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CHEMICAL PROBLEMS 2018 no. 4 (16) ISSN 2221-8688
UDC 541.123/.123.8/9:546.57'81'86/23
PHASE EQUILIBRIA IN THE PbSe-AgSbSei SYSTEM Mansimova Shabnam Hamlet1, Babanly Kamala Nagi2, Mashadiyeva Leyla Farhad2
'Baku State University 23, Z.Khalilov str., AZ-1148, Baku, Azerbaijan 2Acad.M. Nagiyev Institute of Catalysis and Inorganic Chemistry National Academy of Sciences ofAzerbaijan; 113, H.JavidAve., AZ-1143, Baku, Azerbaijan, e-mail: leylafm76@gmail.com
The phase equilibria in the PbSe-AgSbSe2 system were studied by means of differential thermal analysis, powder X-ray diffraction technique and micro-hardness measurement. A T-x diagram and composition dependence graphs of the lattice period and micro-hardness were constructed. It found that the PbSe-AgSbSe2 system is quasibinary and pertaining to the peritectic type. Note that at room temperature the AgSbSe2-based solubility is 80 mol% and the PbSe-based solubility is ~2 mol%.
Keywords: PbSe-AgSbSe2 system, phase diagram, solid solutions, crystal lattice, micro-hardness DOI: https://doi.org/10.32737/2221-8688-2018-4-530-536
1. INTRODUCTION
Heavy metals-based chalcogenides and their derivative complex phases have drawn tremendous attention in recent years because of their potential use in high-technologies due to their outstanding physical properties [1-5]. Various materials such as Ag-BV-X and Ag-AIV-BV-X (where AIV-Sn, Pb; BV-Sb, Bi; X-S, Se, Te) alloys have high ZT values and are among the most promising thermoelectric materials [6-8]. In particular, ternary semiconducting compounds with general formula AgBVX2 excited interest due to their thermoelectric, optical and electronic properties. In addition, they are phase-changing materials and can be used as a switching medium in rewritable optical memories [9-11].
Optimization of functional properties of these materials can be achieved by changing their composition. It is based, in turn, on the
research into phase equilibria in the systems consisting of structural analogues, since they are expected to form wide range solid solutions [12-15].
In this paper, we present the results of the research into phase equilibria in the PbSe-AgSbSe2 system. Some similar systems such as PbSe-AgBiSe2, PbTe-AgBiTe2, SnTe-AgSbTe2, and SnTe-AgBiTe2 were studied earlier and new phases of variable composition discovered therein [16-20].
Lead selenide melts congruently at 1354 K [21] and crystallizes in a cubic NaCl structure (Sp.Gr.Fm3m) with a unit-cell parameter a=6.1243A [22].
The AgSbSe2 compound also melts congruently at 908 K [23] and crystallizes in a cubic structure (Sp.Gr.Fm3m) with a lattice parameter a = 5.786 A [24].
2. EXPERIMENTS
Synthesis
Owing to their congruent character of melting, the PbSe and AgSbSe2 compounds crystallize from a melt of stoichiometric composition upon cooling. For the synthesis, the elementary components with a purity of at
least 99.999% were used. Stoichiometric amounts of the starting components were put into silica tubes with a diameter of about 1.5 cm and sealed under a pressure of 10-2 Pa. AgSbSe2 was synthesized by direct synthesis in a resistance furnace at 950 K followed by
cooling in a switched-off furnace. PbSe was synthesized in a two-zone inclined furnace. The lower hot zone was heated to 1400 K, and the cold one to 900 K, which is somewhat lower than the boiling point of selenium (958 K) [20]. The purity of the synthesized compounds was checked by the by differential-thermal analysis (DTA) and powder X-ray diffraction technique (PXRD).
Methods
Phase equilibria in the PbSe-AgSbSe2 system were investigated by means of DTA and PXRD technique, as well as micro-hardness measurement.
DTA of the equilibrated alloys was carried out using a NETZSCH 404 F1 Pegasus system. The measurement was performed between a room temperature and ~1400 K with heating and cooling rate of 5 K-min"1 under the vacuum. Temperatures of thermal effects were taken mainly from the heating
Melting temperatures and crystallographic parameters of obtained compounds nearly coincide with the literature data [22, 24] (Tables 1 and 2).
The alloys of the 2PbSe-AgSbSe2 system were prepared by melting the starting compounds in quartz ampules under vacuum followed by homogenizing annealing at 800 K (700 h).
curves. NETZSCH Proteus Software was used to measure and evaluate the data obtained.
