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Crystalline–amorphous transitions of Ge–Se alloys by mechanical grinding
Journal of Non-Crystalline Solids 293±295 (2001) 779±784
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Crystalline±amorphous transitions of Ge±Se alloys by mechanical grinding Yasuo Tani 1, Yoshiyuki Shirakawa *, Atsuko Shimosaka, Jusuke Hidaka Department of Chemical Engineering and Materials Science, Doshisha University, Kyotanabe, Kyoto 610-0321, Japan
Abstract X-ray diraction and dierential scanning calorimetry (DSC) measurements of Se and GeSe2 during ball milling have been investigated. In this study, amorphous Se and GeSe2 are synthesized by mechanical grinding (MG) recognized as a method of inducing a crystalline±amorphous phase transition. The amorphization of Se is caused from distortion between helical chains connected by van der Waals forces. The enthalpy DH for crystallization of Se by rapid quench is lower than that by the MG. This result suggests that the dierence of DH corresponds to that of the structures in intermediate range. Monoclinic GeSe2 is amorphized by disordering between the connected tetrahedra. It is considered that the tetrahedral unit of GeSe2 by the MG is distorted with wide distributions. GeSe2 is completely amorphized after 4 h of grinding, and Se after 30 h in the present milling. The dierence indicates that disordering between the connected tetrahedral units in monoclinic GeSe2 is induced more easily than that between chains in trigonal Se. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 61.10.)i; 61.43.Dg; 81.20.Ev
1. Introduction Chalcogenide glasses are applied to switching devices, optical ®bers, memory materials, etc. They are prepared by rapid quench of molten alloys in general. Recently, the mechanical grinding (MG) is used as a method to induce a crystalline±amorphous phase transition by mechanical collision and friction forces in a solid-state reaction [1,2]. It is a convenient method that amorphous materials, which are dicult to produce by rapid quench, can be obtained. On the other hand, the amount of amorphous phase in crystalline matrix is con* Corresponding author. Tel.: +81-774 65 6596; fax: +81-774 65 6842. E-mail address: [email protected] (Y. Shirakawa). 1 Present address: Nissin Foods Inc., 1-1, Nagara-cho, Nakagawa-ku, Nagoya 454-8520, Japan.
trolled by grinding time. Therefore, the MG method has ability to produce new functional materials of chalcogenide glasses such as hybrid materials consisting of crystalline and amorphous phase. In order to design these materials, it is important to understand the crystalline±amorphous phase transitions for chalcogenide glasses caused by the MG. In this study, the transitions of Se and GeSe2 are studied as functions of grinding time in terms of structural and thermodynamic consideration. Se and GeSe2 samples mechanically ground by a planetary ball mill are investigated by using X-ray diractions and dierential scanning calorimetry measurements (DSC). 2. Experimental procedures Amorphous Se and GeSe2 samples were prepared by rapid quenching from the melt compos-
0022-3093/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 1 ) 0 0 7 8 8 - 8
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Y. Tani et al. / Journal of Non-Crystalline Solids 293±295 (2001) 779±784
ing of Se (99.999%) and Ge (99.999%). The molten state of Se was kept at 350 °C for 5 h and that of GeSe2 was kept at 1000 °C for 5 h. The crystalline samples were prepared by annealing process of these amorphous samples. The crystalline Se was obtained by annealing at 150 °C for 5 h. The crystalline GeSe2 was obtained by annealing at 500 °C for 5 h. The crystalline structure of Se and GeSe2 were con®rmed by X-ray diraction analysis. The planetary ball mill (Fritsch, Pulversette 7) is used to give the samples mechanical energy. The milling vial with the inner diameter 40 mm and height 40 mm was made of SUS304 alloy. SUS304 balls with a diameter of 3 mm were used. The powder samples were sieved under a particle size less than 74 lm sealed in the vial with the balls under Ar gas atmosphere. The MG was carried out using the revolutions of 300 rpm and the rotations of 600 rpm. X-ray diraction patterns were measured by means of a powder diractometer with Mo-Ka and Cu-Ka radiation. DSC measurements were performed with a heating rate of 10 °C/min in Ar gas atmosphere. In Se after 50 h and GeSe2 after 10 h of grinding, the amount of impurity Fe from the vial and balls was measured to be less than 0.1 at.%.
