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CHEMICAL PROBLEMS 2019 no. 1 (17) ISSN 2221-8688 41
UDC 541.123/.123.8/9:546.57'81'86/23
PHASE EQUILIBRIA IN THE Ag2Se-PbSe-AgSbSe2 SYSTEM
1Sh.H. Mansimova, 2K.N. Babanly, 2L.F. Mashadiyeva, 1R.J. Mirzoyeva, 2M.B. Babanly
1Baku State University Z. Khalilov, 23, AZ-H48, Baku, Azerbaijan 2Institute of Catalysis and Inorganic Chemistry National Academy of Sciences of Azerbaijan 113, H.JavidAve., AZ-H43, Baku, Azerbaijan; e-mail: leylafm76@gmail.com
Received 16.12.2018
The work presents results of the study into phase equilibriums in the Ag2Se-PbSe-AgSbSe2 through the methods of differential thermal and X-ray diffraction analyses. Based on the experimental data, polythermic sections of AgSbSe2-(PbSe)0,5(Ag2Se)0,5 and Ag2Se-(PbSe) 0,5(AgSbSe2)0,5, isothermal section at 300 K of the phase diagram, as well as projection of the liquidus surface were constructed. It showed that the system cited above is a quasi-ternary plane of the Ag-Pb-Sb-Se quaternary system characterized by the presence of non-invariant transition equilibrium. The fields of primary crystallization of phases, types and coordinates of non- and monovariant equilibria were determined. A wide range (~80 mol%) of solid solutions based on AgSbSe2 along the PbSe-AgSbSe2 section was revealed.
Keywords: phase diagram, solid solutions, liquidus surface, silver-antimony selenide, lead monoselenide.
Doi.org/10.32737/2221-8688-2019-1-41-49
INTRODUCTION
In recent decades, the study of thermoelectric materials has attracted increasing attention both from an energy and environmental point of view. In addition, the development of highly efficient thermoelectric devices for heat waste recovery systems can bring enormous economic benefits [1-5]. Recent studies have shown that Ag-BV-X and Ag-AIV-BV-X (where AIV-Sn, Pb; BV-Sb, Bi; X-S, Se, Te) alloys show high ZT values of thermoelectric figure of merit [6-10]. In particular, ternary compounds with the general formula AgBVX2 attract the attention of researchers due to their thermoelectric, optical, and electronic properties [11-15]. The functional properties of such materials can be improved using such processes as doping, obtaining of related solid solutions and composites. And this, in turn, is based on the study of phase equilibria in the corresponding systems [16-18].
Earlier, we carried out complex studies of phase equilibria and thermodynamic properties of the Ag2Se-PbSe-Bi2Se3 [19],
Ag2Te-PbTe-Bi2Te3 [20], Ag2Te-SnTe-Bi2Te3 [21] and Ag2Te-SnTe-Sb2Te3 [22, 23] systems where wide areas of solid solutions along AIVX-AgBVX2 sections are revealed.
This paper presents new experimental data on phase equilibria in the quasi-ternary Ag2Se-PbSe-AgSbSe2 system.
All the initial components of the title system, which are semiconductors [24], have been studied quite well. Lead monoselenide melts congruently at 1354 K [25] and crystallizes in a NaCl-type crystal lattice (Sp.gr. Fm3m) with the unit cell parameter a = 6.1243A [26].
Silver selenide is characterized by polymorphism and mixed electron-ion conductivity [27]. This compound melts congruently at 1170 K [28]. According to the authors of [29], the a^P phase transition in Ag2Se occurs at 407.7±0.5 K. The low-temperature P-Ag2Se phase crystallizes in an orthorhombic structure (Sp.gr. P 212121) with cell parameters: a = 4.3359 A, b = 7, 0700 A, c = 7.7740 A, Z = 4 [30]. The high-temperature
a-Ag2Se phase forms crystals of the cubic system (Sp.gr. I m3m) with cell parameters a = 5.043 A, Z = 2 [31].
The AgSbSe2 compound also melts congruently at 908 K [32] and forms a cubic crystal lattice of the NaCl type (Sp.gr. Fm3m) with the parameter a = 5.786 A [33].
