1. INTRODUCTION:
Chalcogenide glasses have drawn a lot of attention amongst several investigators because of their immense applications in various fields. These materials are based on the chemical elements of group VI termed as chalcogen elements (S, Se, and Te). Other elements such as Ge, As, Sb, Ga, Sn, Pb etc are added in chalcogens elements to form chalcogenide glasses. These glasses have drawn colossal attention because of their ability to transmit light in the mid to far-infrared region and due to their uses in optoelectronic, microelectronic, holographic applications etc. [1-3]. Se exhibits the property of reversible phase transformation which makes it suitable for the various applications like memory switching, xerography, photocells etc. Se is a very unique element but its pure form has low photo sensitivity and short life time. So some impurity atoms like Bi, Te, Ge, Sb, In, Ag, etc are generally added in Se to overcome these disadvantages. This may lead to reduce ageing effects, enhance sensitivity, and crystallization temperature of the chalcogenide glasses [4].
The incorporation of Bi element into the Ge-Se based system results in raising the glass forming region. It also creates configurational and compositional disorder in the system as well as produces big effect on the various properties such as optical, physical, thermal and structural properties [5-7]. In the present work, we have formulated a quaternary composition of Ge-Bi-Se-Sn. Here we have fixed the atomic % of Ge and Sn and by varying the concentration of Bi from 2 to 16 atomic %, we have studied the effect on some useful parameters. We have established that physical properties in this system are highly composition dependent. This article is concerned with physical parameters such as average coordination number, mean bond energy, glass transition temperature etc. for Ge10BixSe80-xSn10 (x=2, 4, 6, 8, 10, 12, 14, 16 at. %) composition based on the theoretical prediction.
2. MATERIAL AND METHODS:
2.1 Average Coordination Number:
The well known Phillips approach says that the tendency of glass formation would be maximum when the number of degrees of freedom is exactly equals the number of constraints. The average coordination number (Z) was calculated using standard method [8] for the composition Ge10BixSe80-xSn10 and Z is given by
aCGe + bCBi + cCSe + dCSn
Z=----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
a+b+c+d
where a, b, c and d are the at. % of Ge, Bi, Se and Sn respectively and CGe(4), CBi(3), CSe(2), CSn(4) are their respective coordination number [9].
2.2 Deviation from the Stoichiometry of Composition:
The parameter R can be determined for Ge10BixSe80-xSn10 system, by using the relation given by [10]
cC_Se
R= ---------------------------------------------------------------------------------------------------------------------------------------------
aCGe + bCBi + dCSn
where a, b, c, d are atomic fractions of Ge, Bi, Se and Sn respectively. On the basis of the values for R, the chalcogenide systems can be organized into three different categories [11]. The threshold at R=1 marks the minimum selenium content at which a chemically ordered network is possible without metal–metal bond formation. If the value of R>1, the system is chalcogen rich. If the value of R<1, the system is chalcogen poor.
2.3 Mean Bond Energy And Glass Transition Temperature:
Another property of chalcogenide glasses the glass transition temperature depend on overall mean bond energy <E>. As explained by the Tichy and Ticha [12] the value of glass transition temperature should not only be related to connectedness of the network which is related to average coordination number, but should also be related to the quality of connections. The overall mean bond energy for the Ge10BixSe80-xSn10 system is given by
< E > = Ec + Erm
where Ec is overall contribution towards bond energy arising from strong heteropolar bonds and Erm is contribution arising from weaker bonds that remains after the strong bonds have been maximized. These can be calculated by given relations
Ec = 4aEGe -Se + 3bEBi - Se + 4dESe-Sn
And
where a, b, c, d are atomic fractions of Ge, Bi, Se and Sn respectively.
Tichy and Ticha has given an interesting correlation between mean bond energy and glass transition temperature Tg as illustrated below
where <E> refers to the mean bond energy.
3. RESULT AND DISCUSSION:
Fig. 1 shows that the values of average coordination number increases from 2.42 to 2.56 with increase in concentration of Bi from 2 to 16 atomic % for the Ge10BixSe80-xSn10 system. It is quite predictable from fig.2, that our system is chalcogen rich. But it is turning towards chalcogen poor with the increase in content of Bi in the system. So in order to maintain a comfortable chalcogen rich status we have to keep Bi concentration within 2 to 16 atomic % only.
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Fig. 1: Variation of Average Coordination Number Z with Bismuth content. |
Fig. 2: Variation of parameter R with Bi content |
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Fig. 3: Variation of overall mean bond energy <E> with Bi content |
Fig. 4: Variation of glass transition temperature Tg with Bi content |
It is clear from fig. 3 that mean bond energy <E> increases from 2.369 to 2.559 eV/atom with increase in concentration of Bi from 2 to 16 atomic % i.e. selenium rich region. Fig. 4 shows the variation of glass transition temperature Tg with Bi content. It can be clearly interpreted that there is rise in glass transition temperature from 456.995 to 515.869 KJ/mol with the increase in Bi content from 2 to 16 at. %. It has been observed so, because we find increase in mean bond energy of the glassy system.
4. CONCLUSION:
In the present work, we had considered a Ge- Se based quaternary alloy comprising of Ge-Bi-Se-Sn. Here Ge and Sn have been fixed at 10 atomic %, and then studied the variation in some of the important physical parameters by varying the concentration of Bi from 2-16 atomic %. It has been established from the above results that physical properties in this system are highly composition dependent. The Ge10BixSe80-xSn10 glass system is of special interest as it forms glasses over a wide domain of compositions. Adding Bi to this decreases the chalcogen concentration in this system. Almost all the parameters, except R, were found to increase with the variation in Bi content thus making this combination suitable for phase change optical recording.
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Received on 14.12.2018 Accepted on 23.01.2019 ©A&V Publications all right reserved Research J. Engineering and Tech. 2019;10(1):01-03. DOI: 10.5958/2321-581X.2019.00001.1 |
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