Fig. 1. X-ray diraction patterns of Se after 0, 10, 20, 30 and 50 h of grinding.
3. Results X-ray diraction showed the presence of trigonal Se at the start. Fig. 1 shows the X-ray diraction patterns of Se after 0, 10, 20, 30 and 50 h of grinding and amorphous Se by rapid quench from 350 °C. The sharp Bragg peaks become broadened with increasing grinding time, and then disappear after 30 h of grinding. It indicates that trigonal Se is amorphized in the present condition. Fig. 2 shows that the DSC curves of Se after 10, 20, 30 and 50 h of grinding and amorphous Se prepared by rapid quench. As shown in Fig. 3(a), both the crystallization temperature Tc and the glass transition temperature Tg increase with the increase of grinding time. Both Tc and Tg of Se by rapid quench are lower than those of Se after 10 h of grinding. Fig. 4(a)
Fig. 2. DSC curves of Se after 10, 20, 30 and 50 h of grinding.
shows that the enthalpy DH for crystallization of mechanically ground Se increases with increasing grinding time, and approaches saturation for a
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Fig. 3. Tc and Tg of (a) Se and (b) GeSe2 by the MG and by rapid quench.
certain value more than 30 h of grinding. DH of Se by rapid quench is lower than that by grinding for 30 h. The crystalline GeSe2 was con®rmed monoclinic GeSe2 by X-ray diraction analysis. Fig. 5 shows the X-ray diraction patterns of GeSe2 after 0, 0.5, 4 and 10 h of grinding, and that by rapid quench. The sharp Bragg peaks decrease with increasing the grinding time, and then the amorphous phase is formed all over. Monoclinic GeSe2 is completely amorphized after 4 h of grinding in the present condition. As shown in Fig. 3(b), both Tc and Tg of GeSe2 prepared by grinding decrease with increasing grinding time. Both Tc and Tg of GeSe2 by rapid quench are higher than those by grinding for 4 h. Fig. 4(b) shows that DH of mechanically ground GeSe2 increases with increasing grinding time. The DH tends to approach a certain value by grinding like that of Se.
Fig. 4. DH of (a) Se and (b) GeSe2 by the MG.
4. Discussion Trigonal Se is composed of the helical chains in crystalline state. The chain structure is formed for a period of three Se atoms connected helically, and the forth atom coordinates the equal site of the ®rst atom in the next unit in their periodicity. The ®rst nearest atoms of trigonal Se, which are in the same helical chain molecule, are located at 2.33 A from an original atom. The second nearest atoms, which are in the adjacent chain molecules, are lo and the third nearest atoms, which cated at 3.47 A, are the second nearest atoms belonging to the [3]. same chain molecule, are located at 3.69 A Pair distribution functions g r for amorphous Se after 30 h of grinding and rapid quench from
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Y. Tani et al. / Journal of Non-Crystalline Solids 293±295 (2001) 779±784
Fig. 5. X-ray diraction patterns of GeSe2 after 0, 0.5, 4 and 10 h of grinding.
the liquid state at 350 °C derived from the structure factor S Q obtained by X-ray diraction are shown in Fig. 6. The g r of Se by the MG has the in the present remain peaks at 2.33 and 3.67 A sults. The present results agree with the reported results [3]. In comparison with trigonal Se, the ®rst nearest distance of the amorphous Se is almost the same, but the second nearest distance changes Therefore, it is considered from 3.47 to 3.67 A. that trigonal Se is amorphized with disordering interaction of the helical chains. In comparison with the g r of Se prepared by rapid quench, the ®rst and second peak positions are almost the same. However, the structure of amorphous Se by the MG for 30 h is dierent from that by rapid quench because of DSC results. Amorphous Se by rapid quench from the liquid state at 350 °C has the structure-like liquid Se at 350 °C. There are two forms in liquid Se, chain and ring structures. The former is called transconformation in a helical chain, and the latter is called cis-conformation in an Se-8 ring. The difference from the two forms depends on bond angle between the ®fth and forth atoms. The potential
Fig. 6. Pair distribution functions g r for Se by the MG for 30 h of grinding and by rapid quench from liquid state at 350 °C.