The PXRD analysis was performed with the Bruker D8 diffractometer (CuKa radiation), with a step size of 0.02° between 10°< 29 < 70°; the data obtained were collected at room temperature. The unit cell parameters of examined alloys were calculated by indexing of powder patterns and the use of Topas V3.0 software.
The micro-hardness was measured on a PMT-3 microhardometer with a load of 20 g.
3. RESULTS
Based on the obtained experimental data (Table 1), a phase diagram and the dependence graph of the micro-hardness on the composition of the PbSe-AgSbSe2 system is constructed (Fig. 1). This system is a quasibinary cross-section of the Ag2Se-PbSe-Sb2Se3 system and is characterized by peritectic equilibrium. The peritectic coordinates correspond to 18 mol% AgSbSe2 and 1220 K. At a peritectic temperature the solubility based on AgSbSe2 is 87 (y-phase) and based on PbSe is 5 mole% (P-phase), and at room temperature is 80 and ~2 mole%, respectively. The minimum point (M) is observed on liquidus and solidus curves of the y-phase.
The results of micro-hardness measurements are in good agreement with the phase diagram. Fig.1b shows the graph of composition dependence of the micro-hardness for the PbSe-AgSbSe2 alloys hardened from 800 K. The micro-hardness of the y-phase
gradually increases from 1100 to 1550 MPa. Then the value of micro-hardness remains constant. As for alloys of 2, 5 and 10 mol%, the micro-hardness of the ß-phase has a constant value (750 MPa) and is somewhat higher than for the pure PbSe. This shows that in the range of 2-20 mol% AgSbSe2 composition the alloys of the system consist of a two-phase mixture ß+y.
In Fig. 2 presents the powder X-ray diffraction patterns of some annealed PbSe-AgSbSe2 alloys. The compositions of alloys for PXRD analyzes are expressed as 2PbSe-AgSbSe2. This is necessary to determine the possible linear dependence of the lattice period on composition. As can be seen, the diffraction patterns of alloys containing >40 mol% AgSbSe2 are qualitatively similar to those of pure AgSbSe2. Note that X-ray diffraction patterns of alloys with compositions of 10 and 30 mol% AgSbSe2 consist of a set of diffraction lines of two cubic phases.
Hji, MPa 1500 1000 500
T, K
1300
1100
900
b)
"750
* i i i 1
1355 WL+P a)
j.V-V'N p L
1 V^L+y
1 \ 1 ß+r 1 i i i i i i i il
y i i 1
PbSe
20
40
60
1100
905
80
mol% AgSbSe,
AgSbSe2
Fig.1. Phase diagram (a) and composition dependence graph of the micro-hardness of the alloys hardened from 800 K (b) for the PbSe-AgSbSe2 system
^10000 o o
5000 4000 3000
Jl 1 I . .
6 1
.J ................. 1 «
4 1 . .
3 .. J
" 2 ..A...........JL
1 , 1 --A- ------*------ -A................. - —
2-Theta - Scale
Fig.2. XRD powder patterns for starting compounds and some alloys of the 2PbSe-AgSbSe2 system. 1-PbSe; 2-10mol% AgSbSe2; 3-30mol% AgSbSe2; 4-40mol% AgSbSe2; 5-60mol% AgSbSe2; 6-80 mol% AgSbSe2; 7-AgSbSe2.
Table 1. DTA data and micro-hardness measurements for the PbSe-AgSbSe2 system
Compositions, mol% AgSbSei Thermal effect, K Micro-hardness, MPa
0 (PbSe) 1355 650
2 1280; 1345 750
5 1220; 1325 750
10 1220; 1285 750;1550
20 1110;1210 1550
30 860; 1125 1520
40 995; 1080 1500
50 950;1025
60 920; 970 1420
70 900; 925
80 890 1250
90 890
100 (AgSbSe2) 905 1100
Table 2. Phase compositions and crystallographic parameters of phases _for the 2PbSe-AgSbSe2 system_
Compositions, mol% AgSbSei Phase compositions Cubic lattice parameters, Â
0 (PbSe) P a=6.1246(5)
10 p+y a=6.1184(6); a=6.0091(7)
20 P+Y a=6.1187(7); a=6.0101(7)
30 P+Y a=6.1185(6); a=6.0094(7)
40 Y a=5.9882(6)
60 Y a=5.9244(5)
80 Y a=5.8521(5)
100 (AgSbSe2) Y a=5.7882(5)
To determine the mutual solubility of the starting compounds in the analyzed system, we plotted the concentration dependences of the cubic lattices parameters (Fig. 3). The unit cell parameters of intermediate alloys were calculated (Table 2). An accuracy of the crystal lattice parameters is shown in parentheses. It revealed that the dependence has fracture points in the composition ~39 mol% AgSbSe2 which correspond to the limiting composition of y-solid solutions based on AgSbSe2. The composition of 39 mol% AgSbSe2 in the 2PbSe-AgSbSe2 system corresponds to the alloy with ~24 mol% AgSbSe2 on the phase diagram (Fig. 1). It
should be noted that in the P+y two-phase region, the lattice periods of the two coexisting phases have constant values regardless of the overall composition of the alloys while within the limits of homogeneity region of the y phase the lattice period is a linear function of the composition.
