Minimum fragility in GexSe80−xPb20 glasses

PHYSICA@ Physica B 205 (1995)403-407

ELSEVIER

Minimum fragility in GexSe80-xPb20 glasses M . K . R a b i n a l a'*, K . S . S a n g u n n i a, E . S . R . G o p a l b, S.V. S u b r a m a n y a m

a

aDepartment of Physics, Indian Institute of Science, Bangalore 560 012, lndia b National Physical Laboratory, New Delhi 110 012, India

Received 7 October 1993; revised 23 September 1994

Abstract Bulk GexSeso xPb2o glasses, which show a change from p- to n-type conduction around x = 21 at %, have been prepared by quenching the melt. The specific heat at constant pressure (Cp) has been measured for these glasses using a DSC. These glasses exhibit minima in a jump in specific heat (ACp) and the area under the peak at Tg at x = 19 at %, where they show a minimum fragility. These results are discussed in the light of a chemical threshold in these glasses.

1. Introduction Some attractive features possessed by chalcogenides have made them a special class of amorphous semiconductors [1]. The existence of positively charged, triply co-ordinated, and negatively charged, singly co-ordinated, defects are the direct consequences of the p-lone pairs and the low coordination of chalcogenide glasses [2-3]. The large concentration of these defects and the fact that each constituent atom satisfies its valence requirements ensure the strong pinning of the Fermi-level in the middle of the gap; as a result these solids are normally insensitive to an external doping. Thus, it is believed that it is difficult to dope these semiconductors under equilibrium conditions, unlike the crystalline counterparts. However, it is pointed out that certain charged additives could have subtle effects on the transport properties of glasses [4, 5]. Indeed this has been

realized in a few glasses on the addition of quite a few elements, like oxygen and chlorine [6], copper [7], bismuth [8], lead [9], etc. [10]. Among these Bi and Pb in G e - M (M = S, Se, Te) have attracted much attention, since these lead to the unlocking of E v and to the p- to n-type conduction change [8, 11, 12]. The G e - S e - B i glasses have been subjected to intensive investigations to explore this subtle phenomenon, Based on these measurements, it is concluded that phase separation at the microscopic level might play a vital role [13-15]. On the other hand, very little information is available on GexSeso-xPbzo glasses, which also show p- to n-type conduction inversion around x = 21 at %. The present communication reports the first specific heat measurements at constant pressure (Cp) on these glasses.

2. Experimental measurements

*Corresponding author.

The macroscopically homogeneous glasses of G e - S e Pb have been reported earlier [16, 17]. The

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M.K. Rabinal et al./Physica B 205 (1995) 403 407

glasses with x = 17, 19, 21, 23 and 24 at % were prepared by quenching a melt. The required quantities of Ge, Se and Pb electronic grade elements were sealed in quartz ampules under a vacuum of 5 x 10- 5 Torr. The material is heated and held at 1273 K for 24 h. To ensure good mixing the molten liquid is subjected to a continuous rotation for another 12 h. The final melt is quenched in a solution of ice-water + NaOH. The resulting samples are characterized to be amorphous using X-ray diffraction. Our earlier observations of thermal crystallization on these glasses show that these are characterized by a single glass transition followed by a single crystallization exotherm [18]. It has been well established that annealing has subtle effects on the glass transition [19, 20]. Hence to have identical thermal history, these bulk glasses are heated just below their respective Tg'S with a heating rate of 20 K/min and cooled quickly back to room temperature. These samples are used for C o measurements using a ratio method [21]. Measurements have been carried out using a fully automated DuPont Differential Scanning Calorimeter (DSC-610), with a heating rate of 20 K/min under dry N2 atmosphere. The C~ data of ct-A1203 have been used as a standard [-22]. All the thermograms were recorded on bulk chips of roughly 100 mg.

Table 1 Energy involved in the endothermic peak as a function of composition x Composition x (at%)

Cp (323 K) (J/mol deg)

Energy (J/g)

17 19 21 23 24

29.09 16.49 21.82 27.08 22.68

1.298 0.473 0.444 0.946 0.970

z.8

o-x=17 o~x~19

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560

600

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3. Results and discussion

o-x=23

C~

"-~ 36

The Cp for all the glasses have been measured from room temperature to well above their glass transition temperatures. The Cp values at 323 K of the different compositions are given in Table 1. Variations of Cp as a function of temperature around Tg are shown in Figs. l(a) and l(b) for all the samples. These glasses exhibit sharp glass transitions; however, for x = 19 at %, the behaviour is slightly sluggish in comparison with other compositions. These plots are used to calculate the jump ACp ( = C g - - Clp), where C~ and C~p are the specific heats at constant pressure of glass and supercooled liquid, respectively. These are marked in the figures. Plots of A C p a s a function of composition are depicted in Fig. 2, which shows a minimum at x = 19 at %. The total energy involved in

ACp 36 ,~

E

28 28 2O

400

i

~;o

~.;o

,

~io

,

5;0

,

20

600

Tempcroture ~K)

Fig. 1. Variation of specific heat Cp around compositions.

Tg for

different

the endothermic peak has been calculated, by joining lower and higher onsets of peak, which is marked in Fig. 3 for x = 17 at % as a representative of other compositions. Fig. 4 shows the

M.K. Rabinal et al./Physica B 205 (1995) 403 407

405

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18-0

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Ge at. %

Fig. 4. Variation of the total energy of the endothermic peak as a function of composition x.

