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Internal friction behavior of liquid AsxTe1−x mixtures
Journal of Non-Crystalline Solids 353 (2007) 1631–1634 www.elsevier.com/locate/jnoncrysol
Internal friction behavior of liquid AsxTe1x mixtures L.J. Guo *, A.Q. Wu, Z.G. Zhu, W.J. Shan Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Science, Hefei 230031, China Received 30 June 2006; received in revised form 10 January 2007 Available online 28 February 2007
Abstract The internal friction technique was applied to investigate the structure properties of liquid AsxTe1x mixtures. A well-pronounced peak was observed in the curve of internal friction as a function of temperature in the liquid AsxTe1x (x 6 40 at.%), suggesting that an abnormal structural change may take place in this system. The appearance of this internal friction peak during both heating and cooling processes indicates that the abnormal behavior is reversible in the molten As–Te system. Further analysis indicates that the internal friction peak observed in AsxTe1x (x 6 40 at.%) melts may be associated with the decomposition of As2Te3 clusters with the increase in temperature. 2007 Elsevier B.V. All rights reserved. PACS: 61.25.Mv; 62.40.+i; 64.70.Ja Keywords: Liquid alloys and liquid metals; Short-range order
1. Introduction The As chalcogenide glasses, especially the Se and Te based glasses, have been widely used due to their optical properties and fast electrical switching characteristics [1,2]. So it is necessary to take a brief account of the atomic arrangement of their structures in the glassy state. For the glass As40Se60, its structure can be considered to be a modified network of covalently linked AsSe3/2 groupings whose structural unit is an –As–Se–As–Se– spiral chain, the same as the crystalline As40Se60 [3]. Whereas, for the glass As40Te60, its atomic structure is composed of threefold coordinated As atoms and twofold coordinated Te atoms, quite different from its crystalline structure of threefold coordinated Te atoms and the As atoms either in trigonal pyramid coordinated sites or octahedrally coordinated sites [4]. In recent years, considerable attentions have been paid to liquid As chalcogenides because many researches have made the suggestion that ultra-microcrystalline regions or domains of some *
Corresponding author. E-mail address: [email protected] (L.J. Guo).
0022-3093/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2007.01.017
enhanced ordering in the melts may be present in a variety of glasses [5], and some novel phenomena and abnormal physical properties are gained. For liquid As40Te60, an exothermic peak appears at 780 K in the differential scanning calorimetric (DSC) curves under cooling, while the density shows a maximum around the corresponding temperature, indicating that a somewhat structural change occurs [6]. The experimental results from extended X-ray absorption fine structure (EXAFS) studies also suggested that there is a structural transformation above 500 C from the network structure composed of threefold coordinated As atoms and twofold coordinated Te atoms to the chain structure [7]. Moreover, measurements of the electrical conductivity, Hall coefficient and thermoelectric power for liquid As–Te mixtures at different temperatures demonstrate that the liquid mixtures undergo a semiconductor to metal (S–M) transition. The transition temperature moves toward higher temperature with increasing As concentration, which is slightly different from that of networkchain transformation [7,8]. These findings enrich and illuminate the phenomenology of As chalcogenides in the liquid state. Further theoretical and experimental studies may help us explore these structural properties.
L.J. Guo et al. / Journal of Non-Crystalline Solids 353 (2007) 1631–1634
As is well known, the internal friction (IF), tan U (U is the angle by which the strain lags behind the stress), is a structure sensitive physical property and is a powerful technique widely used for studying crystal structures, defects and phase transitions in solids [9]. Moreover, it was often employed to study the structural relaxation in both the glassy state and supercooled liquid state of the metallic glassy ribbons [10–12]. Recently, our groups have adopted the IF technique to investigate the liquid structure of binary alloys, such as Pb–Sn and In–Sn melts, and verified it was suitable for the study of the liquid structure and its transition [13,14]. In the present work, by the IF technique we investigate the temperature-dependent structural properties of As–Te melts. The paper is organized as follows: in Section 2, we describe the experimental method. The results and the corresponding discussions are presented in Section 3 and Section 4, respectively. A short summary is given in Section 5.