The difference between the examined PbSe-AgSbSe2 system and above-mentioned PbSe-AgBiSe2, PbTe-AgBiTe2, SnTe-AgSbTe2 and SnTe-AgBiTe2 [16-20] is that the continuous series of high-temperature solid solutions were detected in them. The presence of a wide interval (3-39 mol% AgSbSe2) of solubility gap in the system PbSe-AgSbSe2
(Fig. 3) is apparently associated with the fact periods of the starting compounds is more than that the difference between the crystal lattice in other systems examined.
a, Â 6,2
6,0
5,8
J 6,115 m m m
i i 6,009
J t i i i i y 1 1 1
2PbSe 20
80
AgSbSe,
40 60 mol%AgSbSe3
Fig.3. Concentration dependences of cubic lattices parameters
3. CONCLUSION
The PbSe-AgSbSe2 system is a quasibinary cross-section of the Ag2Se-PbSe-Sb2Se3 system and is characterized by peritectic equilibrium (18 mol% AgSbSe2 and 1120 K). Formation of a wide area of solid solutions based on AgSbSe2 (~80 mol%) has
been revealed. The solubility based on PbSe is much lower not to exceed 2 mol%. The crystal lattice parameters and micro-hardness values of the obtained solid solutions have been determined.
4. ACKNOWLEDGMENTS
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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PbSe-AgSbSe2 SiSTEMiNDd FAZA TARAZLIQLARI
1§.H. Mansimova, 2K.N. Babanli, 2L.F. Ma§adiyeva
Baki Dovldt Universiteti AZ 1148 Baki, Z.Xdlilov kug., 23 AMEA-nin akad. M.Nagiyev adina Kataliz vd Qeyri-uzvi Kimya institutu AZ1143, Baki, H.Cavidpr., 113; e-mail: eylafm76@gmail.com
Differensial termiki vd rentgenfaza analizi usullari ild, hdmginin mikrobdrkliyin olgulmdsild PbSe-AgSbSe2 sistemindd faza tarazliqlari oyrdnilmi§dir. Sistemin T-x diaqrami, hdmginin qdfds parametrldrinin vd mikrobdrkliyin tdrkibddn asililiq qrafikldri qurulmu§dur. Mudyydn edilmi§dir ki, PbSe-AgSbSe2 sistemi kvazibinardir vd peritektik tipli hal diaqrami dmdld gdtirir. Otaq temperaturunda AgSbSe2 dsasinda hdllolma 80 mol%, PbSe dsasinda isd <2 mol% td§kil edir. Agar sozlar: PbSe-AgSbSe2 sistemi, faza diaqrami, bdrk mdhlullar, kristal qdfds, mikrobdrklik.
ФАЗОВЫЕ РАВНОВЕСИЯ В СИСТЕМЕ PbSe-AgSbSe2
1Ш.Г. Максимова, 2К.Н. Бабанлы, 2Л.Ф. Машадиева
Бакинский государственный университет AZ1148 Баку, ул. З.Халилова, 23 Институт катализа и неорганической химии им. акад. М.Нагиева Национальной АН Азербайджана AZ1143Баку, пр.Г.Джавида, 113; e-mail: eylafm76@gmail.com
Фазовые равновесия в системе PbSe-AgSbSe2 изучены методами дифференциального термического и рентгенофазового анализов и измерением микротвердости. Построены T-x диаграмма и графики зависимости периода решетки и микротвердости от состава. Установлено, что система PbSe-AgSbSe2 является квазибинарной и относится к перитектическому типу. При комнатной температуре растворимость на основе AgSbSe2 составляет 80 мол.%, а на основе PbSe <2 мол.%.
Ключевые слова: система PbSe-AgSbSe2, фазовая диаграмма, твердые растворы, кристаллическая решетка, микротвердость