energy of the cis-conformation is a little higher than that of the trans-conformation. It is reported by Misawa that the calculated ratio of the presence of Se-8 ring is approximately 25% in liquid Se at 350 °C [4]. On the other hand, it seems that there are almost trans-conformations in amorphous Se prepared by grinding [3]. The structure of amorphous Se by rapid quench from 350 °C consists of trans- and cis-conformations. There is the dierence between these amorphous Se caused by the existence of the cis-conformation. Hence, DH by rapid quench should be larger than that by grinding, but less as shown in Fig. 4(a). In the process of the MG, strain is accumulated in samples, and then leads to an amorphization in principle. It seems that there exists the strain in various scales, because amorphous Se is composed primarily of SN long chains with N 105 [5], and the crystallization needs thermal energy in order to release the strain. Therefore, it is considered that the dierence of the intermediate-range order more of the pair distribution functions g r in than 6 A Fig. 6 is concerned with this results. A structural unit of monoclinic crystalline GeSe2 consists of 4 Se atoms coordinated a Ge atom. The tetrahedral units are connected to share their Se
Y. Tani et al. / Journal of Non-Crystalline Solids 293±295 (2001) 779±784
corners each other, and the connected tetrahedral units form the chain structure. Each chain of GeSe2 is situated in parallel. The chains are connected by the tetrahedral units sharing the Se±Se bond [6]. Fig. 7 shows the g r for amorphous GeSe2 by the MG for 4 h and by rapid quench from liquid state at 1000 °C. In the present result of amorphous GeSe2 and the by the MG, the Ge±Se bond length is 2.32 A Se±Se is 3.79 A. On the other hand, the Ge±Se bond and the Se±Se is 3.80 A in amorlength is 2.32 A phous GeSe2 by rapid quench. The results show that the Ge±Se and Se±Se bond length of amorphous GeSe2 by the MG for 4 h are almost the same as those of amorphous GeSe2 by rapid quench. It is reported that amorphous GeSe2 by rapid quench consists of the tetrahedral units [7]. Therefore, it shows that monoclinic GeSe2 is mainly amorphized by disordering the coordination of the tetrahedral structure. The g r of GeSe2 by rapid quench is slightly dierent from that by the MG in the region The structural dierence between more than 5 A. GeSe2 by the MG and by rapid quench appears in Fig. 8 showing the structure factors S Q. The peaks of S Q of GeSe2 by rapid quench are sharper than those by the MG for 4 h. It is clari®ed by the molecular dynamics simulation that the ®rst peak
783
Fig. 8. Structure factors S Q of GeSe2 by the MG for 4 h of grinding and by rapid quench from liquid state at 1000 °C.
1 is mainly concerned with Ge±Ge around Q 1 A 1 correlations and the second one around Q 2 A is mainly due to Se±Se correlations [7]. Therefore, these correlations of GeSe2 by the MG have wide distributions of intra- and inter-tetrahedra. This result shows GeSe2 by the MG has wider energy states than that by rapid MG quench. And the tetrahedral unit of GeSe2 by MG is distorted. DH of GeSe2 by the MG is larger than that by rapid quench. It is considered that the dierence of DH is caused by wide distributions of the Ge±Ge and Se± Se correlations of GeSe2 by the MG. GeSe2 is completely amorphized after 4 h of grinding, and Se is done after 30 h. There is a large dierence between the two samples under the same milling condition. The dierence means that disordering of the connected tetrahedral units in monoclinic GeSe2 is induced more easily than that of the bond between helical chains in trigonal Se. 5. Conclusions
Fig. 7. Pair distribution functions g r for GeSe2 by the MG for 4 h of grinding and by rapid quench from liquid state at 1000 °C.