0.12
cooling heating
, ,
0.09
tan φ
1632
No Sample 0.06
0.03
0.00 400
450
500
550
600
650
Temperature (οC) Fig. 1. Internal friction tan U of the liquid As40Te60 as a function of temperature during a single cooling-heating run at a given frequency of 4 Hz. The cooling or heating rate is 5 C/min. The lines are guides to the eyes.
2. Experimental details
3. Results Fig. 1 shows the IF (tan U) versus temperature (T) for the As40Te60 melt in a single cooling–heating cycle with a given rate of 5 C/min at the frequency of 4 Hz. In the tan U–T curves a notable peak was observed around 505 C in the cooling process and around 506 C in the subsequently heating process. The temperature region of the IF peak during the cooling is from 434 C to 551 C, while during the heating it is from 422 C to 581 C, which are in good agreement with the differential scanning calorimetric results [6]. In addition, we have also measured the internal friction of As40Te60 melt with four different oscillating frequencies of 0.5, 1, 2, and 4 Hz, as shown in Fig. 2. From this figure,
0.12
, , , ,
0.08
tan φ
All the samples used in this work were prepared from 5N purity As and Te. As and Te were weighted, mixed and sealed in a quartz ampoule (inner diameter 10 mm, thickness 2 mm) under a vacuum of 102 Pa, and then melted in at 850 C for 2 h. The melts of As–Te mixtures were cooled down to room temperature in the furnace. Afterwards, the formed mixtures were finely ground in order to mix homogeneously, and then were re-melted and cooled down under the same conditions as above-mentioned except the holding temperature was 400 C. Finally, the re-melted mixtures were ground again and sealed in the quartz ampoules (inner diameter 1 mm, thickness 0.4 mm) under an evacuated vacuum 102 Pa for IF experimental measurements. The apparatus for the IF measurements is a computer-controlled automatic inverted torsion pendulum with the error of accuracy about 0.001. The experimental sample was initially heated to 600 C, and then its internal friction was measured during cooling and subsequent reheating processes with a given rate of 5 C/min. The maximum measurement amplitude is 1 · 105.
0.5H 1Hz 2Hz 4Hz No Sample
0.04
0.00
400
450
500
550
600
650
Temperature (oC) Fig. 2. Internal friction tan U at four different measurement frequencies of the liquid As40Te60 as a function of temperature under the cooling rate of 5 C/min. The lines are guides to the eyes.
it can be seen that there exists a pronounced peak in each curve, but the peak position slightly shifts to the lower temperature with the decrease of frequency: around 506 C for 4 Hz, 501 C for 2 Hz, 495 C for 1 Hz and 488 C for 0.5 Hz. The peak height decreases with the decrease of frequency. These analyzed features in this melt seemingly indicate that this peak is associated with some relaxation behavior. The observed IF behaviors in the As40Te60 melt can be well repeated in the other AsxTe1x (x < 40 at.%) melts except the As50Te50 melt. For convenience, we present the IF experimental results of liquid AsxTe1x only at the frequency of 0.5 Hz under the cooling run in Fig. 3. From this figure, we can see that a prominent IF peak appears in liquid AsxTe1x (x 6 40 at.%), but disappears in liquid As50Te50. In addition, it should be pointed out that the IF peak height drops dramatically from liquid As40Te60
L.J. Guo et al. / Journal of Non-Crystalline Solids 353 (2007) 1631–1634 0.12
As10 Te90 As20 Te80
0.09
tan φ
As30 Te70 As40 Te60 0.06
As50 Te50
0.03
0.00 350
400
450
500
550
600
650
Temperature (oC) Fig. 3. Internal friction tan U of As–Te melts with different compositions as a function of temperature (the cooling rate is 5 C/min and the measurement frequency is 0.5 Hz). The lines are guides to the eyes.
