Microstructure and Thermal Properties of the ...

Vol. 21 　No. 1

Journal of Wuhan University of Technology - Mater. Sci. Ed.

March 2006

Microstructure and Thermal Properties of the GeS22In2 S32CsI Glasses

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MAO Shun 　TAO Haizheng 　ZHAO Xiujian 　DONG Guoping ( Key Laboratory of Silicate Materials Science and Engineering( Wuhan University of Technology) , Ministry of Education ,Wuhan 430070 , China) 　　Abstract : 　The homogeneous GeS 2 2In2 S 3 2CsI glassy samples were prepared by conventional melt2quench2 ing method . When the molar ratio of In2 S 3 to CsI remains 1 , the non2crystalline region can extend to the compo2 sition 0. 4 GeS 2 20. 3 In2 S 3 20. 3 CsI . And with the addition of CsI , the glass2forming ability of this serial glass reaches its maximum at the composition 0. 8 GeS 2 20. 1 In2 S 3 20. 1 CsI . According to the Raman spectra , the mi2 crostructure of these glasses is mainly constituted by [ GeS 4 ] and [ InS 4 2x Ix ] tetrahedra , which are interconnected by the bridging sulf ur atoms ; meanwhile , the ethane2liked structural units [ S 3 Ge2GeS 3 ] can be formed because + of the lacking of sulf ur ; Cs ion , which is added f rom CsI , exists as the nearest neighbor of I2ion in the glassy network . Key words : 　chalcohalide glasses ; 　microstructure ; 　thermal properties

1 　Introduction Chalcogenide glasses exhibit many useful properties , especially their high non2linear optical properties , and have recently drawn great attentions because of their po2 tential applications for all2optical switching , promising [125 ] hosts for rare2earth doped active devices and etc . . Chalcogenide glasses are well known for their high chemical durability and very wide infrared transmittance [6 ] range . However , their relatively high refractive indices lead to large intrinsic losses in the mid2IR. Halide glass2 es , especially non2fluoride glasses , have a low glass tran2 sition temperature and in addition their chemical durabili2 [7 ] ty is relatively low . These factors have limited their practical applications. Our purpose is combining the advantages of the ha2 lide glasses and the chalcogenide glasses. The structural aspect is obvious from a fundamental viewpoint since chal2 cogenide glasses are predominantly covalent and halide glasses are main ionic. Based on these factors , it seems possible to improve the lower chemical durability of halide glasses by incorporating a certain chalcogenide glasses. Simultaneously , a possible decrease in the refractive index of the chalcogenide glasses would reduce the large intrin2 sic attenuation in the mid2IR. Many chalcohalide glasses were researched but the detailed researches of the GeS2 2In2 S3 2CsI glassy system have not been reported. Because of the intimate relation2 ( Received :Feb. 19 ,2005 ;Accepted :Nov. 15 ,2005) MAO Shun ( 毛舜) : E2mail : maoshun223 @tom. com gCorresponding author : ZHAO Ziujian ( 赵修建 ) : Prof. ; Ph D ; E2 mail :opluse @mial. whut. edu. cn 3 Funded by the National Natural Science Foundation of China (No. 50125205) , the Opening Fund of Key Laboratory of Silicate Materials Science and Engineering ( Wuhan University of Technolo2 gy) , Ministry of Education (No. SYSJJ2004214)

ship between the microstructure and the potential applica2 tions , the micro2structural study on these glasses will be worthwhile . In this paper , the ( 122 x ) GeS2 2 x In2 S3 2 x CsI glasses were prepared by the conventional melt2quenching method , and the structural investigations and thermal properties of these glasses were reported.

