Structural and physical properties of Ag–As–Te glasses

Journal of Non-Crystalline Solids 293±295 (2001) 799±805

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Structural and physical properties of Ag±As±Te glasses Takeshi Usuki a,*, Osamu Uemura a, Shigemoto Konno a, Yasuo Kameda a, Masaki Sakurai b a

b

Faculty of Science, Department of Material and Biological Chemistry, Yamagata University, 1-4-12 Koshirakawa, Yamagata 990-8560, Japan Institute for Materials Research, Tohoku University, Katahira 2-1-1, Aobaku, Sendai 980-8577, Japan

Abstract Di€erential scanning calorimetric (DSC), dc electrical conductivity, X-ray di€raction and EXAFS measurements for …Ag2 Te†x …AsTe†1 x glasses with x ˆ 0 to 0.3 have been carried out to investigate physical properties and the co-ordination environment of constituting atoms in ternary Ag±As±Te glasses. Results of the DSC measurement suggest that the glass structure is more thermally unstable with increasing Ag content. The incorporation of Ag2 Te into AsTe glass is responsible for a pronounced increase in the electrical conductivity and corresponding decrease in the electrical activation energy. Least-squares ®t analyses for the observed X-ray structure functions have been carried out under the assumption that the ®rst co-ordination shell is composed of As±Te, Ag±Te and As±As correlations. The results indicate that the interatomic distances of three atomic correlations are all composition-independent. The co-ordination number of As atoms is determined to be about three and remains practically unchanged. Ag atoms are roughly fourfold coordinated with Te atoms, as suggested by the formal valence shell (FVS) model proposed for the chalcogenide glasses. The co-ordination number of Te atoms increases from 1.93 to 3.60 with increasing Ag content. The thermal stability and electrical properties of the present glasses may be associated with the local structure and bonding nature of Ag±Te bonds. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 61.10.Eq; 61.10.Ht; 61.43.Fs; 72.80.Ng

1. Introduction Ternary chalcogenide glasses containing noble metal elements, such as Ag±As±X (X ˆ S, Se) glasses, have been extensively investigated because of their unique optical, electrical and physicochemical properties [1]. Although many techniques have also been used to study the local structure of these glasses [2±4], several models remain for explaining their physical behaviours, * Corresponding author. Tel.: +81-23 628 4582; fax: +81-23 628 4591. E-mail address: [email protected] (T. Usuki).

and there are some discrepancies in co-ordination environments for the metal atoms. On the other hand, ternary telluride glasses have not received much attention, and especially there has been a lack of information concerning the detailed microscopic structure of telluride glasses. The present authors have therefore carried out di€erential scanning calorimetric (DSC), dc electrical conductivity, X-ray di€raction and EXAFS measurements for …Ag2 Te†x …AsTe†1 x glasses with x ˆ 0 to 0.3, in order to investigate physical properties and co-ordination environment of constituting atoms in ternary Ag±As±Te glasses.

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 9 1 - 8

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T. Usuki et al. / Journal of Non-Crystalline Solids 293±295 (2001) 799±805

2. Experimental procedures

3. Results

…Ag2 Te†x …AsTe†1 x glasses with x ˆ 0, 0.1, 0.2 and 0.3 were prepared by melting required amounts of elemental materials (purity: all 99.99%) sealed in an evacuated quartz ampoule at 950 °C followed by ice±water quenching. Using a DSC (Perkin Elmer DSC-7), the glass transition temperature Tg and crystallisation temperature Tc at a heating rate of 10 K/min were determined, respectively, as the extrapolated onset temperatures of the glass transition and the temperatures of maximum crystallisation rate. The electrical conductivity r was measured by a conventional four-terminals method using a dc voltmeter (HP 34420A) from 300 to 370 K on various disk-shaped samples (13 mm in dia. and 3 mm in thickness), which were annealed in advance at their respective Tg for 5 h. The contacts to the electrodes were through silver paste to minimise the e€ect of electrode polarisation. The reproducibility of the data was good on heating and cooling runs with accuracy of 3%. X-ray di€raction measurements were made using a h±h re¯ection-type goniometer (Rigaku) with the Mo-Ka radiation operated at 50 kV and 35 mA. Scattering intensities were counted four times with a dwell time of 40 s at intervals of 2h ˆ 0:2° over the range of 3° 6 2h 6 150°, corresponding to  1 …Q ˆ 4p sin h=k†. the range of 0:46 6 Q 6 17:1 A The procedures of corrections and normalisation of the intensity data are similar to those described previously [5]. EXAFS measurements around the As K-edge were performed in the BL-10B station at KEK-PF (Japan), with a silicon (3 1 1) channelcut monochromator calibrated by the K-edge of a standard Cu foil. All spectra were taken at room temperature covering the energy range between  1 †. 11 370 and 13 300 eV …kmax ˆ 19:4 A