to As30Te70. Moreover, the peak position evidently changes with arsenic contents: the IF peaks of liquid As30Te70, As20Te80 and As10Te90 are located around 451 C, 435 C and 416 C, respectively, which are slightly higher than the corresponding S–M transition temperatures: 450 C, 410 C and 380 C, but are lower than the network-chain transformation temperatures 500–550 C [7]. 4. Discussion Recently, our groups [13] have explored the structural behaviors of the Pb–Sn melts by means of the IF technique. The observed IF peaks in Pb–Sn melts show the same typical features as those related to the structural phase transition in solids, namely, the peak position does not change with frequency and the magnitude of the peaks drops with increasing frequency. However, similar behaviors do not appear in the As40Te60 melt, as shown in Fig. 2. Surprisingly, the features of the IF peak in the As–Te melts resemble quite well those of the thermally activated relaxation process in solids [15]. In order to test whether the present observed IF peak in liquid As–Te is associated with the thermally relaxation process or not, we analyze the relation between the peak temperature and the measurement frequency. For a thermally activated relaxation process, the relaxation time s generally follows the Arrhenius law: [15] s ¼ s0 exp ðQ=k B T Þ; where s0 is the pre-exponential factor, Q denotes the activation energy of the relaxation process, kB is the Boltzmann constant, T is the absolute temperature. It is well known that at the peak the condition xTP = 1 is fulfilled, where x = 2pf is the angular frequency, TP is the temperature at the peak. If a series of peaks are obtained at a number of different frequencies, a plot of lnx versus 1/TP can be displayed. A linear relation would be obtained according to the Arrhenius law, which yields the activation energy
1633
Q and relaxation time s0. Using the data shown in Fig. 2 we obtain the activation energy of 5.76 eV and the preexponential factor of 3.2 · 1039 s. The pre-exponential factor is greatly higher that of normal liquids, suggesting that the IF behaviors presented in the As–Te melts may not originate from a simple structural relaxation process. On the other hand, previous investigators found a network-chain transformation accompanied by the S–M transition in liquid As–Te mixtures by differential scanning calorimetric, density, EXAFS, conductivity and Hall coefficient measurements [6,7] when temperature is varied. Compared to these results, it can be found that the IF peak position corresponds roughly to that of conductivity and Hall coefficient and its temperature range is in good agreement with that of DSC and density. Therefore, the IF behaviors presented in the As–Te melts might be related to the S–M transition. Oberafo have performed experimental investigation of electrical conductivity and thermoelectric power of As–Te molten system [8]. They found that the As–Te molten system exhibits two conductivity regimes. One is the low-temperature constant activation energy regime, another is the high-temperature variable and decreasing activation energy regime. They also suggested that As2Te3 clusters and some Te clusters in As–Te melts are responsible for the electrical properties in the low temperatures, while metallic Te and As atoms account for the electrical behaviors in the high temperatures. The number of As2Te3 clusters varies with As content and reaches the maximum in As40Te60. These clusters will break up as the temperature raise to the S– M transition temperature, so that the electrical properties change from the behavior controlled by As2Te3 clusters to the behavior controlled the mixture of Te and As atoms. As for liquid As50Te50, Hodgkinson [16] considered that As2Te3 clusters are more stable in the presence of arsenic than in the presence of tellurium. As a result, no decomposition of the clusters takes place within the experimental temperature range and thus no obvious transition occurs in As50Te50 melt. Based on the conclusions by Oberafo and Hodgkinson, we consider that the microscopic structures in the As–Te system experience such process. When temperature is elevated, the As2Te3 clusters are firstly broken, and at the same time, the atomic bond builds up and forms new local network clusters. With temperature continuously increased, the kinetic energy of the atoms is high enough to readjust the atomic configuration and new chainlike clusters are then constructed. And that this process is reversible, which is suggested by our experiments and differential scanning calorimetric studies [6]. 5. Conclusion In summary, in the curve of internal friction versus temperature we have observed a well-pronounced IF peak in the AsxTe1x (x 6 40 at.%) liquids but have not probed in liquid As50Te50. The peak position shifts to lower temperature and the peak height decreases when As contents
1634
L.J. Guo et al. / Journal of Non-Crystalline Solids 353 (2007) 1631–1634
is decreased. At a fixed composition, the peak position moves toward higher temperature and the peak height decreases with increasing frequency. The IF peaks observed in AsxTe1x (x 6 40 atomic percent) melts may be associated with the decomposition of As2Te3 clusters with increasing temperature. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant Nos. 10374089 and 10674135) and the Knowledge Innovation Project of Chinese Academy of Science (Grant No KJCX2-SW-W17). References [1] S.R. Ovshinsky, Phys. Rev. Lett. 21 (1968) 1450. [2] N.F. Mott, E.A. Davis, Electronic Processes in Non-Crystalline Materials, Clarendon, Oxford, 1979.