The boundary quasi-binary systems of Ag2Se-PbSe, Ag2Se-AgSbSe2 and PbSe-AgSbSe2 have been studied in a number of works. The Ag2Se-PbSe system has a simple eutectic type phase diagram. Eutectics melts at 933 K and contains 25 mol. % PbSe [34] (22 mol.% PbSe according to [35]).
The Ag2Se-AgSbSe2 system forms an
eutectic-type phase diagram [31]. The eutectic melt contains 46 mol. % AgSbSe2 and crystallizes at 813 K.
The PbSe-AgSbSe2 system was recently studied in [36] and its phase diagram was constructed. It is shown that the system is quasi-binary and belongs to the peritectic type (Fig. 1). Peritectics has coordinates of 18 mol.% AgSbSe2 and 1220 K. Solubility on the basis of AgSbSe2 and PbSe at the peritectic temperature is 87 (y-phase) and 5 mol% (P-phase), and at room temperature 80 and ~2 mol%, respectively. There is a minimum point (M) on the curves of the liquidus and solidus.
AgSbSe,
Fig.1. Phase diagram of the PbSe-AgSbSe2 system [36]
EXPERIMENTAL PART
For the experiments, the initial compounds of the studied system of Ag2Se, PbSe and AgSbSe2 were obtained. The synthesis was carried out by fusing the corresponding elementary components in stoichiometric ratios in evacuated to
~10"z Pa
and sealed quartz ampoules at temperatures 50° above the melting points of the compounds. Simple substances from the company EVOCHEM ADVANCED MATERIALS GMBH (Germany) of high purity were used for the synthesis: silver in granules (Ag-00047; 99.999%), antimony in
granules (Sb-00002; 99.999%), lead in granules (Pb-00005; 99.9995%), selenium in granules (Se-00002; 99.999%). Taking into account the high vapor pressure of selenium at the melting temperatures of binary selenides of Ag2Se and PbSe, the synthesis of both compounds was performed in the two-zone mode. The hot zone temperature was 1150 K for Ag2Se and 1390 K for PbSe, and the cold zone was 900 K, which is slightly below the boiling point of selenium (958 K) [37]. According to the recommendations of
the authors of [38], the ampoule with Ag2Se melt was quenched from a temperature of 1100 K into cold water after the synthesis in order to obtain a uniform stoichiometric composition of this compound. All the synthesized compounds were checked by the methods of DTA and PXRD. The obtained values of melting points and lattice parameters for all synthesized compounds were close to the above literature data within the error (±3 K and ±0,0003 A).
For experiments, a series of alloys along the AgSbSe2-[A] and Ag2Se-[B] sections (where [A] and [B] are alloys with composition (PbSe)0,5(Ag2Se)0,5 and
(PbSe)0,5(AgSbSe2)0,5, respectively), as well as a number of additional alloys outside of them were prepared by melting the initial
compounds under vacuum. Cast non-homogenized samples were annealed at 770 K (500 h) in order to achieve a state closest to the equilibrium state in alloys.
DTA was performed in range from room temperature to 1400 K with a heating rate of 10 K- min-1 on a NETZSCH 404 F1 PEGASUS SYSTEM differential scanning calorimeter. The measurement results were processed using the NETZSCH Proteus Software. The accuracy of temperature measurement was within ±2 K.
X-ray phase analysis was carried out at room temperature on a BRUKER D8 ADVANCE diffractometer with CuKa1 radiation. X-ray patterns were indexed using BRUKER TOPAS V3.0 Software.
RESULTS AND DISCUSSION
By means of joint processing of obtained experimental results and literature data on the boundary quasi-binary systems of Ag2Se-PbSe [34,35], Ag2Se-AgSbSe2 [32] and
PbSe-AgSbSe2 [36], we obtained a mutually agreed picture of phase equilibria in the Ag2Se-PbSe-AgSbSe2 system.
Ajt.St
PbSiï 20 40 60 KO A^SbSC-
moil% AgSbSe,
Fig. 2. Isothermal section at 300 K of the phase diagram of the Ag2Se-PbSe-AgSbSe2 system. # 1 and # 2 - alloys with powder X-ray patterns are presented in Fig.3.
