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AZERBAIJAN CHEMICAL JOURNAL № 3 2019 ISSN 2522-1841 (Online)
ISSN 0005-2531 (Print)
UDC 546 (72.682.23)
PHASE DIAGRAM OF THE FeSe-In2Se3 SYSTEM F.M.Mammadov
M.Nagiev Institute of Catalysis and Inorganic Chemistry, NAS of Azerbaijan
[email protected] Received 03.04.2019
The quasi-binary system FeSe-In2Se3 was re-examined by the methods of DTA and XRD and its T-x phase diagram was constructed. It was established that the diagram is somewhat different from the earlier one given by us in the literature. FeIn2Se4 compound, which congruently melts at 1195 K, was found in the system. This compound forms eutectics with both the initial binary compounds. The crystal structure of FeIn2Se4 is refined by using the powder diffraction data by the Rietveld method.
Keywords: FeSe-n2Se3 system, phase diagram, FeIn2Se4 compound, eutectic, Rietveld method, crystal structure.
https://doi.org/10.32737/0005-2531-2019-3-62-67
Introduction
Complex chalcogenides and phases based on them have long attracted attention due to a number of promising functional properties [14]. Recent studies have shown that some of them are topological insulators and extremely promising for use in spintronics in quantum computing [5-7].
Materials which include AB2X4 com-
1 ^
pounds (A - Mn, Fe, Co, Ni; B - p -p elements, X - chalcogen) are the group known for their anisotropic physical properties that find potential applications in the fields of photode-tectors, switches, and photovoltaic as well as many semiconducting and magnetic applications [8-17].
Design and development of novel methods for the directed synthesis of new complex phases and materials require the study of phase equilibria in the corresponding systems [18-20].
The studies of the phase equilibria in systems MX-Ga2X3-In2X3 (M - Mn, Fe; X - S, Se, Te) were undertaken by us in order to search and develop the physicochemical basis for obtaining new magnetic semiconductors. The preliminary results obtained [21, 22] showed the formation of wide regions of solid solutions along the MGa2X4-MIn2X4 sections of these systems. However, an analysis of the literature data showed that the phase diagrams of some of the boundary quasi-binary components of these quasi-ternary systems require clarification. In [23], new refined data on phase
equilibria in the systems FeSe-Ga2Se3 u Ga2Se3-In2Se3 were presented.
This paper presents the results of repeated studies of the system FeSe-In2Se3.
Experimental part
Synthesis
FeSe and In2Se3 used in research were synthesized using high purity iron (99.995%), indium (99.999%), and selenium (99.99%) purchased from Evochem. The calculated amounts of elements were sealed in a vacuum quartz tube with a residual pressure
of ~10-2 Pa. Then the ampoules were placed in two-zone furniture. The hot zone was maintained at 1300 K (FeSe) or 1200 K (In2Se3), and the temperature of the cold zone was about 900 K, which is slightly below the boiling point of selenium (958 K) [24].
Intermediate samples of the FeSe-In2Se3 system were prepared from pre-synthesized compounds in evacuated quartz ampoules at 1250 K followed thermal annealing at 1000 K (200 h), and then at 700 K (300 h). Some alloys after annealing were quenched in cold water in order to investigate by XRD method.
Analysis
In order to analyze the samples, the differential thermal analysis (DTA) and X-ray powder diffraction (XRD) were used. DTA was carried out using a NETZSCH 404 F1 Pegasus system in the range within room temperature (~1400) K at a heating rate of 10 K min-1.
The crystal structures of the initial compounds and intermediate samples were investigated by powder X-ray diffraction analysis on a Bruker D2 Phaser powder diffractometer (CuXa-radiation) in the 20 range of 10-750. The lattice parameters were indexed using the TOPAS-4.2 software.
Two thermal effects were present on the FeSe thermogram; they corresponded to the peritectoid reaction of formation of P-FeSe (730 K) and melting (1345 K). According to DTA data, synthesized In2Se3 melts at 1160 K. Obtained data are in accordance with the data of [25, 26].
The powder patterns of FeSe and In2Se3 showed the formation of single-phase materials. The calculated lattice constants of tetragonal FeSe (Sp.gr. P4-nm, a=0.37743(4), c=0.55301 (7) nm), and hexagonal In2Se3 (Sp.gr. R3m, a=0.405(2), c=2.877(4), z=3) nm were consistent with the literature data [27, 28].
Results and discussion
Phase diagram of the FeSe-In2Se3 system
The phase diagram constructed (Figure 1) refers to the dystectic type. The intermediate y-phase with the stoichiometric composition of FeIn2Se4 melts congruently at 1195 K. This compound forms eutectics with both the initial compounds of the system, which have the com-
positions 40 (ei) and 70 (e2) mol% In2Se3 and crystallize at 1170 and 1070 K, respectively. At the eutectic temperature e1, the area of homogeneity of solid solutions on the basis of a high-temperature modification of iron selenide is about 3 mol%, and the y-phase is about 5 mol% (48-53 mol% In2Se3). At the eutectic temperature e2, the homogeneity region of the p4-phase on the basis high-temperature modification of In2Se3 reaches 9 mol%, and the y-phase is about 7 mol% (48-55 mol% In2Se3). The system is characterized by a series of solid-phase reactions associated with the polymorphism of FeSe and In2Se3. Eutectoid equilibrium a ^ (FeSe)1 + y
occurs at 727 K.