[3] A.L. Renninger, B.L. Averbach, Acta Crystaalogr. B 29 (1973) 1583. [4] Q. Ma, D. Raoux, S. Benezath, Phys. Rev. B 48 (1993) 16332. [5] A.R. Ubbelohde, The Molten State of Matter: Melting and Crystal Structure, Wiley, New York, 1978. [6] Y.S. Tver’yanovich, V.M. Ushakov, A. Tverjanovich, J. Non-Cryst. Solids 197 (1996) 235. [7] H. Endo, H. Hoshino, H. Ikemoto, T. Miyanaga, J. Phys.: Condens. Mat. 12 (2000) 6077. [8] A.A. Oberfo, J. Phys. C: Solid State Phys. 8 (1975) 469. [9] A.S. Nowick, B.S. Berry, Anelastic Relaxation in Crystalline Solids, Academic, New York, 1972. [10] C.M. Mo, J.P. Shui, Y.Z. He, J. Phys. Colloq. 42 (C5) (1981) 523. [11] H.R. Sinning, J. Non-Crystal. Solids 110 (1989) 195. [12] B. Cai, L.Y. Shang, P. Cui, Phys. Rev. B 70 (2004) 184208. [13] F.Q. Zu, Z.G. Zhu, L.J. Guo, B. Zhang, J.P. Shui, C.S. Liu, Phys. Rev. B 64 (2001) 180203. [14] F.Q. Zu, Z.G. Zhu, L.J. Guo, X.B. Qin, H. Yang, W.J. Shan, Phys. Rev. Lett. 89 (2002) 125505. [15] A.S. Nowick, B.S. Berry, Anelastic Relaxation in Crystalline Solids, Academic, New York, 1972. [16] R.J. Hodgkinson, J. Phys. C: Solid State Phys. 9 (1976) 1467.
Internal friction behavior of liquid AsxTe1x mixtures L.J. Guo *, A.Q. Wu, Z.G. Zhu, W.J. Shan Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Science, Hefei 230031, China Received 30 June 2006; received in revised form 10 January 2007 Available online 28 February 2007
Abstract The internal friction technique was applied to investigate the structure properties of liquid AsxTe1x mixtures. A well-pronounced peak was observed in the curve of internal friction as a function of temperature in the liquid AsxTe1x (x 6 40 at.%), suggesting that an abnormal structural change may take place in this system. The appearance of this internal friction peak during both heating and cooling processes indicates that the abnormal behavior is reversible in the molten As–Te system. Further analysis indicates that the internal friction peak observed in AsxTe1x (x 6 40 at.%) melts may be associated with the decomposition of As2Te3 clusters with the increase in temperature. 2007 Elsevier B.V. All rights reserved. PACS: 61.25.Mv; 62.40.+i; 64.70.Ja Keywords: Liquid alloys and liquid metals; Short-range order
1. Introduction The As chalcogenide glasses, especially the Se and Te based glasses, have been widely used due to their optical properties and fast electrical switching characteristics [1,2]. So it is necessary to take a brief account of the atomic arrangement of their structures in the glassy state. For the glass As40Se60, its structure can be considered to be a modified network of covalently linked AsSe3/2 groupings whose structural unit is an –As–Se–As–Se– spiral chain, the same as the crystalline As40Se60 [3]. Whereas, for the glass As40Te60, its atomic structure is composed of threefold coordinated As atoms and twofold coordinated Te atoms, quite different from its crystalline structure of threefold coordinated Te atoms and the As atoms either in trigonal pyramid coordinated sites or octahedrally coordinated sites [4]. In recent years, considerable attentions have been paid to liquid As chalcogenides because many researches have made the suggestion that ultra-microcrystalline regions or domains of some *
Corresponding author. E-mail address: [email protected] (L.J. Guo).