Amorphous Se and GeSe2 have been obtained by the MG method. The amorphization of Se is
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caused from distortion between helical chains connected by van der Waals forces. There is not a large dierence of the bonding length between Ge and Se atoms in the tetrahedral unit of GeSe2 for the amorphization. The amorphization of monoclinic GeSe2 is caused from disordering between the connected tetrahedral units. However, it is considered that they are distorted in amorphous Se and GeSe2 by grinding, and then the distortions cause a rise of the enthalpy for crystallization. GeSe2 is completely amorphized after 4 h of grinding, and Se after 30 h in the present milling. The dierence indicates that disordering between the connected tetrahedral units forming chain structure in monoclinic GeSe2 is induced more easily than that between each helical chain in trigonal Se, because these samples are given the same amount of mechanical energy by the planetary ball mill.
Acknowledgements The authors would like to thank Dr Umesaki for X-ray diraction experiments. This work was supported by Hosokawa Powder Technology Foundation, and a grant to RCAST at Doshisha University from the Ministry of Education, Japan. References [1] R.B. Schwarz, Mater. Sci. Eng. 97 (1988) 71. [2] R.B. Schwarz, C.C. Koch, Appl. Phys. Lett. 49 (1986) 146. [3] T. Fukunaga, M. Utsumi, H. Akatsuka, M. Misawa, U. Mizutani, J. Non-Cryst. Solids 205±207 (1996) 531. [4] M. Misawa, K. Suzuki, J. Phys. Soc. Jpn. 44 (1978) 1612. [5] R. Zallen, The Physics of Amorphous Solids, Wiley, New York, 1998. [6] O. Matsuda, K. Inoue, K. Murase, Solid State Commun. 75 (1990) 303. [7] P. Vashishta, R.K. Kalia, G.A. Antonio, Phys. Rev. Lett. 62 (1989) 1651.
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Crystalline±amorphous transitions of Ge±Se alloys by mechanical grinding Yasuo Tani 1, Yoshiyuki Shirakawa *, Atsuko Shimosaka, Jusuke Hidaka Department of Chemical Engineering and Materials Science, Doshisha University, Kyotanabe, Kyoto 610-0321, Japan
Abstract X-ray diraction and dierential scanning calorimetry (DSC) measurements of Se and GeSe2 during ball milling have been investigated. In this study, amorphous Se and GeSe2 are synthesized by mechanical grinding (MG) recognized as a method of inducing a crystalline±amorphous phase transition. The amorphization of Se is caused from distortion between helical chains connected by van der Waals forces. The enthalpy DH for crystallization of Se by rapid quench is lower than that by the MG. This result suggests that the dierence of DH corresponds to that of the structures in intermediate range. Monoclinic GeSe2 is amorphized by disordering between the connected tetrahedra. It is considered that the tetrahedral unit of GeSe2 by the MG is distorted with wide distributions. GeSe2 is completely amorphized after 4 h of grinding, and Se after 30 h in the present milling. The dierence indicates that disordering between the connected tetrahedral units in monoclinic GeSe2 is induced more easily than that between chains in trigonal Se. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 61.10.)i; 61.43.Dg; 81.20.Ev
1. Introduction Chalcogenide glasses are applied to switching devices, optical ®bers, memory materials, etc. They are prepared by rapid quench of molten alloys in general. Recently, the mechanical grinding (MG) is used as a method to induce a crystalline±amorphous phase transition by mechanical collision and friction forces in a solid-state reaction [1,2]. It is a convenient method that amorphous materials, which are dicult to produce by rapid quench, can be obtained. On the other hand, the amount of amorphous phase in crystalline matrix is con* Corresponding author. Tel.: +81-774 65 6596; fax: +81-774 65 6842. E-mail address: [email protected] (Y. Shirakawa). 1 Present address: Nissin Foods Inc., 1-1, Nagara-cho, Nakagawa-ku, Nagoya 454-8520, Japan.