Typical DSC curves and characteristic temperatures are given in Fig. 1 and Table 1, respectively. A single glass transition is observed for all samples, indicating that they represent homogeneous glasses. Values of Tg and Tc for AsTe glass are in good agreement with those reported by Titus et al. [6]. The present DSC study indicates that Tg of …Ag2 Te†x …AsTe†1 x glasses with x > 0 is practically independent of x, in contrast to the case of Cu± As±Te glasses which exhibit a gradual increase of Tg with increasing Cu content [7]. Double crystallisation peaks are observed in the glasses with x > 0:10. Such composition-independent Tg and several crystallisation peaks have also been reported for Ag±As±Se(Te) glasses [8,9]. The ®rst crystallisation temperature shows a monotonic

Fig. 1. Typical DSC curves for …Ag2 Te†x …AsTe†1

Table 1 The characteristic temperatures obtained by DSC measurements and conduction parameters for …Ag2 Te†x …AsTe†1

x

x

glasses

x

0

0.1

0.2

0.3

Tg …K† Tc …K† DT …K† DE …eV† r0 …X 1 cm 1 †

405(3) 517(3) 84(3) 0.38(2) 30.2(2)

390 503 87 0.28 11.0

390 491 73 0.24 5.4

390 479 58 0.21 7.0

glasses.

T. Usuki et al. / Journal of Non-Crystalline Solids 293±295 (2001) 799±805

decrease from 517 to 479 K with increasing x. In addition, DT ˆ Ti Tg , where Ti is the onset temperature of the ®rst crystallisation, decreases with increasing x, implying that the glass forming ability or thermal stability …DT † of the present glasses decreases with increasing Ag2 Te content. As shown in Fig. 2, the dc conductivity r of the present glasses is represented by a single activation process according to a well-known equation, r ˆ r0 exp… DE=kT †, at all compositions. Values of the electrical activation energy DE and conductivity pre-exponential factor r0 are listed in Table 1. The incorporation of Ag2 Te into AsTe glass is responsible for a pronounced increase in r by about two orders of magnitude with corresponding decreases in both DE and r0 . These changes in electrical transport properties with the incorporation of noble metals into parent glasses have also been observed in other Ag±(Cu±) telluride glasses [7,10]. Structure factors S…Q† obtained by X-ray diffraction measurements for …Ag2 Te†x …AsTe†1 x glasses are shown in Fig. 3. A pre-peak in S…Q† at  1 in AsTe glass, which is related about Q ˆ 1:2 A to the medium-range order (MRO) constructed by As-pyramidal network units, disappears gradually

Fig. 2. Temperature dependences of the dc conductivity for …Ag2 Te†x …AsTe†1 x glasses.

801

with increasing Ag2 Te content. Fig. 4 gives pair distribution functions g…r† obtained from the Fourier transformation of S…Q†. A well-resolved  in AsTe glass. With ®rst peak is found at 2.62 A increasing x, this peak clearly shifts toward a larger r side due to the formation of longer Ag±Te bonds. In addition, the intensity of the second and third peaks in g…r† systematically decreases with increasing x, suggesting that the MRO in the present glasses changes gradually with increasing Ag2 Te content. This may be associated with the disappearance of the pre-peak in S…Q† mentioned above. Fig. 5 shows magnitudes of radial distribution functions jF …R†j obtained by the Fourier transformation of the EXAFS oscillation functions k 3 v…k† around the As K-edge. A functional form of a main peak in jF …R†j does not change in its position and shape with glass composition. The standard curve ®tting analysis was applied to the ®ltered k 3 v…k† spectra. The ®tting procedure in the present study was all identical to that described in our previous paper [11]. The best-®tted curve was obtained assuming that two atomic pair cor-

Fig. 3. Structure factors, S…Q†, for …Ag2 Te†x …AsTe†1

x

glasses.

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T. Usuki et al. / Journal of Non-Crystalline Solids 293±295 (2001) 799±805

Fig. 4. Pair distribution functions, g…r†, for …Ag2 Te†x …AsTe†1 x glasses. Contributions of As±As, As±Te and Ag±Te correlations, estimated from the present least-squares analysis, are represented by grey lines.

Table 2 Bond lengths obtained by the EXAFS measurement for …Ag2 Te†x …AsTe†1 x glasses x

0

0.1

0.2

0.3

 rAs±As …A†  rAs±Te …A†

2.49(2) 2.66(2)

2.50(2) 2.67(2)

2.50(2) 2.67(2)

2.50(2) 2.67(2)

Fig. 5. Magnitudes of radial distribution functions, jF …R†j, obtained by the Fourier transformation of the EXAFS oscillation functions around the As K-edge.

using the observed total S…Q† under the assumption that partial structure factors Si j …Q† are identical at any composition. In a poly component system, S…Q† is written in the Faber±Ziman scheme as X S…Q† ˆ Wi j Si j …Q†; …1† Wi

relations, namely As±As and As±Te, contribute to the ®rst co-ordination shell. Results of the bond lengths are listed in Table 2. As±As and As±Te  bond lengths, rAs±As ˆ 2:50 and rAs±Te ˆ 2:67 A, which are in good agreement with those reported in amorphous [12] and liquid As±Te alloys [13], remain almost constant at all compositions. 4. Discussion We can attempt to obtain quasi-partial structure factors DS…Q† of …Ag2 Te†x …AsTe†1 x glasses

j

ci cj fi fj ˆ P ; 2 … ci fi †

…2†

where ci and fi are the concentration and atomic scattering factor of atom i, respectively. Using S…Q† for AsTe glass (denoted by S 0 …Q†) and the following equation, we can derive the quasi-partial DS x …Q† in which Ag±Te and Te±Te correlations are emphasised because of a cancellation of the As±Te correlation, !   x x WAs±Te WAs±Te x x 0 DS …Q† ˆ S …Q† S …Q† 1 : 0 WAs±Te W0As±Te …3†