0022-3093/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2007.01.017
enhanced ordering in the melts may be present in a variety of glasses [5], and some novel phenomena and abnormal physical properties are gained. For liquid As40Te60, an exothermic peak appears at 780 K in the differential scanning calorimetric (DSC) curves under cooling, while the density shows a maximum around the corresponding temperature, indicating that a somewhat structural change occurs [6]. The experimental results from extended X-ray absorption fine structure (EXAFS) studies also suggested that there is a structural transformation above 500 C from the network structure composed of threefold coordinated As atoms and twofold coordinated Te atoms to the chain structure [7]. Moreover, measurements of the electrical conductivity, Hall coefficient and thermoelectric power for liquid As–Te mixtures at different temperatures demonstrate that the liquid mixtures undergo a semiconductor to metal (S–M) transition. The transition temperature moves toward higher temperature with increasing As concentration, which is slightly different from that of networkchain transformation [7,8]. These findings enrich and illuminate the phenomenology of As chalcogenides in the liquid state. Further theoretical and experimental studies may help us explore these structural properties.
L.J. Guo et al. / Journal of Non-Crystalline Solids 353 (2007) 1631–1634
As is well known, the internal friction (IF), tan U (U is the angle by which the strain lags behind the stress), is a structure sensitive physical property and is a powerful technique widely used for studying crystal structures, defects and phase transitions in solids [9]. Moreover, it was often employed to study the structural relaxation in both the glassy state and supercooled liquid state of the metallic glassy ribbons [10–12]. Recently, our groups have adopted the IF technique to investigate the liquid structure of binary alloys, such as Pb–Sn and In–Sn melts, and verified it was suitable for the study of the liquid structure and its transition [13,14]. In the present work, by the IF technique we investigate the temperature-dependent structural properties of As–Te melts. The paper is organized as follows: in Section 2, we describe the experimental method. The results and the corresponding discussions are presented in Section 3 and Section 4, respectively. A short summary is given in Section 5.
0.12
cooling heating
, ,
0.09
tan φ
1632
No Sample 0.06
0.03
0.00 400
450
500
550
600
650
Temperature (οC) Fig. 1. Internal friction tan U of the liquid As40Te60 as a function of temperature during a single cooling-heating run at a given frequency of 4 Hz. The cooling or heating rate is 5 C/min. The lines are guides to the eyes.
2. Experimental details
3. Results Fig. 1 shows the IF (tan U) versus temperature (T) for the As40Te60 melt in a single cooling–heating cycle with a given rate of 5 C/min at the frequency of 4 Hz. In the tan U–T curves a notable peak was observed around 505 C in the cooling process and around 506 C in the subsequently heating process. The temperature region of the IF peak during the cooling is from 434 C to 551 C, while during the heating it is from 422 C to 581 C, which are in good agreement with the differential scanning calorimetric results [6]. In addition, we have also measured the internal friction of As40Te60 melt with four different oscillating frequencies of 0.5, 1, 2, and 4 Hz, as shown in Fig. 2. From this figure,
0.12
, , , ,
0.08
tan φ
All the samples used in this work were prepared from 5N purity As and Te. As and Te were weighted, mixed and sealed in a quartz ampoule (inner diameter 10 mm, thickness 2 mm) under a vacuum of 102 Pa, and then melted in at 850 C for 2 h. The melts of As–Te mixtures were cooled down to room temperature in the furnace. Afterwards, the formed mixtures were finely ground in order to mix homogeneously, and then were re-melted and cooled down under the same conditions as above-mentioned except the holding temperature was 400 C. Finally, the re-melted mixtures were ground again and sealed in the quartz ampoules (inner diameter 1 mm, thickness 0.4 mm) under an evacuated vacuum 102 Pa for IF experimental measurements. The apparatus for the IF measurements is a computer-controlled automatic inverted torsion pendulum with the error of accuracy about 0.001. The experimental sample was initially heated to 600 C, and then its internal friction was measured during cooling and subsequent reheating processes with a given rate of 5 C/min. The maximum measurement amplitude is 1 · 105.