trolled by grinding time. Therefore, the MG method has ability to produce new functional materials of chalcogenide glasses such as hybrid materials consisting of crystalline and amorphous phase. In order to design these materials, it is important to understand the crystalline±amorphous phase transitions for chalcogenide glasses caused by the MG. In this study, the transitions of Se and GeSe2 are studied as functions of grinding time in terms of structural and thermodynamic consideration. Se and GeSe2 samples mechanically ground by a planetary ball mill are investigated by using X-ray diractions and dierential scanning calorimetry measurements (DSC). 2. Experimental procedures Amorphous Se and GeSe2 samples were prepared by rapid quenching from the melt compos-
0022-3093/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 1 ) 0 0 7 8 8 - 8
780
Y. Tani et al. / Journal of Non-Crystalline Solids 293±295 (2001) 779±784
ing of Se (99.999%) and Ge (99.999%). The molten state of Se was kept at 350 °C for 5 h and that of GeSe2 was kept at 1000 °C for 5 h. The crystalline samples were prepared by annealing process of these amorphous samples. The crystalline Se was obtained by annealing at 150 °C for 5 h. The crystalline GeSe2 was obtained by annealing at 500 °C for 5 h. The crystalline structure of Se and GeSe2 were con®rmed by X-ray diraction analysis. The planetary ball mill (Fritsch, Pulversette 7) is used to give the samples mechanical energy. The milling vial with the inner diameter 40 mm and height 40 mm was made of SUS304 alloy. SUS304 balls with a diameter of 3 mm were used. The powder samples were sieved under a particle size less than 74 lm sealed in the vial with the balls under Ar gas atmosphere. The MG was carried out using the revolutions of 300 rpm and the rotations of 600 rpm. X-ray diraction patterns were measured by means of a powder diractometer with Mo-Ka and Cu-Ka radiation. DSC measurements were performed with a heating rate of 10 °C/min in Ar gas atmosphere. In Se after 50 h and GeSe2 after 10 h of grinding, the amount of impurity Fe from the vial and balls was measured to be less than 0.1 at.%.
Fig. 1. X-ray diraction patterns of Se after 0, 10, 20, 30 and 50 h of grinding.
3. Results X-ray diraction showed the presence of trigonal Se at the start. Fig. 1 shows the X-ray diraction patterns of Se after 0, 10, 20, 30 and 50 h of grinding and amorphous Se by rapid quench from 350 °C. The sharp Bragg peaks become broadened with increasing grinding time, and then disappear after 30 h of grinding. It indicates that trigonal Se is amorphized in the present condition. Fig. 2 shows that the DSC curves of Se after 10, 20, 30 and 50 h of grinding and amorphous Se prepared by rapid quench. As shown in Fig. 3(a), both the crystallization temperature Tc and the glass transition temperature Tg increase with the increase of grinding time. Both Tc and Tg of Se by rapid quench are lower than those of Se after 10 h of grinding. Fig. 4(a)
Fig. 2. DSC curves of Se after 10, 20, 30 and 50 h of grinding.
shows that the enthalpy DH for crystallization of mechanically ground Se increases with increasing grinding time, and approaches saturation for a
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781
Fig. 3. Tc and Tg of (a) Se and (b) GeSe2 by the MG and by rapid quench.
certain value more than 30 h of grinding. DH of Se by rapid quench is lower than that by grinding for 30 h. The crystalline GeSe2 was con®rmed monoclinic GeSe2 by X-ray diraction analysis. Fig. 5 shows the X-ray diraction patterns of GeSe2 after 0, 0.5, 4 and 10 h of grinding, and that by rapid quench. The sharp Bragg peaks decrease with increasing the grinding time, and then the amorphous phase is formed all over. Monoclinic GeSe2 is completely amorphized after 4 h of grinding in the present condition. As shown in Fig. 3(b), both Tc and Tg of GeSe2 prepared by grinding decrease with increasing grinding time. Both Tc and Tg of GeSe2 by rapid quench are higher than those by grinding for 4 h. Fig. 4(b) shows that DH of mechanically ground GeSe2 increases with increasing grinding time. The DH tends to approach a certain value by grinding like that of Se.