2 　Experimental The ( 122 x ) GeS2 2 x In2 S3 2 x CsI samples , where x = 0. 05 ,0. 1 ,0. 15 ,0. 2 ,0. 25 ,0. 3 ,0. 35 ,0. 4 ( molar cont2 ent ) were prepared by the melt2quenching technique from the elements Ge , In , S ( 99. 999 % purity) and the com2 pound CsI ( 99. 9 % purity ) . All investigated samples , which were prepared according to their batches , were heated at 950 ℃ in an evacuated quartz tube ( 10 mm in inner diameter) for 12 h and subsequently quenched with water. Details of the preparations were similar to the pro2 [8210 ] cedure in our previous works . The homogeneity of the quenched products was stud2 ied by optical microscopy and X2ray diffraction. Charac2 teristic temperatures of the glasses were obtained by DSC and TG analysis with a heating rate of 10 KΠmin for sam2 ple weights of approximately 50 mg. The errors in deter2 mining characteristic temperatures by this method were ± 2 ℃. The Raman investigations were conducted by a mi2 cro Raman Spectrometer ( Renishaw inVia Type) using the 632. 8nm laser line. For details , see our previous wo2 [8 ,10 ] rk .

3 　Results and Discussion 3. 1 　X2ray diffraction investigations Fig. 1 presents the X2ray diffraction patterns of the (122 x ) GeS2 2 x In2 S3 2 x CsI samples where x = 0. 3 ,0. 35 ,

Vol. 21 　No. 1 　　　　　　MAO Shun et al :Microstructure and Thermal Properties of the. . . .

0. 4. According to JCPDF Cards , for the samples 0. 3 GeS2 20. 35In2 S3 20. 35CsI and 0. 2 GeS2 20. 4In2 S3 20. 4CsI , certain diffraction peaks appear corresponding to the characteristic peaks of the crystals CsI and In2 S3 . However , the crystallization level is alleviated for the sample x = 0. 35. For the sample x = 0. 3 , it represents the classic characteristic profile of non2crystal phases. In conclusion , as shown from the X2ray diffraction patterns , crystalline phases of the sample 0. 3 GeS2 20. 35In2 S3 20. 35CsI have been identified and the amorphous region can extend to the sample x = 0. 3. [11 ,12 ] According to the previous studies , the micro2 structure of GeS2 is constituted by the basic units [ GeS4 ] , which are connected through bridging sulfur at2

Fig. 1 　X2ray patterns of the samples (122 x ) GeS22 x In2 S32 x CsI

3. 2 　Differential scanning calorimetry and thermal gravity The characteristic DSC and TG thermograms of these glasses are shown in Fig. 2. Two characteristic phenomena are clear in the studied temperature region. The first dis2 tinct endothermic peak corresponding to the glass transi2 tion temperature Tg located at about 350 ℃. And from the thermogram , the first distinct crystallization exother2 mic peak Tc , which corresponds to the temperature of the crystallizing peak , located at 480 ℃. Besides , the TG curve has shown that the gravity of the sample begins to decrease at about 500 ℃, then drops slightly and finally at 700 ℃ reaches about 92. 1 % of its original weight . According to the DSC and TG thermograms , the val2 ues of characteristic temperatures of the ( 122 x ) GeS2 2 x In2 S3 2 x CsI glassy system are given in Table 1. In the studied glassy system , the glass transition temperatures Tg exceed 335 ℃ for all the samples and have a monotonous decreasing tendency with the increased content of CsI. The criterion Tc2 Tg is critical to analyze the thermal stability of a glass and its fiber drawing abili2 ty ; glasses with high Tc2 Tg values , e g , > 100 ℃, are usually drawn into fibers. From Table 1 , values of Tc2 Tg for most of the glasses in this study are generally > 100 ℃, indicating that these glasses are easily obtained in

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oms to form a three2dimensional network. And for the GeS2 2In2 S3 pseudo2binary glassy system , the basic units are [ GeS4 ] and [ InS4 ] . With the incorporation of CsI , + 2I ion enters the glassy network and replaces S ion of + the [ InS4 ] tetrahedra gradually ; in other words , I ion breaks the glassy network because of its network terminat2 ing effect . And this influence has resulted in the amor2 phous region limitation because the network can be de2 stroyed due to the breaking effect . In conclusion , accord2 ing to the X2ray diffraction investigation , it is verified that the short2range order of the studied glasses has changed with the incorporating of CsI , which will be further testi2 fied with the Raman investigations.