0.5H 1Hz 2Hz 4Hz No Sample
0.04
0.00
400
450
500
550
600
650
Temperature (oC) Fig. 2. Internal friction tan U at four different measurement frequencies of the liquid As40Te60 as a function of temperature under the cooling rate of 5 C/min. The lines are guides to the eyes.
it can be seen that there exists a pronounced peak in each curve, but the peak position slightly shifts to the lower temperature with the decrease of frequency: around 506 C for 4 Hz, 501 C for 2 Hz, 495 C for 1 Hz and 488 C for 0.5 Hz. The peak height decreases with the decrease of frequency. These analyzed features in this melt seemingly indicate that this peak is associated with some relaxation behavior. The observed IF behaviors in the As40Te60 melt can be well repeated in the other AsxTe1x (x < 40 at.%) melts except the As50Te50 melt. For convenience, we present the IF experimental results of liquid AsxTe1x only at the frequency of 0.5 Hz under the cooling run in Fig. 3. From this figure, we can see that a prominent IF peak appears in liquid AsxTe1x (x 6 40 at.%), but disappears in liquid As50Te50. In addition, it should be pointed out that the IF peak height drops dramatically from liquid As40Te60
L.J. Guo et al. / Journal of Non-Crystalline Solids 353 (2007) 1631–1634 0.12
As10 Te90 As20 Te80
0.09
tan φ
As30 Te70 As40 Te60 0.06
As50 Te50
0.03
0.00 350
400
450
500
550
600
650
Temperature (oC) Fig. 3. Internal friction tan U of As–Te melts with different compositions as a function of temperature (the cooling rate is 5 C/min and the measurement frequency is 0.5 Hz). The lines are guides to the eyes.
to As30Te70. Moreover, the peak position evidently changes with arsenic contents: the IF peaks of liquid As30Te70, As20Te80 and As10Te90 are located around 451 C, 435 C and 416 C, respectively, which are slightly higher than the corresponding S–M transition temperatures: 450 C, 410 C and 380 C, but are lower than the network-chain transformation temperatures 500–550 C [7]. 4. Discussion Recently, our groups [13] have explored the structural behaviors of the Pb–Sn melts by means of the IF technique. The observed IF peaks in Pb–Sn melts show the same typical features as those related to the structural phase transition in solids, namely, the peak position does not change with frequency and the magnitude of the peaks drops with increasing frequency. However, similar behaviors do not appear in the As40Te60 melt, as shown in Fig. 2. Surprisingly, the features of the IF peak in the As–Te melts resemble quite well those of the thermally activated relaxation process in solids [15]. In order to test whether the present observed IF peak in liquid As–Te is associated with the thermally relaxation process or not, we analyze the relation between the peak temperature and the measurement frequency. For a thermally activated relaxation process, the relaxation time s generally follows the Arrhenius law: [15] s ¼ s0 exp ðQ=k B T Þ; where s0 is the pre-exponential factor, Q denotes the activation energy of the relaxation process, kB is the Boltzmann constant, T is the absolute temperature. It is well known that at the peak the condition xTP = 1 is fulfilled, where x = 2pf is the angular frequency, TP is the temperature at the peak. If a series of peaks are obtained at a number of different frequencies, a plot of lnx versus 1/TP can be displayed. A linear relation would be obtained according to the Arrhenius law, which yields the activation energy
1633
Q and relaxation time s0. Using the data shown in Fig. 2 we obtain the activation energy of 5.76 eV and the preexponential factor of 3.2 · 1039 s. The pre-exponential factor is greatly higher that of normal liquids, suggesting that the IF behaviors presented in the As–Te melts may not originate from a simple structural relaxation process. On the other hand, previous investigators found a network-chain transformation accompanied by the S–M transition in liquid As–Te mixtures by differential scanning calorimetric, density, EXAFS, conductivity and Hall coefficient measurements [6,7] when temperature is varied. Compared to these results, it can be found that the IF peak position corresponds roughly to that