Fig. 4. DH of (a) Se and (b) GeSe2 by the MG.
4. Discussion Trigonal Se is composed of the helical chains in crystalline state. The chain structure is formed for a period of three Se atoms connected helically, and the forth atom coordinates the equal site of the ®rst atom in the next unit in their periodicity. The ®rst nearest atoms of trigonal Se, which are in the same helical chain molecule, are located at 2.33 A from an original atom. The second nearest atoms, which are in the adjacent chain molecules, are lo and the third nearest atoms, which cated at 3.47 A, are the second nearest atoms belonging to the [3]. same chain molecule, are located at 3.69 A Pair distribution functions g r for amorphous Se after 30 h of grinding and rapid quench from
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Fig. 5. X-ray diraction patterns of GeSe2 after 0, 0.5, 4 and 10 h of grinding.
the liquid state at 350 °C derived from the structure factor S Q obtained by X-ray diraction are shown in Fig. 6. The g r of Se by the MG has the in the present remain peaks at 2.33 and 3.67 A sults. The present results agree with the reported results [3]. In comparison with trigonal Se, the ®rst nearest distance of the amorphous Se is almost the same, but the second nearest distance changes Therefore, it is considered from 3.47 to 3.67 A. that trigonal Se is amorphized with disordering interaction of the helical chains. In comparison with the g r of Se prepared by rapid quench, the ®rst and second peak positions are almost the same. However, the structure of amorphous Se by the MG for 30 h is dierent from that by rapid quench because of DSC results. Amorphous Se by rapid quench from the liquid state at 350 °C has the structure-like liquid Se at 350 °C. There are two forms in liquid Se, chain and ring structures. The former is called transconformation in a helical chain, and the latter is called cis-conformation in an Se-8 ring. The difference from the two forms depends on bond angle between the ®fth and forth atoms. The potential
Fig. 6. Pair distribution functions g r for Se by the MG for 30 h of grinding and by rapid quench from liquid state at 350 °C.
energy of the cis-conformation is a little higher than that of the trans-conformation. It is reported by Misawa that the calculated ratio of the presence of Se-8 ring is approximately 25% in liquid Se at 350 °C [4]. On the other hand, it seems that there are almost trans-conformations in amorphous Se prepared by grinding [3]. The structure of amorphous Se by rapid quench from 350 °C consists of trans- and cis-conformations. There is the dierence between these amorphous Se caused by the existence of the cis-conformation. Hence, DH by rapid quench should be larger than that by grinding, but less as shown in Fig. 4(a). In the process of the MG, strain is accumulated in samples, and then leads to an amorphization in principle. It seems that there exists the strain in various scales, because amorphous Se is composed primarily of SN long chains with N 105 [5], and the crystallization needs thermal energy in order to release the strain. Therefore, it is considered that the dierence of the intermediate-range order more of the pair distribution functions g r in than 6 A Fig. 6 is concerned with this results. A structural unit of monoclinic crystalline GeSe2 consists of 4 Se atoms coordinated a Ge atom. The tetrahedral units are connected to share their Se
Y. Tani et al. / Journal of Non-Crystalline Solids 293±295 (2001) 779±784
corners each other, and the connected tetrahedral units form the chain structure. Each chain of GeSe2 is situated in parallel. The chains are connected by the tetrahedral units sharing the Se±Se bond [6]. Fig. 7 shows the g r for amorphous GeSe2 by the MG for 4 h and by rapid quench from liquid state at 1000 °C. In the present result of amorphous GeSe2 and the by the MG, the Ge±Se bond length is 2.32 A Se±Se is 3.79 A. On the other hand, the Ge±Se bond and the Se±Se is 3.80 A in amorlength is 2.32 A phous GeSe2 by rapid quench. The results show that the Ge±Se and Se±Se bond length of amorphous GeSe2 by the MG for 4 h are almost the same as those of amorphous GeSe2 by rapid quench. It is reported that amorphous GeSe2 by rapid quench consists of the tetrahedral units [7]. Therefore, it shows that monoclinic GeSe2 is mainly amorphized by disordering the coordination of the tetrahedral structure. The g r of GeSe2 by rapid quench is slightly dierent from that by the MG in the region The structural dierence between more than 5 A. GeSe2 by the MG and by rapid quench appears in Fig. 8 showing the structure factors S Q. The peaks of S Q of GeSe2 by rapid quench are sharper than those by the MG for 4 h. It is clari®ed by the molecular dynamics simulation that the ®rst peak
783
Fig. 8. Structure factors S Q of GeSe2 by the MG for 4 h of grinding and by rapid quench from liquid state at 1000 °C.
