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AZ0RBAYCAN KiMYA JURNALI № 4 2012
UDC 541.123.3.546.85
THE ISOTHERMAL SECTION OF THE SYSTEM As-Te-J TERNARY SYSTEM AT 300 K AND THERMODYNAMIC PROPERTIES OF ARSENIC TELLUROIODIDES
Z.S.Aliev, I.R.Amiraslanov, Z.l.lsmayilov, K.Z.Mustafayeva, M.B.Babanly
Baku State University [email protected] Received 19.12.2011
The phase relations of the As-Te-J ternary system at 300 K have been investigated mainly by means of X-ray powder diffraction and EMF measurements with an arsenic electrode. The section consists of twelve three-phase and two two-phase regions. The isothermal section at 300 K confirms the formation of ternary compounds As5Te7J, As4Te5J2 and As8Te7J5. From the X-ray powder diffraction (XRD) analysis, the crystallographic parameters of As4Te5I2 and As8Te7I5 were determined. From the EMF measurements, the partial molar functions of arsenic (AG, AH, AS) as well as standard integral thermodynamic functions of ternary compounds were calculated.
Keywords: arsenic telluroiodides, As-Te-J system, phase relations, thermodynamic properties.
For many years there has been an interest in the vast family of arsenic, antimony and bismuth chalcogen-halides. These compounds exhibit properties that make them a good base for creating various functional materials, including ferro- and piezoelectrics, piezoelastics, thermoelectrics, and photo-conductors [1-3]. The preparative routes to these compounds, and especially to their large crystals, are not always straightforward; in many cases they require the knowledge of the respective phase diagrams.
The ternary As-Te-J system was investigated along the polythermal sections As2Te3-AsJ3, AsJ3-TeJ4, and in the composition area As-As2Te3-AsJ3 [4-6]. The AsJ3-TeJ4 system was reported to of a simple eutectic type [4]. The eutectic composition has the melting point of 405 K at 5 mole % TeJ4. According to the literature [5], the section As2Te3-AsJ3 includes the only compound As4Te5J2 that melts congruently at 563 K, whereas the eutectic compositions were found to be 23 and 92 mole % AsJ3 at the temperatures 553 and 403 K respectively. In the system there are solid solubility ranges on the base of both As2Te3 and As4Te5J2. According to the literature [6], the composition area As-As2Te3-AsJ3 of the system As-Te-J contains the ternary compounds As8Te7J5. The phase diagram of the system As-As4Te5J2 was constructed from the DTA results. It was established that this section is quasi-binary and characterized by monotectic and eutectic equilibriums. The eutectic composition has the melting point of 553 K at 87.5 mole % As4Te5J2. The monotectic temperature is achieved at 583 K, and the immiscibility area ranges from 2 to 70 mole % As4Te5J2. The projection of the liquidus surface of subsystem As-As2Te3-AsJ3 consists of five areas of the primary crystallization fields (As, As2Te3, AsJ3, As4Te5J2 and As8Te7J5). The area of the primary crystallization of arsenic occupies up to 95% of the As-As2Te3- AsJ3 triangle area. There is a wide immiscibility area in the subsystem. The comparative analysis of the system As-As4Te5J2 and the liquidus surface shows that the borders of the immiscibility area in the subsystem are shown incorrectly. Thus, according to [6], the immiscibility area on the liquidus surface ranges from 21 to 95 mole % As4Te5J2. Also, eutectic and monotectic equilibriums of the system As-As4Te5J2 are not shown on the projection of the liquidus surface.
Three phases are reported to exist in the system As-Te-J; they are As4Te5J2, As8Te7J5, and As5Te7J. All three compounds possess different structures. The crystal structure of As5Te7J is well-explored [7, 8]. It crystallizes monoclinically with the following lattice parameters: a = 14.5520 A, b = 4.0335 A; c = 13.8440 A; p = 1120, z = 2, and the space group either Cm or C2/m, depending on whether the Te/J joint occupancy of same crystallographic positions allowed or not. In this structure, the As-Te-J slabs run along the b axis in such a way that the As atoms inside the slab are surrounded by six Te and/or J atoms while the As atoms at the sides of the slab are three-coordinated. This structure forms its own structure type, though some similarity to the crystal structure of BinSe9Cli2 can be noticed [9, 10]. Less is known about the crystal structure of two other compounds. As4Te5J2 is reported to have the face-centered cubic structure with the unit cell parameter of 5.7825 A [6]. Arguably, it crystallizes with the defect zinc-blend type. No crystallographic data are available for As8Te7J5 save for the XRD pattern [6].
