PbSe-PbTe SYSTEM

V. Vassilev, K. Tomova, E. Fidancevska, Vachkov, M. Milosevski, L. Aljihmani Journal S. ofParvanov, the University of Chemical TechnologyV.and Metallurgy, 42, 1, 2007, 61-66

PHYSICOCHEMICAL PROPERTIES OF GLASSES IN THE GeSe2-PbSe-PbTe SYSTEM V. Vassilev1, S. Parvanov1, K. Tomova1, E. Fidancevska2, V. Vachkov3, M. Milosevski2, L. Aljihmani1

1

University of Chemical Technology and Metallurgy 8 Kl. Ohridski, 1756 Sofia, Bulgaria E-mail: [email protected] 2 Faculty of Technology and Metallurgy, Ruger Boshkovik 16, Skopje, Macedonia 3 South-West University ,,N. Rilski“, Faculty of Mathematics and Natural Science, 66 Hr. Michailov, 2700 Blagoevgrad, Bulgaria

Received 11 October 2006 Accepted 12 November 2006

ABSTRACT Chalcogenide glasses from the GeSe2-PbSe-PbTe system were synthesized. Microhardness, HV; density, d and the glass transition temperature, Tg were measured. Some thermomechanical characteristics, such as volume, Vh and formation energy, Eh of micro-voids in the glassy network, as well as the module of elasticity, E were calculated. The mean values of the overall bond energy, ; coordination number, ; bond energy of the average cross-linking/atom, E c and that of the ,,remaining matrix”, E rm were determined. The average heteropolar bond energy, Ehb and the degree of ,,crosslinking/atom”, Pp were also calculated. A correlation between the composition and properties of the GeSe2-PbSe-PbTe glasses was established and comprehensively discussed. Keywords: chalcogenide glasses; physico-chemical properties, bond energy.

INTRODUCTION Ion-selective electrodes (ISE) with functional membranes on the basis of chalcogenide glasses (CG) are widely used in the last decade. The membranes for heavy metal ions detection are prepared by melting and fast quenching or by pressing of insoluble sulfides, or selenides [1]. The glassy chalcogenides are more stable in electrolyte solutions than polycrystalline chalcogenides due to their resistance and chemical durability in aggressive media [2]. These properties make the chalcogenide glasses a suitable material for membranes preparation in ion-selective electrodes for Ag(I)-, Pb(II), Fe(III)- and Cu(II)- detection [2-5].

The investigation of new glassy materials on the basis of semiconductor oxide [6] Ge- and (or) As-chalcogenides [7] provokes the scientific interest for new membrane materials production. The investigation involves: 1) synthesis of new chalcogenide glasses; 2) investigation of their physico-chemical properties; 3) direct current (d.c.) and alternating current (a.c.) conductivity, diffusion coefficient determination of the charge carriers, proposition of a model of ionic transport in the glass; 4) investigation of the morphology using Xray diffraction and mass-spectroscopy; 5) investigation of the composition using Auger electron spectroscopy. The aim of the present work is to synthesize multicomponent chalcogenide glasses in the system GeSe2-

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Journal of the University of Chemical Technology and Metallurgy, 42, 1, 2007

PbSe-PbTe, and to investigate their physico-chemical characteristics. It is expected the new system synthesized glasses to be a suitable material for Pb(II) ion-selective electrodes in conformity with the above mentioned consequence. To be suitable for this purpose the glassy phases should contain Pb and have a high enough ionic conductivity [8]. Following these requirements Pb as PbTe and PbSe is added in the chalcogenide glass. The glass-forming region in the system GeSe2PbSe-PbTe has been studied in our previous work – Fig. 1 [9]. It is extended to the area rich in GeSe2 and partially lies on the faces GeSe2-PbSe (40 – 57 mol % PbSe) and GeSe2-PbTe (15,0 – 57,5 mol % PbTe) of the Gibbs’concentration triangle. No glasses were obtained in the system PbSe-PbTe. The starting elements were with purity: Ge, Se and Te – 5N, Pb – 3N8, respectively.