1 is mainly concerned with Ge±Ge around Q 1 A 1 correlations and the second one around Q 2 A is mainly due to Se±Se correlations [7]. Therefore, these correlations of GeSe2 by the MG have wide distributions of intra- and inter-tetrahedra. This result shows GeSe2 by the MG has wider energy states than that by rapid MG quench. And the tetrahedral unit of GeSe2 by MG is distorted. DH of GeSe2 by the MG is larger than that by rapid quench. It is considered that the dierence of DH is caused by wide distributions of the Ge±Ge and Se± Se correlations of GeSe2 by the MG. GeSe2 is completely amorphized after 4 h of grinding, and Se is done after 30 h. There is a large dierence between the two samples under the same milling condition. The dierence means that disordering of the connected tetrahedral units in monoclinic GeSe2 is induced more easily than that of the bond between helical chains in trigonal Se. 5. Conclusions
Fig. 7. Pair distribution functions g r for GeSe2 by the MG for 4 h of grinding and by rapid quench from liquid state at 1000 °C.
Amorphous Se and GeSe2 have been obtained by the MG method. The amorphization of Se is
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caused from distortion between helical chains connected by van der Waals forces. There is not a large dierence of the bonding length between Ge and Se atoms in the tetrahedral unit of GeSe2 for the amorphization. The amorphization of monoclinic GeSe2 is caused from disordering between the connected tetrahedral units. However, it is considered that they are distorted in amorphous Se and GeSe2 by grinding, and then the distortions cause a rise of the enthalpy for crystallization. GeSe2 is completely amorphized after 4 h of grinding, and Se after 30 h in the present milling. The dierence indicates that disordering between the connected tetrahedral units forming chain structure in monoclinic GeSe2 is induced more easily than that between each helical chain in trigonal Se, because these samples are given the same amount of mechanical energy by the planetary ball mill.
Acknowledgements The authors would like to thank Dr Umesaki for X-ray diraction experiments. This work was supported by Hosokawa Powder Technology Foundation, and a grant to RCAST at Doshisha University from the Ministry of Education, Japan. References [1] R.B. Schwarz, Mater. Sci. Eng. 97 (1988) 71. [2] R.B. Schwarz, C.C. Koch, Appl. Phys. Lett. 49 (1986) 146. [3] T. Fukunaga, M. Utsumi, H. Akatsuka, M. Misawa, U. Mizutani, J. Non-Cryst. Solids 205±207 (1996) 531. [4] M. Misawa, K. Suzuki, J. Phys. Soc. Jpn. 44 (1978) 1612. [5] R. Zallen, The Physics of Amorphous Solids, Wiley, New York, 1998. [6] O. Matsuda, K. Inoue, K. Murase, Solid State Commun. 75 (1990) 303. [7] P. Vashishta, R.K. Kalia, G.A. Antonio, Phys. Rev. Lett. 62 (1989) 1651.