In this work we report the results of the complete investigation of phase equilibriums in the As-Te-J system at 300 K.
EXPERIMENTAL PART
As2Te3, AsJ3, TeJ4, TeJ, TeJ4, As5Te7J, As4Te5J2, and As8Te7J5 were synthesized from elements of high purity grade in evacuated (~10-2 Pa) sealed silica ampoules according to the following schemes. As2Te3 was prepared by a one-step annealing of the stoichiometric mixture of the elements at 700 K, which is above the melting points of As2Te3 (654 K), followed by cooling with the furnace. For the preparation of AsJ3, TeJ4, TeJ, TeJ4, As5Te7J, As4Te5J2, and As8Te7J5, the specially designed method was used taking into account high volatility of iodine. The synthesis was performed in the inclined three-zone furnace, with two hot zones kept at 410-630 K, whereas the temperature of the cold zone was about 400 K. After the main portion of iodine reacted at about 470 K, the ampoules were relocated such that the products melted at 550630 K. The melts were stirred at these temperatures and then cooled with the furnace.
Most of the samples, having masses of 0.5 g, were pre-prepared from the above mentioned binary and ternary compounds. After determining the solidus temperatures, the sintering temperatures were adjusted to be 20-30 K below the solidus. Subsequently, they were annealed for 800-1000 h at 500 K within the As-As2Te3-As8Te7J5 field and at 380 K for the other fields.
X-ray powder diffraction and differential thermal analysis were used to analyze the samples. The XRD analysis was performed on a Bruker D8 ADVANCE diffractometer with Cu^a-radiation. The lattice parameters were refined by using the Topas V3.0 software.
For the electromotive force (EMF) measurements, the following concentration chains were used:
Saturated glycerin solution of KJ with the addition of 0.1 mass % of AsJ3 was used as the electrolyte. EMF was measured by the compensation method in the temperature range of 300-400 K, with the accuracy of the temperature control being 0.2 K. In each experiment the first reading was performed after approximately 30 h after the start of the experiment, and then 4-5 h after reaching the desired temperature, which ensures the achievement of equilibrium.
The isothermal section of the As-Te-J ternary system at 300 K (Figure 1) was determined from the results of powder XRD and EMF methods. There are twelve three-phase and two two-phase regions (a+As and a+Te) in this system. The isothermal section confirms the formation of ternary compounds As5Te7J, As4Te5J2 and As8Te7J5.
(-) As (solid) / glycerin + KJ + AsJ3 / (As-Te-J) (solid) (+).
(1)
RESULTS AND DISCUSSION
J
Figure 1. Isothermal section at 300 K of the phase diagram of the system As-Te-J. The EMF values (mV) of the concentration chains of type (1) in some phase areas are indicated in parenthesis.
As
20
40
As2Te3
80
Te
at. % Te
Figure 1 shows that AsJ3, being the more thermodynamically stable compound of the system, has the critical influence on the distribution of the phase areas in subsolidus. This compound forms connod lines with tellurium, tellurium iodides (TeJ4 and TeJ), and also with ternary compounds As8Te7J5 and As4Te5J2.
The EMF method allows to clearly differentiating these areas. Figure 1 shows the values of the EMF (mV) chains of the mode (1) in certain phase areas of the system As-Te-J at 300 K. The measurements show that under specified temperature, in the range of each of three-phase areas, the EMF has a strict constant value which is independent of the general composition of alloys, but at transition from one three-phase area to another it is changed jump-likely.
From the X-ray powder diffraction analysis, the lattice parameters of the As4Te5J2 and As8Te7J5 were determined. The XRD patterns of As4Te5J2 and As8Te7J5 are shown in Figure 2.