Phase equilibria at room temperature. The obtained diagram of solidphase equilibria (Fig. 2) in the Ag2Se-PbSe-AgSbSe2 system clearly shows the location of phase regions at 700 K. As can be seen, Ag2Se forms connodes with the P- and y-phases based on PbSe and AgSbSe2, respectively. As a
result, the concentration triangle is divided into two two-phase and one three-phase areas. The phase compositions of the alloys are confirmed by PXRD technique. For example, Fig. 3 shows powder diffraction patterns of two alloys: # 1 with a composition of 20
mol.% Ag2Se, 62 mol.% AgSbSe2, 18 mol.% PbSe and # 2 with a composition of 50 mol.% Ag2Se, 6 mol.% AgSbSe2, 44 mol.% PbSe. As can be seen from Figure 3, alloy # 1 consists
of a two-phase mixture (Ag2Se)i+y, and alloy # 2 consists of a three-phase mixture (Ag2Se)i+p+y.
Fig. 3. Powder X-ray patterns of alloys # 1 (20 mol.% Ag2Se, 62 mol.% AgSbSe2, 18 mol.% PbSe) and # 2 (50 mol.% Ag2Se, 6 mol.% AgSbSe2, 44 mol.% PbSe)
As,Se
Ail«)
so/ '(tooA
An ©\
M 60A V v / K
$ [A]/ \ $ A / % V \/ v. \ 7 A % \
20/ % / I \
'f100 rl 1 ® \VA 1 AVA
PbSe P 20 40 |u| 60 8(1 M AgSbSe;
MOJI'% A^SbSc.
Fig. 4. Projection of the liquidus surface of the Ag2Se-PbSe-AgSbSe2system. Primary crystallization fields: 1 - a, 2 - p, 3 - y. Dotted lines - cuts AgSbSe2-[A] and Ag2Se-[B]
Liquidus surface. The liquidus of the Ag2Se-PbSe-AgSbSe2 system (Fig. 4) consists of three fields of the primary crystallization of the a-, P- and y-phases (respectively, fields 1, 2 and 3 on Fig. 4). The liquidus surface of the y-phase (AgSbSe2) has the greatest width. The fields of primary crystallization of phases are separated by one peritectic (pU) and two
eutectic (eU, Ue2) curves. All three curves converge at a transition point U, corresponding to the composition of the melt that is in invariant equilibrium L+P^-a+y.
The types and coordinates of invariant equilibria, as well as temperature ranges of monovariant equilibria are listed in the Table.
Table. Non- and monovariant equilibria in the Ag2Se-PbSe-AgSbSe2 system
Point or Composition, mol%
curve in Equilibrium T, K
Fig.4 Ag2Se AgSbSe2 PbSe
ei L~a+ß 73 - 27 933
e2 L^-a+y 54 46 - 793
P L+ß~y - 82 18 1220
M L^-y - 85 15 890
U L+ß^-a+y 70 8 22 910
eiU L~a+ß 933-910
pU L+ß~y 1220-910
e2U L^-a+y 793-910
Polythermal sections. We analyzed and constructed two polythermal sections AgSbSe2-[A] and Ag2Se-[B].
20 40 60 80 AgShSe.
moji.% AgSbSe,
Fig. 5. Polythermal section AgSbSe2-[A] of the phase diagram of the system Ag2Se-PbSe-AgSbSe2.
The AgSbSe2-[A] section (Fig. 5). The
liquidus of this section consists of two branches corresponding to the primary crystallization of the P-phase based on PbSe and y-phase based on AgSbSe2. A flat
minimum is observed on the liquidus curve of the y-phase which is probably due to the presence of a minimum point on the PbSe-AgSbSe2 boundary system. Curves below the liquidus correspond to monovariant peritectic
(pU) and eutectic (e1U; e2U) equilibria (Table, Fig.4). The horizontal line at 910 K characterizes the nonvariant transition equilibrium L+P^-a+y (point U in Fig. 4). After completion of this process a three-phase The Ag2Se-[B] section (Fig. 6). This section passes through the primary crystallization fields of the a- and y-phases. Then joint secondary crystallization of these
a+P+y area is formed at the excess of P-phase. As for the excess of liquid, the transition reaction finished by the formation of the three-phase L+a+y region.
phases occurs and as a result, a two-phase a+y area is formed in a sub-solidus. The horizontal line at 397 K corresponds to the phase transition of the a-phase.