Fig. 1. Glass-forming region in the three-component system GeSe2-PbSe-PbTe

EXPERIMENTAL The microhardness (HV) by the Vickers method (microscope MIM-7 and microhardness meter PMT-3 at 10 g loading) and the density by the hydrostatic method (with toluene as a reference liquid) were measured and the softening temperature, Tg was determined by differential thermal analysis (DTA) for samples of the examined system (GeSe2)x(PbSe)y(PbTe)z. Table 1 presents their composition. For the DTA we have used an apparatus of the system F.Paulik-J.Paulik-L.Erdey of the company MOM-Hungary, Stepanov’s vacuumed

62

containers, sample mass 0,3 g, heating rate 16 oC min-1 and a reference substance - calcinated α-Al2O3. The module of elasticity (E), the minimum volume of micro-voids (Vh) and the energy of their creation (Eh) were calculated by the equations 1 [10]:

Vh = 5,04

E = 15HV , E h = 30,729Tg

Tg HV

, (1)

RESULTS AND DISCUSSION The softening temperatures of the chalcogenide glasses for the studied system were determined from the DTA curves. The results are presented in Table 1, where m means the relation [PbSe]/([GeSe2 + PbSe]), i.å. m = y/(x+y). Tg depends on the glass composition (Table 1), the PbTe effect being definitely stronger. Moreover, PbSe and PbTe act in different ways. PbSe introduction (at constant PbTe concentration) results in slight Tg increase, while with the increase of the PbTe content (at constant PbSe concentration), Tg decreases considerably. Such an effect is expected, having in mind the following features of PbSe and PbTe: • in the order PbS → PbSe → PbTe the metal component of the chemical bond grows and in the same order the trend for glass formation decreases; • PbTe causes considerably stronger restructuring of the glass structure compared to PbSe, as in the Table 1. Physico-chemical properties of some glassy phases from the system (GeSe2)x(PbSe)y(PbTe)z. ¹ 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

composition, mol % x y z 45 45 10 54 36 10 63 27 10 48 32 20 56 24 20 64 16 20 72 8 20 80 0 20 49 21 30 56 14 30 63,7 6,3 30 70 0 30 48 12 40 54,6 5,4 40 60 0 40

m 0,5 0,4 0,3 0,4 0,3 0,2 0,1 0,0 0,3 0,2 0,1 0,0 0,2 0,1 0,0

Tg, °C 317 315 311 282 277 272 268 263 258 253 250 248 244 239 235

d, g cm-3 6,34 5,86 5,57 6,20 5,86 5,55 5,34 5,30 6,13 5,77 5,52 5,48 6,40 6,18 6,05

HV, N mm-2 467,0 480,7 525,8 419,9 426,7 439,5 453,2 470,9 356,1 353,2 366,9 371,8 288,4 292,3 315,9

V. Vassilev, S. Parvanov, K. Tomova, E. Fidancevska, V. Vachkov, M. Milosevski, L. Aljihmani

glass network two new elements (Pb and Te) are incorporated, which is also reflected in the glass formation in the system; • it is quite probable that PbSe fulfills the functions of a modifier in the studied system improving the GeSe2 glass formation ability by analogy with the couple SiO2 and PbO, in the silicate glasses. The microhardness of the studied glasses is within the range of 284 – 530 N mm-2 and depends slight on the composition of glases – Table 1. The changes in the dependence HV(m, z) are expected and it seems logical

having in view that HVGeSe 2 > HVPbSe ( HVPbTe ): HVGeSe 2 =981-1962 N mm-2 [11], HVPbSe = 544,5 N mm-2 [12] è HVPbTe = 392 – 589 N mm-2 [13]). The curves of the dependences Å(m) (Fig. 2) and Eh(m) (Fig. 3) at z=const, and E(z) and Eh(z), at m=const are analogical to the dependences HV(m) and Tg(m), at z=const, and HV(z) and Tg(z), at m=const, respectively (in compliance with eq. 1). The minimum volume of the micro-voids changes within the range of (29 - 42).10-3 nm3 and depends on the composition of glasses (Fig. 4). When increasing the PbSe content (m grows) at constant PbTe content (z=const) and the PbTe content (z grows) at constant PbSe quantity (m=const), the volume of the micro voids grows. The introduction of PbSe (PbTe) leads to increase of the average molecular mass and in accordance with equation 2, the X-ray density (dR) also grows:

dR =

z Mm H V

(2)

where: z-number of atoms in the elementary cell; Ì -average molecular mass; mH-mass of the hydrogen atom; V-volume of the elementary cell. It is logical to presume that the measured sample densities will also increase in spite of the fact that they

Fig. 2. Dependence of E from the composition of the investigated glasses.

Fig. 3. Dependence of Eh from the composition of the investigated glasses.

Fig. 4. Dependence of Vh from the composition of the investigated glasses.

