CC BY
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ISSN 2522-1841 (Online) ISSN 0005-2531 (Print)
AZERBAIJAN CHEMICAL JOURNAL № 4 2021
67
UDC 544.344.3:546.56123
SOLUBILITY OF GERMANIUM DISELENIDE IN THE Cu3AsSe4
Z.T.Hasanova
Baku State University, Baku, Azerbaijan roya707@mail.ru Received 14.07.2021 Accepted 25.08.2021
Differential thermal analysis and powder X-ray diffraction methods were used for phase equilibria studying in the Cu3AsSe4-GeSe2 system. It was established that wide (up to 30 mol.%) solid solutions based on Cu3AsSe4 are formed. GeSe2-rich alloys consist of various heterogeneous mixtures, including phases outside the T-x plane of this section.
Keywords: copper-based chalcogenides, copper-arsenic selenide, solid solutions, phase equilibria. doi.org/10.32737/0005-2531-2021-4-67-70 Introduction
Ternary and complex copper chalcogenides attract attention as promising functional materials due to their photoelectric, thermoelectric, nonlinear optical, and other properties [1-3]. In particular, copper-based chalcogenides with arsenic subgroup elements are synthetic analogs of multiple copper minerals, such as tetraedrite (Cu12Sb4S13), tennantite (Cu12As4S13), famatinite (Cu3SbS4), enargite (Cu3AsS4), etc., are promi-sing thermoelectric (TE) materials due to their excellent transport properties and relatively low thermal conductivity [4-10]. These phases are one of the most studied in recent years' compounds, since their good thermoelectric performance, as well as widespread inexpensive and non-toxic constituents. However, the thermoselectric figure of merit of such phases is too low to use in practice due to the low concentration of charge carriers and, as a consequence, poor electrical conductivity. Substitution doping and obtaining non-stoichiometric phases is the best and proven strategy for changing the electronic structure and TE performance of a material [11]. For this purpose, it is necessary to investigate phase equilibria in the corresponding systems [12, 15]. In a number of works [16, 17], complex systems based on copper chalcogenides were studied and new nonstoichiometric phases were discovered in them.
In this work, the Cu3AsSe4-GeSe2 section of the Cu-Ge-As-Se system was investigated to search and study solid solutions with As^Ge substitution. Similar studies were carried out in Cu3SbX4-GeX2 (X - S, Se) systems [18, 19], and a region of solubility based on Cu3SbX4
up to 15 mol% was found in both systems.
Here, some physico-chemical properties of the starting compounds Cu3AsSe4 and GeSe2. Cu3AsSe4 melts congruently at 773 K and has a polymorphic transition at 715 K [20]. The high-temperature modification crystallizes in the cubic system, while the low-temperature modification crystallizes in a tetragonal structure (Sp. gr. I42m) with lattice parameters: a = b = 5.53 A; c = 11.83 A; a = P=y = 90 [21].
Germanium diselenide melts on distectic reaction at 1015 K [22]. GeSe2 crystallizes in a monoclinic structure (space group P21/c) with lattice parameters: a=7.016(5) A; b=16.796(8) A; c=11.831(5) A; p = 90.65(5)0 [23].
Experimental part
The initial Cu3AsSe4 and GeSe2 compounds were preliminarily synthesized for the experiments from 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%), Germanium pieces (Ge-00003; 99.9999%), selenium granules (Se-00002; 99.999%). Synthesis was performed by fusion of simple substances in stoichiometric ratios in evacuated to ~10-2 Pa and sealed quartz ampoules at temperatures 50° higher than the melting temperatures 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.
All synthesized compounds were control-
led by differential thermal analysis (DTA) and PXRD methods. The obtained values of the melting temperatures and the crystal lattices parameters for all synthesized compounds within the error limit (±3 K and ±0.0003 A) were close to the above literature data.
About fifteen Cu3AsSe4-GeSe2 alloys were prepared by alloying the initial compounds under vacuum conditions. Further, cast alloys obtained by rapid cooling of melts were annealed at 650 K within 700 hours in order to achieve a state as close as possible to equilibrium.
The prepared samples were investigated by DTA and PXRD methods. DTA was performed on a 404 F1 PEGASUS SYSTEM differential scanning calorimeter (Netzsch). The heating rate was 10 K-min-1. The DTA measurement results were processed using the NETZSCH Proteus Software. The temperature measurement accuracy
was within ±2°.
PXRD was carried out at room temperature on a D8 ADVANCE diffractometer (Bruker) with CuKa1 radiation. The X-ray patterns were indexed using the Topas V3.0 Software Bruker.