Horizontal lines at 980, 915 and 483 K correspond to eutectoid
P4 ~ P3 + Y, P3 ^ P2 + Y
and peritectoid
P2 + Y ~P1
equilibria; herein p1, p2, p3, and p4 - solid solutions based on various crystalline modifications of In2Se3).
The ternary compound of composition Fe2In2Se5, described in [29], was not found by us. XRD data showed that the alloy of this composition is a two-phase mixture of FeIn2Se4 and FeSe.
Fig.1. Phase diagram of the FeSe-In2Se3
The refined T-x diagram of the FeSe-In2Se3 system (Figure 1) is slightly different from that given in [30]. First, the melting point of the compound FeIn2Se4 determined by us is significantly higher than the value (1163 K) specified in [30]. There are also discrepancies in the coordinates of the eutectic, eutectoid and pe-ritectoid equilibria and the length of the homogeneity regions of the p1, p2, p3, and p4 phases.
Refinement of the crystal structure of the FeIn2Se4
The lattice type and unit cell parameters for the powder sample FeIn2Se4 were determined based on the obtained powder XRD pattern. Figure 2 displays the XRD pattern and the intensities differences between the experimental and calculated by Rietveld method data.
The refined unit cell parameters are shown in Table 1, and the atomic positional parameters are given in Table 2.
The crystal structures of MB2X4-type layered compounds consist of seven-layer packets [31].
180,000 170,000 160,000 150,000 140,000 130,000 120,000 110,000 100,000 90,000 80,000 70,000 60,000 50,000 40,000 30,000 20,000 10,000
-10,000 -20,000
Table 1. Refined structure parameters and interatomic
distances of the FeIn2Se4
Structure parameters FeIn2Se4
Space group R-3m
Lattice parameters: a (nm) c (nm) 0.40219(13) 3.9161(25)
Cell volume(nm3) 0.5486(50)
Density (g/cm3) 5.4604(50)
R-Bregg (%) 2.964
These packets are formed from atomic layers in the following sequence: X-B-X-M-X-B-X. If we characterize the structure of these packets in polyhedra, it can be shown that they consist of three polyhedral layers. In this case, the central layer consists of octahedra connected by common edges This layer of octahedra is surrounded on two sides by layers of tetrahedra that are connected by vertices. Tetrahedral positions are occupied by B atoms, and the central layer of octahedra is only occupied by M atoms.
Fig.2. Observed and calculated powder diffraction pattern and difference plot for FeIn2Se4. Table 2. Atomic positional parameters in FeIn2Se4
Atom Multiplicity of positions X y z Atom type and relative occupation
Fe 3 0.00000 0.00000 0.00000 Fe+2 1
In 6 0.33333 0.66667 0.0896(12 In+3 1
Se1 6 0.33333 0.66667 0.0276(14) Se 1
Se2 6 0.66667 0.33333 0.1187(16) Se 1
Fig.3. Occupancy of metal positions in the FeIn2Se4 crystal structure.
References
Conclusion
A new scheme of phase relations in the quasi-binary system FeSe-In2Se3, somewhat different from the previously given in the literature, has been constructed. It was shown that the only ternary compound of the FeIn2Se4 system melts congruently at 1195 K and forms eu-tectics with both the initial binary compounds. The eutectic coordinates determined by us differ from the literature data. The crystal structure of FeIn2Se4 is refined by treatment of the powder diffractogram using the Rietveld method.
Obtained experimental results can be used for choosing the composition of solution-melt for the growth of the high-quality crystals of intermediate phases which are of interest as magnetic semiconductors.
Acknowledgment
This work was supported by the Science Development Foundation under the President of the Republic of Azerbaijan - Grant № EIF/MQM/Elm-Tehsil-1-2016-1(26)-71/01/4-M-33.
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FeSe-In2Se3 SÎSTEMÎNÎN FAZA DiAQRAMI F.M.Mammadov
FeSe-In2Se3 kvazibinar sistemi DTA va RFA metodlari ils yenidan tadqiq edilmiç va onun T-x faza diaqrami qurulmuçdur. Müayyan edilmiçdir ki, diaqram avvaldan adabiyyatda verilandan bir qadar farqlanir. Sistemda 1195 K-da konqruent ariyan FeIn2Se4 birlaçmasi amala galir. Bu birlaçma har iki llkin binar birlaçmalarla evtektika amala gatirir. FeIn2Se4 birlaçmasinin kristal quruluçu ovuntu rentgenoqraminin naticalari asasinda Ritveld metodu ila daqiqlaçdirilmiçdir.
Açar sözlar: FeSe-In2Se3 sistemi, faza diagrami, FeIn2Se4 birh§masi, evtektika, Rietveld metodu, kristal qurulu§.
ФАЗОВАЯ ДИАГРАММА СИСТЕМЫ FeSe-In2Se3 Ф.М.Мамедов
Квазибинарная система Ре8е-1п28е3 была повторно исследована методами БТЛ и РФА и построена ее фазовая диаграмма. Установлено, что диаграмма несколько отличается от ранее приведенной в литературе. В системе обнаружено соединение Бе1п28е4, конгруэнтно плавящееся при 1195 К. Это соединение образует эвтектику с обоими исходными бинарными соединениями. Методом Ритвельда на основании порошковых дифракционных данных уточнена кристаллическая структура Ре1п28е4.
Ключевые слова: система ЕеБе-1п28е3, фазовая диаграмма, соединение Ее1п2Бе4, эвтектика, метод Ритвельда, кристаллическая структура.