Fig. 2 　Thermograms of the sample 0. 8 GeS2 20. 1In2 S320. 1CsI

bulk forms and have adequate stabilities for fiberization. Besides , the characteristic temperature Tc2 Tg , varies from 101. 4 ℃ to 122. 3 ℃ and a maximum point at x = 0. 1 can be obtained. So , according to Table 1 , the com2 position 0. 8 GeS2 20. 1In2 S3 20. 1CsI has the best glass2 forming ability for this serial of glasses. Table 1 　Characteristic temperatures of the ( 122 x) GeS22 x In2 S32 x CsI glasses Compositions

TgΠ℃

TcΠ℃

Tc2Tg

X = 0. 05

404. 8

505. 2

101. 4

X = 0. 1

358. 5

480. 8

122. 3

X = 0. 2

341. 7

456. 2

114. 5

X = 0. 25

335. 5

447. 3

111. 8

Fig. 3 　Raman spectra of the samples (122 x) GeS22 x In2 S32 x CsI

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Journal of Wuhan University of Technology - Mater. Sci. Ed. 　　　　　　　　　　March 2006

3. 3 　Raman spectral analysis Fig. 3 shows the Raman spectra of GeS2 and the samples ( 122 x ) GeS2 2 x In2 S3 2 x CsI where x = 0. 1 ,0. 15 , 0. 2 ,0. 25. The Raman spectrum of the GeS2 glass has the [11213 ] similar features with those reported previously . The basic structure units are GeS4Π2 tetrahedra , which are con2 nected through the bridging sulfur atoms to form a three2 - 1 dimensional network. The dominant band at 340 cm is due to the symmetric bond2stretching vibration of the tet2 - 1 rahedra[ GeS4 ] . The shoulder at 372 cm is known as a companion band , which is due to the vibration of two - 1 edge2shared tetrahedra. The band at about 432 cm orig2 inates from the vibration of two tetrahedra connected [13 ] through bridging sulfur at the corner ; but another theo2 ry pointed out that this band was due to S S bond or [11 ] some multi2sulfur bonds . Hence , we do not explain in detail about this band. For all the samples , there is a dominant band at - 1 about 340 cm just as viewed in the above spectrum of the GeS2 glass. And the intensity of this peak decreased with the decreasing of the GeS2 content and this was easily understood according to the content of GeS2 , which has a linearity law as shown on the spectra. - 1 Another obvious band at 300 cm , which has not emerged in the spectrum of the GeS2 glass , can be clearly differentiated from the spectra. According to the [14 ] reference , the symmetric stretching vibrating mode of - 1 the [ InS4 ] tetrahedra locates at about 305 cm and , fur2 thermore , the symmetric bond2stretching vibration of the - 1 [ InS42 x Cl x ] tetrahedra locates at about 296 cm . Be2 cause of the Cl and I are in the same main group of the periodic table of elements , it is comfortable to predicate that the symmetric bond2stretching vibration of the [ InS4 2 x I x ] tetrahedra has the same effect as the [ InS42 x Cl x ] tetrahedra. So it is suggested that the intense peak at 300 - 1 cm has a certain relationship with the symmetric bond2 stretching vibration of the [ InS42 x I x ] tetrahedra. As a re2 - 1 sult , we ascribe the peak at 300 cm to the symmetric bond2stretching vibration of the [ InS42 x I x ] tetrahedra. In the GeS2 2In2 S3 glasses , most In exist as the form of + [ InS4 ] tetrahedra and with the combining of CsI , I ion 2has taken the place of S ion and forms the [ InS4 - x I x ] tetrahedra. However , the symmetric stretching vibrating - 1 mode of the [ InS4 - x Ix ] tetrahedra has shifted for 5 cm towards low frequency compared with that of the [ InS4 ] [10 ] tetrahedra. Based on our previous research , the mode shifting is related to the bond intensity and the discount mass of the corresponding tetrahedra. The intensity of this peak decreases with the decreasing content of In2 S3 and this can be explained by the same explanation according - 1 to the discussion on the band at 340 cm . - 1 In addition , a broad band at about 260 cm is rec2