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Journal of the University of Chemical Technology and Metallurgy, 42, 1, 2007

are vitreous (dR>dcrystal>dglass). This hypothesis is verified by the experimental values of dglass (Table 1). As d=m/(Vh+Vg), where Vh and Vg are the volume of the micro-voids and the volume occupied by the glass, respectively, it is quite probable, that when PbSe (PbTe) is introduced, the result will be the densification of the glass, i.å. with the decrease of Vg, in a certain concentration interval, Vh+Vg to decrease, correspondingly - d to increase. The properties of the chalcogenide glasses are closely related to the overall mean bond energy which represents a function of the mean coordination number, the type and energy of the chemical bonds between atoms forming the glass. When examining the composition/property dependences of glasses of the (GeSe2)x(PbSe)y(PbTe)z system we mark the concentration of the components that compose it by x, y and z. For calculation of the average energy of the bonds and its components in case of complicated chalcogenide systems, the following presentation is much more convenient: Gex’Pby’Sez’Tew’, where x’ + y’ + z’ + w’ = 1. We have calculated the average coordination number of the glasses, of the examined system, written in the form Gex’Pby’Sez’Tew’, by the formula (3) suggested by Tanaka [14], by using the coordination numbers ZGe = 4, ZPb = 4, ZSe = 2 [15] and ZTe = 2 [16]:

< Z >= x ' Z Ge + y ' Z Pb + z ' Z Se + w Z Te

(5) E c = Pp E hb where Ehb is the average heteropolar bond energy for glasses with the composition Gex’Pby’Sez’Tew’. It is calculated with the equation (6):

(

(

)(

R = z ' ZC + w ' ZD / x ' ZA + y' ZB

)

)

(7)

When R > 1 the system is rich in chalcogen and has heteropolar bonds and chalcogen-chalcogen bonds. When R = 1 the system has a stoichiometric composition because it has heteropolar bonds only. When R < 1 the system is deficient in chalcogen and has heteropolar bonds and metal-metal bonds. The studied system is deficient in chalcogen, because R < 1 R = (2.0,55 + 2.0,44) / (4.0,18 + 4.0,22 ) ≅ 0,74 . In this case the degree of cross-linking/atom Pp (for R < 1) is calculated with equation (8):

(

)(

Pp = z ' Z Se + w ' Z Te / x ' + y ' + z ' + w '

(3)

)

(8)

The average bond energy per atom of the ,,re-

We determined the overall mean bond energy with Tichy’s equation [17] for the complex chalcogenide systems: (4)

< E >= E c + E rm

)(

Ehb = x' ZAEA−C + y' ZBEB−C + x' ZAEA−D + y' ZBEB−D / x' ZA + y' ZB (6) where EA-C, EB-C, EA-D and EB-D are the heteropolar bond energy Ge-Se, Pb-Se, Ge-Te and Pb-Te (Table 2). We calculated the R coefficient determining the content of chalcogen in glasses with equation (7):

where E c is the average energy of cross-linking/atom calculated with equation (5):

maining matrix” E rm (for R < 1) is:

E rm = 2(0,5 < Z > − Pp )E / < Z > E = (E A − A + E B− B + E A − B ) / 3

(9) (10)

In equation (10) Å is the average bond energy Me-Me (Me + Me’) in the chalcogen-poor region in the glass-forming region and EA-A, EB-B and EA-B are respec-

Table 2. Bond energy of the glasses from the system GeSe2-PbSe-PbTe [18].

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bond

Se-Se

Te-Te

Å, eV

2,14

1,65

Ge-Ge Pb-Pb 2,13

0,89

Ge-Se

Ge-Te

Pb-Se

Pb-Te

Ge-Pb

Se-Te

2,44

1,87

2,40

1,90

1,51

2,00

V. Vassilev, S. Parvanov, K. Tomova, E. Fidancevska, V. Vachkov, M. Milosevski, L. Aljihmani

Table 3. Physico-chemical properties of the samples of the system (GeSe2)x(PbSe)y(PbTe)z or (Gex’Pby’Sez’Tew’). ¹ 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

x' 0,180 0,210 0,240 0,190 0,220 0,240 0,260 0,290 0,197 0,219 0,242 0,260 0,193 0,210 0,230

composition, mol % y' z' 0,220 0,550 0,180 0,567 0,140 0,580 0,210 0,520 0,172 0,530 0,140 0,546 0,100 0,560 0,070 0,570 0,205 0,478 0,172 0,492 0,138 0,506 0,110 0,520 0,210 0,436 0,180 0,450 0,154 0,462

w' 0,400 0,039 0,038 0,080 0,078 0,076 0,070 0,070 0,120 0,117 0,114 0,110 0,161 0,160 0,154

2,780 2,772 2,756 2,800 2,784 2,764 2,706 2,720 2,804 2,782 2,760 2,740 2,806 2,780 2,768

tively the bond energies Ge-Ge, Pb-Pb and Ge-Pb – Table 2. The values for , E c, E rm and are summarized in Table 3. The most general analysis of the results given in Table 3 shows that the total average energy of the bonds, , as well as of its components - E c and E rm, depend on the glass compositions, which in implicit form is reflected in the average coordination number, . It is logical to presume that between the energies ,

