THE PHASE EQUILIBRIA IN THE Sb–Sb2S3–SbI3 TERNARY SUBSYSTEM Текст научной статьи по специальности «Химические науки»

AZ9RBAYCAN KIMYA JURNALI № 2 2015

57

UDC 54L123.3:546.86'15'22

THE PHASE EQUILIBRIA IN THE Sb-Sb2S3-SbI3 TERNARY SUBSYSTEM

Z.S.Aliev1, S.S.Musayeva2, F.Y.Jafarli2, M.B.Babanly1,2

1 Nagiyev Institute of Catalysis and Inorganic Chemistry Azerbaijan NAS

2Baku State University

[email protected]

Received 15.01.2015

The phase equilibria in the Sb-Sb2S3-SbI3 ternary subsystem have been investigated by the differential thermal analysis (DTA) and X-ray diffraction (XRD) analyses. The quasi-binary section SbI3-Sb2S3 res-tudied and ternary compound SbSI reported earlier was confirmed. Several polythermal sections and the projection of the liquidus surface have been revised. The fields of the primary crystallization, as well as the types and coordinates of non- and monovariant equilibria were determined.

Keywords: phase diagram, antimony(III) sulfide, antimony sulfohalides, antimony(III) iodide.

Introduction

The great interest to the AVBVICVII-type ternary compounds is caused by the fact that they possess high ferroelectric, piezoelectric, thermoelectric, photoconductorand piezoelastic properties both in the single crystal form and thin film [1-9]. SbSI is the most studied compound of its kind and at present there is much information on its physical properties [8]. It is applied to developing optical light modulators, electroacousto-optical transformers, piezoele-ments, sensitive low-pressure gauges, and so on. Three phases of the SbSI have been reported: ferroelectric (T<295 K), antiferroelectric (295<T<410 K) and paraelectric (T<410 K) [7].

The novel preparative routes to these type compounds either melting congruently or incongruently, and especially to their bulk crystals, are not always straightforward; in all cases it requires the detailed investigation of phase equilibria in the corresponding systems.

Therefore, the phase relationships in the AV-X-I (AV - As, Sb, Bi; X - S, Se, Te) ternary system are well-worth studying. It can provide valuable information for improving material properties and developing new materials. Up to now, some ternary systems of AV-X-I type [1-4] have been investigated, but the phase diagram of the Sb-S-I ternary system is limited. In the literature there are two different reports on the phase diagram of this system, devoted to the SbI3-Sb2S3 section [10, 11]. Belyayev and co-authors [10] reported that this section includes the only compound SbSI that melts congruently at 650 K. The

eutectic composition between SbSI and Sb2S3 has the melting point of 595 K at about 75 mol % Sb2S3, whereas the eutectic between SbSI and Sb2S3 is degenerated nearest of SbI3 at 443 K. Ryazantsev and co-authors [11] report slightly different data on the melting point of SbSI and eutectic compositions. According to this work SbSI melts congruently at 675 K has narrow homogeneity range. The eutectic was found to be 73 and 5 mol % Sb2S3 at temperatures 660 and 430 K, respectively.

The crystal structures of SbSI are reported in [12, 13]. It crystallizes in orthorhombic structure with lattice constants: a = 8.49 A, b = 10.1 A, and c = 4.16 A, c - axis being the polar axis in paraelectric phase.

In this paper, we studied the phase equilibria in the Sb-S-I ternary system in order to confirm the existence of ternary phases and phase relationship in this system, to provide useful information for searching new materials or preparing pure and high quality materials.

