been established that the system as a whole is nonquasibinary due to the incongruent melting of Ag8GeTe6, and is characterized by the formation of continuous solid solutions with cubic structure. The T-x-diagram of the title system was plotted. The lattice parameters of the starting ternary compounds and solid solutions were calculated by the Rietveld method. The obtained solid solutions are interesting as potential environmentally friendly thermoelectric materials.
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Ag8SiTe6-Ag8GeTe6 SÎSTEMÎNDO FAZA TARAZLIQLARI G.M.Oçirov
Ag8SiTe6-Ag8GeTe6 sisteminda faza tarazliqlan differensial termiki va ovuntu rentgenfaza analizi üsullan ils ôyranilmiçdir. Müayyan edilmiçdir ki, sistemda Si^Ge avazlamali fasilasiz bark mahlullar этэ1э galir. Sistemin T-x diaqrami va bark mahlullarin qafas parametrlarinin tarkibdan asliliq qrafiki qurulmuçdur.
Açar sözlar: gümüg-germanium-silisium telluridbri, faza tarazligi, Ьэгк m3hlullar, T-x diaqrami.
ФАЗОВЫЕ РАВНОВЕСИЯ В СИСТЕМЕ Ag8SiTe6-Ag8GeTe6
Г.М.Аширов
Методами дифференциального термического и рентгенофазового анализов экспериментально исследованы фазовые равновесия в системе Ag8SiTe6-Ag8GeTe6. Установлено, что в системе образуется непрерывный ряд твердых растворов с Si^Ge замещением. Построены Т-х-диаграмма исследуемой системы и график концентрационной зависимости параметров решетки полученных твердых растворов.
Ключевые слова: теллуриды серебра-германия-кремния, фазовые равновесия, твердые растворы, Т-х-диаграмма.