ognized and ascribed to the vibration of ethane2liked [13 ] structure units [ S3 Ge2GeS3 ] , which is formed when GeS2 is melt2quenched and the microstructure is partly fluctuated. And the existence of ethane2like structure [ S3 Ge2GeS3 ] implies that the formed glasses become defi2 ciency of sulfur according to the chemistry prescription. Furthermore , the intensity of this band increases with the increasing contents of CsI and In2 S3 . This phenomenon can be explained by the two following reasons. In the first place , as we know , the basic structure units GeS4Π2 tetra2 hedra are connected through a bridging sulfur to form a three2dimensional network in the glassy system. However , 3+ with the combining of In2 S3 , In ions , which exist as the form of [ InS4 ] tetrahedra in glassy network ( ratio of 2In to S is 1Π2 ) , link more S ions according to the chemistry prescription of In2 S3 ( ratio of In to S is 2Π3 ) , causing the sulfur deficiency and forming some ethane2like units [ S3 Ge2GeS3 ] . The ethane2like units [ S3 Ge2GeS3 ] are undoubtedly of sulfur economization. Secondly , with 2the addition of CsI , I ions can take the place of S ion gradually and [ InS4 - x I x ] tetrahedra can be formed. And 2this process can free some S ions and alleviates the sul2 fur deficiency. These two factors have contradictive ef2 fects. According to the spectra ( Fig. 3) , the former has a more influence. But this band is not so acute which means that just a little ethane2like units [ S3 Ge2GeS3 ] ex2 ist in the glassy system. The above analysis presents an2 other powerful proof of the existence of [ InS4 - x I x ] tetra2 hedra and strengthens our ascription of the intense peak at - 1 300 cm . According to the results of microstructural study , the origin of the maximum glass forming ability for the sample 0. 8 GeS2 20. 1In2 S3 20. 1CsI can be deduced from two cont2 radictive factors. First , with the increasing content of CsI , some [ InS4 - x I x ] tetrahedra and some ethane2like structures [ S3 Ge2GeS3 ] can be formed and enter into the glassy network as we discussed on the Raman spectra ; these disordered micro2structural units can enhance the confusion degree of the glass network. Secondly , CsI is a representative ionic crystal and it has a great tendency to form a crystalloid. In addition , with the combining of I ion , which represents a glassy net terminating effect , the dimensionality of the glassy network has been decreased. All these factors can reduce the glass2forming ability. Consequently , based on the above2mentioned analysis , it successfully explains the origin of the maximum glass2 forming ability for the sample x = 0. 1 on this serial of glasses.

4 　Conclusions The ( 122 x ) GeS2 2 x In2 S3 2 x CsI samples ( x = 5 ,10 ,

Vol. 21 　No. 1 　　　　　　MAO Shun et al :Microstructure and Thermal Properties of the. . . .

15 ,20 ,25 ,30 ,35 ,40) were prepared by the conventional melt2quenching method. The composition , for having the best glass formation ability , is 0. 8 GeS2 20. 1In2 S3 20. 1CsI. This phenomenon can be explained by the coopera2 tion of the following two aspects : on the one hand , the mixed tetrahedra [ InS4 - x I x ] and some ethane2like struc2 tures [ S3 Ge2GeS3 ] can enhance the confusion degree of the glassy network ; on the other hand , the adding of I , which represents a glassy network ending effect and de2 creases distinctly the connectivity of the glassy net , can be a adverse factor on the glass formation. Based on the above discussion , it can be concluded that the microstructure of the ( 122 x ) GeS2 2 x In2 S3 2 x CsI glasses can be depicted as follows : the glassy net is con2 stituted mainly by [ GeS4 ] and [ InS42 x I x ] tetrahedra and all these structural units are interconnected by sulfur bridges. Some little part of Ge exists in the form of the ethane2like structural units [ S3 Ge2GeS3 ] because of the lacking of S , and these units are homogeneously dispersed + in the disordered polymer network. Cs ion , which was added from CsI , exists as the nearest neighbor of I ion.

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