Experimental Part

Synthesis. Binary SbI3 and ternary SbSI were synthesized from the elements of a high purity grade (not less than 99.999%) in sealed (~10-2 Pa) silica ampoules following a specially designed method, which takes into account high volatility of iodine and sulfur. The synthesis was performed in an inclined two-zone furnace, with the hot zone kept at a temperature 20-30 K higher than the corresponding melting point of a synthesized compound, whereas the temperature of the cold zone was about 400 K. After the

main portion of iodine and sulfur had reacted, the ampoule was relocated in such a manner that the product could melt at 500 K (SM3) and 700 K (SbSI). The melt was stirred at this temperature by shaking the furnace and then cooled with the furnace. Sb2S3 was prepared by a one-step annealing of the stoichiometric mixture of the antimony and sulfur at 850 K, which is above the melting point of Sb2S3 (823 K), followed by cooling with the furnace. We have prepared more than thirty five alloys from the composition area Sb-SbI3-Sb2S3. As a rule, the all samples (total mass, 0.5 g) were prepared from initial elemental components or preliminary synthesized compounds. After melting most of the alloys were annealed at 400 K and 360 K for 1000 h.

Analysis. X-ray powder diffraction (XRD) and differential thermal analysis (DTA) were used to analyze the samples. The XRD analysis was performed on a Bruker D8 ADVANCE diffractometer with CuK^-radiation. For the DTA measurements, the THERMOSCAN-2 device equipped with chro-mel-alumel thermocouples was used. The ramp rate was 5 K m-1. Temperatures of thermal effects were taken mainly from the heating curves. XRD confirmed that the pre-synthesized binary compounds were phase - pure. For the electro-motive force (EMF) measurements, the following concentration chains were assembled: (-)Bi(solid)/glycerin+KI+Bil3/(Bi-S-I)(solid)(+). (1)

In the chains of type (1), metallic bismuth was the left (negative) electrode, while equilibrium alloys of the Bi-S-I system were exploited as right (positive) electrodes. A saturated glycerine solution of KI with the addition of 0.1 mas.% of BiI3 was used as the electrolyte. The assembly of an electrochemical cell and measurements are described in detail elsewhere [14]. EMF was measured by the compensation method in the temperature range of 300-390 K with the accuracy of ±0.1 mV, using the high-resistance universal B7-34A digital voltmeter.

Results and discussion

The section SbI3-Sb2S3 (Fig. 1) was first reported by Ryazantsev and co-authors [11].

Our DTA, XRD data and the results of the EMF measurements for the selected compositions

E, mV

280 T, K

sbh

40 60

mol%Sbb

Figure 1. The phase diagram of the SbI3-Sb2S3.

confirm the literature data. The compounds SbSI is well determined in the E-x diagram (Figure 1b). The isothermal curve of E-x at 300 K consists of three horizontal straights with the EMF values of 301±1 and 286±1 mV. Such stepwise changing of the EMF values at the composition of both compounds indicates that these compounds are stable at room temperature.

The section 3Sb-SbSI (Fig. 2) is characterized by a monotectic (m3m3) and eutectic (e5) equilibria.

T, K

40 60

mol % SbSI

SbSI

Figure 2. The phase diagram of the 3Sb-SbSI.

At the monotectic temperature of 895 K, the immiscibility field ranges from 6 to 91 mol% SbSI. The eutectic contains approximately 94 mol% SbSI and crystallizes at 655 K.

The liquidus surface of the Sb-Sb2S3-SbI3 subsystem consists of four fields corresponding to the primary crystallization of the two binary, one ternary compounds and elemental antimony (Figure 3, Table 1).

Figure 3. Projection of liquidus surface of the subsystem Sb-SbI3-Sb2S3. Areas of primary crystallization of phases: 1 - Sb, 2 - Sb2S3, 3 -SbSI, 4 - SbI3. Quasi-binary sections are shown by dashed lines.