Fig. 2. ACp as a function of composition x.

x:17 -7-

E

v

-9-

-r -II

-13 Z,33

z053

~ 473

I I 493 513 Temperoture (K)

I 533

553

Fig. 3. Calculation of the area under the endothermic peak for x = 17 a t % .

variation of this energy with the composition: again it shows a minimum between x = 19 and 21 at %. The numerical data are given in Table 1. Experimental investigations reveal that the phenomenon of glass transition is a confluence of thermodynamic and kinetic effects. Various theoretical models have been put forward to account for it [23]. However, till now there is no single theory to

account for all the aspects of glass transition. The basic properties, such as the shear viscosity, density, specific heat, electrical conductivity, etc., undergo dramatic changes when the material is either cooled or heated through the glass transition [24]. The viscosity of these materials is often taken to understand the phenomenon. Many of the glass forming liquids can be described by the simple empirical Vogel-Tammann-Fulcher equation of viscosity [23]. The first attempt to interlink the viscosity and the configurational entropy goes to the credit of Adam and Gibbs [25], who found an expression r1 = rlo e x p ( B / T

So),

(1)

where ~/is the viscosity, r/o, B are constants, T is the temperature and Sc is the configurational entropy given by S~ =

f;

ACp dln T.

(2)

o

Thus, these two equations connect the tl and ACp changes at the glass transition. The lower limit To is called the ideal glass transition temperature or

406

M.K. Rabinal et al./Physica B 205 (1995) 403 407

the Kauzmann point. From Eq. (2) it is clear that for very small ACp, Sc becomes temperature independent and hence Eq. (1) would be Arrhenius-like. For large ACp, Sc contributes a temperature-dependent term to Eq. (1) and hence the behaviour would be non-Arrhenius-like. Based on extensive investigations, Angell and colleagues [26] classified most of the glass forming liquids into two extremes: strong liquids and fragile liquids. This model has been considered by others also E27, 28]. The strong liquids are characterized by Arrhenius-like behaviour and have a small jump in ACp. Examples are SiO2, GeO2, etc. The fragile liquids are characterized by non-Arrhenius-like behaviour with a large jump in ACp, the examples being Ca(NO3)2, organic glasses, etc. Therefore, ACp is an important experimental parameter to check the fragility at chemical and mechanical thresholds. The G e - S e - P b glasses have been subjected to close scrutiny by many authors [9, 18,29-31]. According to Tohge et al. [9], these glasses show p- to n-type conduction change at x = 21 at % monitored by thermopower measurements. They exhibit anomalous behaviours at this critical composition. It is found that, at ambient conditions, the electrical conductivity shows a maximum and the corresponding activation energy shows a minimum. On the other hand, the drift mobility of charge carriers goes to a deep minimum around x = 19 at%. Similarly, our recent measurements on the glass transition and crystallization kinetics on these glasses also reflect such features. The Tg, crystallization activation energy and its frequency factor show maxima at x = 21 a t % [18]. Bhatia et al. have carried out the AC conductivity measurements on x = 19, 21 and 23 a t % glasses. It has been inferred that at x = 21 at% the dielectric constant and the defect densities reach minima [29]. Measurements on the X-ray radial distribution function (RDF) on a few compositions of these glasses have been carried out [32]. The Pb atoms go to Pb 2+ configuration by probably forming ionic bonds with non-bridging Se- ions. In addition, some of the very recent results on microstructure, such as X-ray absorption edge spectroscopy [33] and far IR transmittance [30], give the signature of iono-covalent bonding in these materials.

To explain the p- to n-type conduction inversion and some of the observed anomalous features Tohge et al. [9] have used chemical bond statistics. It is assumed that Pb-Se ionic bonds are fixed. With Ge percentage, GexSeso xPb2o glasses reach a chemically ordered phase at x - - 2 0 at % and mainly contain the high energy Ge-Se heteropolar bond. Additional support has been provided by EPR measurements on these glasses [31]. Thus, the anomalous features observed by many authors could be attributed to the existence of a chemical threshold in these glasses. The present experimental results could also be accounted for by the chemical threshold present in these glasses. At a lower Ge percentage, say x = 17 at %, keeping P b - S e ionic bonds constant, these glasses contain GeSe4/2 tetrahedral units dissolved in the rest of the Se matrix. Therefore the system can be considered as fragile. Hence a greater jump in ACp is seen. With an increase in x the concentration of GeSe4/2 units builds up at the expense of Se chains. At x -- 20 at %, the material becomes chemically ordered and the cohesive energy of the system reaches a maximum. As a result it should show the minimum fragility at this composition. On further addition, the high energy Ge-Se bonds would be replaced by low energy G e - G e bonds and hence could lead to fragility. The observed minimum fragility occurs at x = 19 at%, which is quite close to the chemical threshold x¢ = 20 at %. The minimum fragility has been observed in G e - A s - S e glasses at a mechanical threshold [34]. There is no quantitative structural model on the phenomenon of glass transition. As a result it is difficult to understand the energy involved in glass transition as a function of composition. However, the changes in it, which shows a minimum in between x = 19 and 21 at%, could be related to similar changes in the structure of these glasses.

4. Conclusions

We have undertaken the first time the Cp measurements on GexSeso xPb2o (17 ~< x ~< 24) glasses using a DSC. These measurements have been carried out well above their respective glass

M.K. Rabinal et al./Physica B 205 (1995) 403 407

transition temperatures. ACp and the energy per gram of endothermic peak at Tg are calculated and they show a deep minimum around x = 19 at%, which is quite close to the chemical threshold x = 20 at% in these glasses. These features could be predicted using the Adam-Gibbs theory of entropy, which shows the minimum fragility at chemical threshold.

Acknowledgements The authors wish to thank Mr. R.S. Vaidyanathan for the recording of thermograms. They also acknowledge Mr. N. Ramesh Rao and other colleagues for assistance during the course of experimental measurements, and IUC-DAE, Government of India, for the financial assistance.

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