Results and discussion
XRD analysis of the annealed Cu3AsSe4-GeSe2 alloys showed that samples containing no more than 30 mol.% GeSe2 are single-phase. Their PXRD patterns have diffraction peaks identical for the pure Cu3AsSe4 compound with a slight shift to the big angles (i.e. right) (Figure 1).
This indicates the formation of up to 30 mol% solid solution based on the Cu3AsSe4 compound. The following lattice parameters were calculated by indexing these powder diffraction patterns:
Cu3AsSe4 a=5.5427 A
(Cu3AsSe4)0.1(GeSe2)0.9 a=5.6389 A
(Cu3AsSe4)0.15(GeSe2)0.85 a=5.6305 A
(Cu3AsSe4V20(GeSe2V80 a=5.6235 A
(Cu3AsSe4)0.30(GeSe2)0.70 a=5.6158 A
c=11.8411 A c=11.8382 A c=11.8309 A c=11.8245 A c=11.8129 A
Fig. 1. PXRD patterns for the CusAsSe4-GeSe2 alloys.
SOLUBILITY OF GERMANIUM DISELENIDE IN THE Cu3AsSe4
69
The formation of solid solutions in the Cu3AsSe4-GeSe2 system was also confirmed by the DTA results (Figure 2). However, we found that the Cu3AsSe4-GeSe2 system is generally non-quasi-binary and is characterized by a complex interaction. The DTA data for alloys with a high GeSe2 content could not be interpreted. Therefore, Figure 2 shows a fragment of the phase diagram of the Cu3AsSe4-GeSe2 system.
Fig. 2. A fragment of the phase diagram for the Cu3AsSe4-GeSe2 system.
The nature of phase equilibria in a similar system Cu3SbSe4-GeSe2 was studied in more detail in [18]. It has been shown that the Cu3SbSe4-GeSe2 section is located in 4-phase regions Cu3SbSe4+Cu2GeSe3+Sb2Se3+Se and Cu2GeSe3+Sb2Se3+GeSe2+Se of the concentration tetrahedron Cu2Se-GeSe2-Sb2Se3-Se. These areas are delimited by a stable concentration triangle Cu2GeSe3-Sb2Se3-Se. Our preliminary data suggest that the nature of phase equilibria in the Cu3AsSe4-GeSe2 system is qualitatively similar to the indicated system.
Conclusion
An experimental study of phase equilibria in the Cu3AsSe4-GeSe2 system has been carried out by means of DTA and PXRD methods. The solubility based on Cu3AsSe4 reaches up to 30 mol% GeSe2. Phase equilibria in the GeSe2-rich area are complex and alloys consisting of various heterogeneous mixtures, including phases outside the T-x plane of this section. The obtained solid solutions are interesting as potential environmentally friendly functional materials.
Acknowledgment
This work was supported by the Science Development Foundation under the President of the Republic of Azerbaijan - Grant № EiF-BGM-4-RFTF-1/2017-21/11/4-M-12.
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GERMANiUM DiSELENiDiN Cu3AsSe4 BiRLO§MOSiNDO HOLL OLMASI
Z.T.Has3nova
Differensial termiki analiz va toz rentgenoqrafiyasi üsullan ila Cu3AsSe4-GeSe2 sisteminda faza tarazliqlan öyranilmiijdir Müayyan edilmi§dir ki, sistemda Cu3AsSe4 birla§masi asasinda geni§ (30 mol%-a qadar) bark mahlul sahasi mövcuddur. GeSe2 ila daha zangin xalitalar müxtalif fazalarin, o cümladan tarkibca bu kasiyin T-x müstavisinda olmayan fazalarin heterogen qari§iqlanndan ibaratdir.
Agar sözlar: mis asasinda xalkogenidlar, mis-arsen selenidlari, bark mahlullar, faza tarazliqlan.
РАСТВОРИМОСТЬ Д^ЕЛЕНИДА ГЕРМАНИЯ В Cu3AsSe4
З.Т.Гасанова
Методами дифференциального термического анализа и порошковой рентгенографии изучены фазовые равновесия в системе Cu3AsSe4-GeSe2. Установлено, что в системе образуются широкая (до 30 мол.%) область твердых растворов на основе Cu3AsSe4. Сплавы с большим содержанием GeSe2 состоят из различных гетерогенных смесей, включая фазы вне T-x плоскости этого сечения.
Ключевые слова: халькогениды на основе меди, селениды меди-мышьяка, твердые растворы, фазовые равновесия.