m,Jl JrCmi \ \2 .aoo V V •

sb m' io 20 30 40 50 mi Si D, at. % S 70 80 90 S

Invariant equilibria in the system Sb-Sb2S3-SbI3

Point in Fig. 3 Equilibrium Composition, at.% T, K

Sb S I

Dj L » Sb2S3 40 60 - 828

d2 L » SbI3 25 - 75 443

D3 L » SbSI 33.3 33.3 33.3 675

Ej L » Sb + SbSI + Sb2S3 37 41 22 650

E2 L » Sb + SbI3 + SbSI 26.5 ~1.5 72 428

ei L » Sb2S3 + Sb 43 57 - 75

e2 L » SbI3 + Sb 26 - 74 440

e3 L » SbSI + Sb2S3 35.5 42 22.5 660

e4 L » SbSI+SbI3 26 ~2 72 430

e5 L » Sb+SbSI 36 32 32 655

mi(ml) L1 » L2 + Sb 43(95) 57(5) - 892

m2(m2) L1 » L2 + Sb 95(30) - 5(70) 900

m3(m/; L1 » L2 + Sb 95(38) ~2.5(31) ~2.5(31) 895

Notes: Conjugate invariant points and their compositions are in parentheses.

The fields of the primary crystallization of Sb2S3, SbSI and elemental antimony are very large and occupy more than 98% of the total area of this subsystem. The fields of the primary crystallization of Sb2S3 and SbSI occupy a very narrow strip along quasi-binary section SbI3-Sb2S3. It was revealed that this subsystem is characterized by invariant eutectic and mono-

tectic equilibria. Wide immiscibility field is observed in this system. It starts from the Sb-Sb2S3 subsystem (m1m1/) and Sb-SbI3 subsystem (m2m2/) and spreads into the Sb-SbI3-Sb2S3 subsystem. All primary crystallization fields observed in the system are bordered by the following monovariant curves:

eiEi LeiEi ^ Sb + Sb2S3 (795-650 К) (1)

e3Ei Le3El ^ SbSI + Sb2S3 (660-650 К) (2)

esEi L^e ^ SbSI+Sb (655-650 К) (3)

e5E2 Le^2 ^ SbSI+Sb (655-428 К) (4)

e4E2 Le4E2 ^ SbSI+SbI3 (430-428 К) (5)

e2E2 Le2E2 ^ Sb + SbI3 (440-428 К) (6)

Crystallization processes end by the following nonvariant eutectic reaction: L ^ Sb + SbSI + SbS

i 2S3

L^ ^ Sb + SbI3 + SbSI

(7)

(8)

Table summarizes the types and coordinates of nonvariant equilibria, including the binary border subsystems.

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Sb-Sb2S3-SbI3 SISTEMINDO FAZA TARAZLIQLARI

Z.S.OHyev, S.S.Musayeva, F.Y.Cafarli , M.B.Babanli

Sb—S—I sisteminda faza tarazliqlan Sb-Sb2S3-SbI3 qatiliq sahasinda DTA va RFA usullan ils ôyranilmiçdir. Bu qatiliq sahasinda SbSI uçlu birlaçmasinin môvcudlugu tasdiq edilmiç, arima xarakteri daqiqlaçdirilmiçdir. Sb-SbSI politermik kasiyinin faza diaqrami va hamçinin likvidus sathinin proyeksiyasi qurulmuçdur. Sistemda butun komponentlarin ilkin kristallaçma sahalari, non- va monovariant tarazliqlarin tiplari va koordinatlari tayin edilmiçdir.

Açar sozlar: faza diaqrami, stibium sulfid(III), stibium sulfohalogeni, stibium(III) yodid.

ФАЗОВЫЕ РАВНОВЕСИЯ В СИСТЕМЕ Sb-Sb2S3-SbI3

З.С.Алиев, С.С.Мусаева, Ф.Я.Джафарлы, М.Б.Бабанлы

Методами ДТА и РФА изучены фазовые равновесия в системе Sb-S-I в области составов Sb-Sb2S3-SbI3. Подтверждено существование тройного соединения SbSI, уточнен характер плавления. Построен политермический разрез Sb- SbSI фазовой диаграммы, а также проекция поверхности ликвидуса. Определены области первичной кристаллизации всех компонентов, типы и координаты нон- и моновариантных равновесий.

Ключевые слова: фазовая диаграмма, сульфид сурьмы(Ш), сульфогалогениды сурьмы, иодид сурьмы(Ш).