As4Te5J2 a
-
т1,
30
~ J
4 p
40
Т1,
50
Position [°2Thota]
Т1,
60
70
As8Te7J5 b
30
=±=
2 sj s й
I \o \o
Position [=2The1a]
Figure 2. The XRD patterns of As4Te5J2 (a) and As8Te7J5 (b) compounds.
It is confirmed that As4Te5J2 crystallizes in the face-centered cubic structure with the unit cell parameter a = 5.854(1) A, which is in a good agreement with the literature data [6]. The XRD pattern of As8Te7J5 was indexed in the trigonal unit cell with the lattice constants a = 4.075(3) A, c = 20.36(1) A. No extra systematic conditions were observed. The crystal structure of As8Te7J5 requires further investigation, however, based on the lattice constants and symmetry it is possible to propose a similarity with bismuth telluride-halides, which crystallize in trigonal space groups with the a parameter slightly exceeding 4 A [11].
The EMF measurement results for the chains of type (1) allowed confirming the correctness of all drawn solid-state equilibria and also served as the basis for the calculation of the thermodynamic functions for As5Te7J, As4Te5J2 and As8Te7J5.
The analysis showed the linearity of the EMF dependances upon temperature for various alloys belonging to the heterogeneous fields C1+C2+Te, AsJ3+C2+Te and AsJ3+C2+C3. Accordingly, the linear least-square treatment of the data was performed [11] and the results were expressed according to the literature recommendations [12] as
E = a + bT ± t
S 2
+
Sb T - T ):
1/2
(2)
where n is the number of pairs of E and T values; SE and Sb are the error variances of the EMF readings and the b coefficient respectively; T is the mean absolute temperature; t is the Student's test. At the confidence level of 95% and n > 20, the Student's test is t < 2 [13]. Using this model (Table 1) and common thermodynamic functions the partial molar functions of arsenic at 298 K were calculated, and the values are shown in Table 2.
Table 1. Temperature dependencies of the EMF for the chains of type (1)
Phase area on Figure 1 E = a + bT ± t + Sj (T - T)2 n 1/2
Ci+C2+Te E = 66.80 + 0.021T ± 2 024 + 2.4 -10-5(Г 360.4) 2 1/2
AsJ3+C2+Te E = 103.63+ 0.012Г ± 2 " 0 21 . + 2.2-10-5(Г 361.1)2 25 -j 1 / 2
ASJ3+C2+C3 E = 81.20 - 0.011T ± 2 А О/" . + 3.5 -10-5(Г 358.1)2 25 1/2
n
Table 2. Relative partial thermodynamic functions of arsenic in the alloys of the As-Te-J system at 298 K
Phase area on Figure 1 agAs АЯ As AS As
kJ/mole JK"1mole"1
Ci+C2+Te 21.15±0.19 19.34±1.2 6.08±2.87
AsJ3+C2+Te 31.03±0.18 30.00±0.96 3.47±2.73
ASJ3+C2+C3 22.55±0.23 23.50±1.23 -3.18±3.42
According to the phase diagram of the As-Te-J system, the partial molar functions of arsenic in the Ci+C2+Te, AsJ3+C2+Te and AsJ3+C2+C3 subsystems are the thermodynamic functions of the following potential forming reactions [14]:
As(solid)+0.167As4Te5J2(solid)+1.5Te(solid)=0.333As5Te7J(solid), (3)
As(solid)+0.2AsJ3(solid)+1.5Te(solid)=0.3As4Te5J2(solid), (4)
As(solid)+0.44AsJ3(solid)+ 0.84As4Te5J2(solid)=0.6As8Te7J5(solid). (5)
Using these equations, the integral thermodynamic functions of formation of SbSeJ can be calculated as:
A/Z0(As5Te7J)=3 AZas +0.5A/Z°(As4Te5J2), (6)
A/Z0(As4Te5J2)=3.333 AZAs +0.667A/Z°(AsJ3), (7)
A/Z°(As8Te7J5)=1.667 AZAS +0.733A/Z°(AsJ3)+1.4A/Z°(As4Te5J2), (8)
where A/Z0 are AjG° and A/H0 values for the corresponding compound, and AZAs is AGAs , A#As . For the calculations according to (6-8), the thermodynamic parameters of AsJ3 were taken from the literature [15,16] (Table 3).