E c and E rm, on the one hand and on the other, a correlation should exist, not taking into consideration Gibbs’ concentration triangle, which we have used in the examination of the properties as function of m and z. This hypothesis has found confirmation by the elaboration of the following correlation equations for the dependences (), E c() and E rm():

E c = 15,93 − 3,85 < Z >

(11)

E rm = 1,43 < Z > −3,79

(12)

< E >= 12,23 − 2,45 < Z >

(13)

In the case of increasing the Se content, Te or their sum, respectively, the coordination number decreases, i.g. the probability for cross-linking/atom decreases which is reflected in the average energy of cross-

Çc, eV 5,0800 5,2180 5,3226 5,1658 5,2356 5,3570 5,4530 5,5144 5,1487 5,2442 5,3399 5,4268 5,1399 5,2526 5,3050

Çrm, eV 0,2300 0,1896 0,1556 0,2157 0,1909 0,1508 0,0971 0,0889 0,2219 0,1878 0,1532 0,1212 0,2249 0,1847 0,1658

, eV 5,3100 5,4076 5,4782 5,3815 5,4265 5,5078 5,5501 5,6034 5,3706 5,4320 5,4931 5,5480 5,3648 5,4373 5,4708

Na

Nb

Nc

1,390 1,386 1,378 1,400 1,392 1,382 1,535 1,360 1,402 1,391 1,380 1,370 1,403 1,390 1,384

1,853 1,848 1,837 1,867 1,856 1,843 1,804 1,813 1,869 1,855 1,840 1,827 1,871 1,853 1,845

3,243 3,234 3,215 3,267 3,248 3,225 3,157 3,173 3,271 3,246 3,220 3,197 3,274 3,243 3,229

linking/atom E c, which grows, as E c is a limit so the average energy of the bonds also increases, which is seen in equations (11) and (13). According to Tanaka [14] the glass viscosity (ì) is well described by the Arrhenius dependence of the type:

µ (Tg ) = µ 0 exp(E µ / kTg )

(14)

The same author assumes that ì(Tg) = 1013 Poise and ì0=100-10-5 Poise. Assuming ì(Tg) = 1013 Poise and ì0=10-3 Poise, gives the following dependence between Tg and Eì:

Tg = 314E µ

(15) Our experimental values of Tg and submit to the dependence Tg = 1083 – 99. By these two equations we define Eì:

E µ = 3,45 − 0,32 < E >

(16)

By analogy we have calculated the values of the constants A and  in the dependence:

Tg = A(3,45 − 0,32 < E > ) + B Tg = 322(3,45 − 0,32 < E > ) − 6,2

(17) (17, a)

i.e.

Tg = 319(3,45 − 0,32 < E > )

(17, b)

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Journal of the University of Chemical Technology and Metallurgy, 42, 1, 2007

• The main thermo-dynamical characteristics of the chemical bonds are calculated and linear dependences on the one hand between E c, E rm and on and on the other hand - between Tg and are determined. Acknowledgements The authors acknowledge greatefully the financial support for this work from the Ministry of education and science (Fund “Scientific investigations”-contract TN-1503/05).

Fig. 5. Dependence of (3,45 – 0,32) on Tg

The dependence Tg() [actually Tg(3,45 – 0,32)] is shown in Fig. 4. Althought the Tg values seem to be rather scattered, a good correlation between Tg and (3,45 – 0,32) is observed. This dependence has a accuracy not worse than ± 0,1Tg and it is quite reasonable [14]. The obtained values of the line slope (319 Ê eV-1) correspond exactly to the theoretically obtained values (314 K eV-1), which is calculated through Tanaka’s model. Probably the relation between Tg and is linear for all types of chalcogenide glasses, including oxychalcogenide, chalcohalide and oxychalcohalide, and when the glass undergoes a transformation from one class to another, the activation energy of viscosity, Åì will change. Fig. 5 presents one more interesting pattern. The straight lines 1, 2, 3 and 4, which correspond to 10, 20, 30 and 40 mol % PbTe, respectively, are practically parallel to the dependence Tg=319(3,45 – 0,32), which means that the dependence Tg(z) has a more universal character than the dependences Tg(m) at z = const. CONCLUSIONS • Basic physico-chemical parameters (Tg, d and HV) of glasses of the system GeSe2-PbSe-PbTe are measured and through them the thermo-mechanical characteristics (E, Eh, Vh) of the glasses are calculated. • A correlation is istablished between the basic physico-chemical properties of the examined chalcogenide glasses of the composition.

66

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