of conductivity and Hall coefficient and its temperature range is in good agreement with that of DSC and density. Therefore, the IF behaviors presented in the As–Te melts might be related to the S–M transition. Oberafo have performed experimental investigation of electrical conductivity and thermoelectric power of As–Te molten system [8]. They found that the As–Te molten system exhibits two conductivity regimes. One is the low-temperature constant activation energy regime, another is the high-temperature variable and decreasing activation energy regime. They also suggested that As2Te3 clusters and some Te clusters in As–Te melts are responsible for the electrical properties in the low temperatures, while metallic Te and As atoms account for the electrical behaviors in the high temperatures. The number of As2Te3 clusters varies with As content and reaches the maximum in As40Te60. These clusters will break up as the temperature raise to the S– M transition temperature, so that the electrical properties change from the behavior controlled by As2Te3 clusters to the behavior controlled the mixture of Te and As atoms. As for liquid As50Te50, Hodgkinson [16] considered that As2Te3 clusters are more stable in the presence of arsenic than in the presence of tellurium. As a result, no decomposition of the clusters takes place within the experimental temperature range and thus no obvious transition occurs in As50Te50 melt. Based on the conclusions by Oberafo and Hodgkinson, we consider that the microscopic structures in the As–Te system experience such process. When temperature is elevated, the As2Te3 clusters are firstly broken, and at the same time, the atomic bond builds up and forms new local network clusters. With temperature continuously increased, the kinetic energy of the atoms is high enough to readjust the atomic configuration and new chainlike clusters are then constructed. And that this process is reversible, which is suggested by our experiments and differential scanning calorimetric studies [6]. 5. Conclusion In summary, in the curve of internal friction versus temperature we have observed a well-pronounced IF peak in the AsxTe1x (x 6 40 at.%) liquids but have not probed in liquid As50Te50. The peak position shifts to lower temperature and the peak height decreases when As contents
1634
L.J. Guo et al. / Journal of Non-Crystalline Solids 353 (2007) 1631–1634
is decreased. At a fixed composition, the peak position moves toward higher temperature and the peak height decreases with increasing frequency. The IF peaks observed in AsxTe1x (x 6 40 atomic percent) melts may be associated with the decomposition of As2Te3 clusters with increasing temperature. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant Nos. 10374089 and 10674135) and the Knowledge Innovation Project of Chinese Academy of Science (Grant No KJCX2-SW-W17). References [1] S.R. Ovshinsky, Phys. Rev. Lett. 21 (1968) 1450. [2] N.F. Mott, E.A. Davis, Electronic Processes in Non-Crystalline Materials, Clarendon, Oxford, 1979.
[3] A.L. Renninger, B.L. Averbach, Acta Crystaalogr. B 29 (1973) 1583. [4] Q. Ma, D. Raoux, S. Benezath, Phys. Rev. B 48 (1993) 16332. [5] A.R. Ubbelohde, The Molten State of Matter: Melting and Crystal Structure, Wiley, New York, 1978. [6] Y.S. Tver’yanovich, V.M. Ushakov, A. Tverjanovich, J. Non-Cryst. Solids 197 (1996) 235. [7] H. Endo, H. Hoshino, H. Ikemoto, T. Miyanaga, J. Phys.: Condens. Mat. 12 (2000) 6077. [8] A.A. Oberfo, J. Phys. C: Solid State Phys. 8 (1975) 469. [9] A.S. Nowick, B.S. Berry, Anelastic Relaxation in Crystalline Solids, Academic, New York, 1972. [10] C.M. Mo, J.P. Shui, Y.Z. He, J. Phys. Colloq. 42 (C5) (1981) 523. [11] H.R. Sinning, J. Non-Crystal. Solids 110 (1989) 195. [12] B. Cai, L.Y. Shang, P. Cui, Phys. Rev. B 70 (2004) 184208. [13] F.Q. Zu, Z.G. Zhu, L.J. Guo, B. Zhang, J.P. Shui, C.S. Liu, Phys. Rev. B 64 (2001) 180203. [14] F.Q. Zu, Z.G. Zhu, L.J. Guo, X.B. Qin, H. Yang, W.J. Shan, Phys. Rev. Lett. 89 (2002) 125505. [15] A.S. Nowick, B.S. Berry, Anelastic Relaxation in Crystalline Solids, Academic, New York, 1972. [16] R.J. Hodgkinson, J. Phys. C: Solid State Phys. 9 (1976) 1467.