Table 3. Standard integral thermodynamic functions of the compounds in the As-Te-J system
Compounds -AfG0 (298 K) -Afl° (298 K) (298 K)
kJ/mole JK-1mole-1
AsJ3 65.8±3.2 [21, 22] 64.9±3.9 [21, 22] 213.1±5.1 [21, 22]
As5Te7J 137.1±1.4 129.7±5.9 609.4±16.4
As4TesJ2 147.3±2.7 143.3±5.8 523.1±13.6
As8Te7Js 292.1±6.5 287.4±13.1 942.5±28.5
The standard entropy of the ternary compounds was calculated using the following equations: ^(AssTeyJH AS (As)+35°(As)+0.55°(As4Te5J2)+ 4.5S°(Te), (9)
5°(As4Te5J2)=3.333 AS (As)+3.333S°(As)+0.667S°(AsJ3)+ 4.5S°(Te), (10)
5°(As8Te7J5)=1.667 AS (As)+1.6675°(As)+0.7335°(AsJ3)+ ^(As^M). (11)
For the calculations, the standard entropies of arsenic and tellurium were taken from the database [16] as 36.6±2.2 and 49.5±0.3 JK._1mole-1, respectively. The results of the calculations are presented in Table 3. In all cases the estimated standard deviations were calculated by accumulation of errors.
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As-Te-J UCLU SiSTEMiNiN 300 K-DO iZOTERMiK KOSiYi VO ARSEN TELLURYODiDLORiNiN
TERMODiNAMiK XASSOLORi
Z.S.Oliyev, i.R.Omiraslanov, Z.i.ismayilov, K.Z.Mustafayeva, M.B.Babanli
As-Te-J sisteminda 300 K-da faza tarazliqlan rentgen faza analizi va e.h.q. metodu ila tadqiq edilmi§dir. Muayyan edilmi§dir ki, 300 K-da bu sistem 12 ugfazali va 2 ikifazali sahadan ibarat olub, As5Te7J, As4Te5J2 va As8Te7J5 uglu birla§malarinin amala galmasini tasdiq edir. RFA naticalarina asasan har ид birla§manin kristal qafas parametrlari hesablnmi§dir. E.h.q. naticalarina asasan arintilarda arsenin parsial molyar va As5Te7J, As4Te5J2, As8Te7J5 uglu birla§malarinin standart amalagalma termodinamik funksiyalari (AG, AH, AS) hesablanmi§dir.
Agar sozlzr: arsen telluryodidhri, As-Te-J sistemi, faza tarazliqlari, termodinamik xassabr.
ИЗОТЕРМИЧЕСКОЕ СЕЧЕНИЕ ТРОЙНОЙ СИСТЕМЫ As-Te-J ПРИ 300 К И ТЕРМОДИНАМИЧЕСКИЕ СВОЙСТВА ТЕЛЛУРИОДИДОВ МЫШЬЯКА
З.С.Алиев, И.Р.Амирасланов, З.И.Исмайлов, К.З.Мустафаева, М.Б.Бабанлы
Методами э.д.с. и РФА изучены фазовые равновесия в системе АБ-Те—Г при 300 К. Установлено, что при 300 К эта система состоит из 12 трехфазных и 2 двухфазных областей. Подтверждено существование тройных соединений - АБ5Те7.Г, АБ4Те5.Г2 и АБдТеГ На основании результатов РФА вычислены кристаллографические параметры всех тройных соединений. Из измерений э.д.с. вычислены парциальные молярные функции мышьяка в сплавах и стандартные термодинамические функции образования (АО, АН, А?) тройных соединений - АБ5Те71, АБ4Те5.Г2 и АБ8Те715.
Ключевые слова: теллуриодиды мышьяка, система Л^—Тв-^ фазовые равновесия, термодинамические свойства.