1170
1100
1000
900
800 400
K L L+y }
L+u
/ ' \J L+a+y
.397 a+y -—■- ^ 1
(Ag.Se),+7
Ag.Se
SO
60 40 MOji"i A&Se
20
[B
Fig. 6. Polythermal section Ag2Se-[B] of the phase diagram of the Ag2Se-PbSe-AgSbSe2 system..
New experimental data on phase equilibria in the quasi-ternary Ag2Se-PbSe-AgSbSe2 system were obtained. Two polythermal sections, an isothermal section at 300 K of the phase diagram, and a liquidus surface projection were constructed. Broad
range of solid solutions based on AgSbSe2 (y-phase) and insignificant solubility on the basis of Ag2Se (a-phase) and PbSe (P-phase) were revealed in the system, and the fields of primary crystallization of these phases determined.
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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Ag2Se-PbSe-AgSbSe2 SÎSTEMÎND3 FAZA TARAZLIQLARI
1§.H. Mansimova, 2L.F. Maçadiyeva, 2K.N. Babanli2, 1R.C. Mirzayeva, 2M.B. Babanli
1Baki Dövlst Universiteti Az 1148 Baki, Z.Xdlilov kuç, 23; e-mail: info@,bsu.az
2AMEA akademik M.Nagiyev adina Kataliz vs Qeyri-üzvi Kimya institutu AZ 1143 Baki, H.Cavidpr.113; e-mail: leylafm76@gmail.com
i$da Ag2Se-PbSe-AgSbSe2 sisteminda faza tarazliqlarmm differensial-termiki va rentgenfaza analizi usullari ila tadqiqinin naticalari verilir. Tacrubi naticalar asasinda sistemin faza diaqraminin AgSbSe2-(PbSe)05(Ag2Se)05 va Ag2Se-(PbSe)05(AgSbSe2)0,5 politermik kasiklari, 300 K-da izotermik kasiyi va likvidus sathinin proyeksiyasi qurulmuçdur. Gôstarilmiçdir ki, o, Ag-Pb-Sb-Se dordkomponentli sisteminin kvaziuçlu mustavisidir va nonvariant keçid tarazligi ila xarakteriza olunur. Sistemda fazalarin ilkin kristalla§ma sahalari, non- va monovariant tarazliqlarin tiplari va koordinatlari tayin edilmiçdir. AgSbSe2 birlaçmasi asasinda PbSe-AgSbSe2 kasiyi boyunca geniç (~80 mol%) bark mahlul sahasi açkar olunmuçdur.
Açar sôzfor: faza diaqrami, bark mahlullar, likvidus sathi, gumuç-stibium selenidi, qurguçun monoselenid.
ФАЗОВЫЕ РАВНОВЕСИЯ В СИСТЕМЕ Ag2Se-PbSe-AgSbSe2
1Ш.Г. Мансимова, 2Л.Ф. Машадиева, 2К.Н. Бабанлы, 1Р.Дж. Мирзоева, 2М.Б. Бабанлы
1 Бакинский Государственный Университет AZ1073 Баку, ул. З. Халилова 23, e-mail: info@bsu.az 2Институт Катализа и Неорганической Химии им. акад. М. Нагиева Национальной АН Азербайджана, AZ1143 Баку, пр.Г. Джавида,113; e-mail: leylafm76@gmail.com
В работе представлены результаты исследования фазовых равновесий в системе Ag2Se-PbSe-AgSbSe2 методами дифференциального термического и рентгенофазового анализов. На основании экспериментальных данных построены политермические сечения AgSbSe2-(PbSe)0,5(Ag2Se)0,5 и Ag2Se-(PbSe)05(AgSbSe2)05, изотермическое сечение при 300 K фазовой диаграммы, а также проекция поверхности ликвидуса. Показано, что данная система является квазитройной плоскостью четверной системы Ag-Pb-Sb-Se и характеризуется наличием нонвариантного переходного равновесия. Определены поля первичной кристаллизации фаз, типы и координаты нон- и моновариантных равновесий. Выявлена широкая область (~80 мол%) твердых растворов на основе AgSbSe2 вдоль разреза PbSe-AgSbSe2.
Ключевые слова: фазовая диаграмма, твердые растворы, поверхность ликвидуса, селенид серебра-сурьмы, моноселенид свинца.