T. Usuki et al. / Journal of Non-Crystalline Solids 293±295 (2001) 799±805

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atomic correlations, namely As±As, As±Te and Ag±Te ones. Under this assumption, structural parameters for these correlations can be determined by the least-squares analysis for the total S…Q† obtained by the present X-ray di€raction measurements. The partial structure factor for i±j pair Si±j …Q† can be expressed as   ci ni±j fi fj 1 2 2 l Q : exp Si±j …Q† 1 ˆ …2 dij † P 2 2 i±j … ci fi † sin…ri j Q† ; …4†  ri±j Q where ri±j , li±j and ni±j denote respectively the bond length, root mean square displacement of i±j pair and number of j atoms around a given atom i, and dij ˆ 1 for i ˆ j and 0 for i 6ˆ j, respectively. The least-squares ®tting was performed in the range of  1. QP8 A

Fig. 6. Quasi-partial pair distribution functions, Dgx …r†, obtained by the Fourier transformation of DS x …Q† for …Ag2 Te†x …AsTe†1 x glasses.

Fig. 6 illustrates the corresponding quasi-partial pair distribution functions Dgx …r† calculated by the Fourier transformation of derived DS x …Q†. Obtained Dgx …r† for three di€erent compositions shows an excellent agreement with each other and  has the ®rst and second peaks at 2.85 and 4.5 A, respectively, which are close to the ®rst nearest Ag±Te and Te±Te distances in AgTe4 tetrahedra of high-temperature crystalline a-Ag2 Te [14]. This fact strongly suggests that the present glasses are regarded as pseudo binary homogeneous mixtures of AsTe and Ag2 Te structural units. Moreover, a  beside small hump is clearly found at about 3:3 A the ®rst peak in Dgx …r†. This hump is probably identi®ed with the presence of possible Ag±Ag correlations, because Ag±Ag nearest-neighbour  for highcorrelations have been found at 3.3 A temperature crystalline Ag2 Te [15], although the position is di€erent from the result obtained by the method of isotopic substitution in neutron diffraction for AgAsTe2 glass [16]. Therefore, it is reasonable to assume that the  for ®rst co-ordination shell in the range of r < 3 A …Ag2 Te†x …AsTe†1 x glasses is composed of three

The results are summarised in Table 3, and contributions of As±As, As±Te and Ag±Te correlations to the ®rst peak in g…r†, obtained by the Fourier transformation of Si j …Q†, are represented in Fig. 4. It should be noted that numerical changes of structural parameters for three atomic correlations were less than 2% even though the ®tting calculation was performed by considering the Ag±Ag nearest-neighbour correlation. The results indicate that the bond-lengths of three atomic correlations are all composition-independent, val for As± ues of which are determined to be 2.50 A   As, 2.66 A for As±Te and 2.85 A for Ag±Te, respectively. Values of rAs±Te and rAs±As are in good agreement with those obtained from the present EXAFS study within a ®tting accuracy. In addition, the total co-ordination number of As atoms nAs is about three at all composition having two As±Te and one As±As bonds. These facts allow us to propose that the network matrix of the system is made by covalent As2 Te4=2 pyramidal units. Both rAg±Te and nAg also remain practically unchanged, implying that a signi®cant number of Ag atoms have the tetrahedral co-ordination with Te atoms similar to that in crystalline a-Ag2 Te [14]. The structure of …Ag2 Te†x …AsTe†1 x glasses can be, therefore, described as the pseudo binary mixture of As2 Te4=2 pyramidal network units and percolated AgTe4 tetrahedra, where the incorporation

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T. Usuki et al. / Journal of Non-Crystalline Solids 293±295 (2001) 799±805

Table 3 Structural parameters of As±As, As±Te and Ag±Te correlations in the ®rst co-ordination shell for …Ag2 Te†x …AsTe†1 x

0

0.1

0.2

0.3

 rAs±As …A†  lAs±As …A† nAs±As

2.50(2) 0.118(2) 0.97(5)

2.50(2) 0.116(2) 0.95(6)

2.50(2) 0.120(2) 0.94(6)

2.50(2) 0.118(4) 0.94(5)

 rAs±Te …A†  lAs±Te …A† nAs±Te

2.66(2) 0.134(3) 1.93(3)

2.66(2) 0.132(5) 1.91(3)

2.66(2) 0.132(3) 1.89(4)

2.66(2) 0.124(2) 1.89(4)

 rAg±Te …A†  lAg±Te …A† nAg±Te

± ± ±

2.85(2) 0.158(3) 3.83(5)

2.85(2) 0.162(4) 3.79(6)

2.85(2) 0.158(3) 3.80(6)

nAs nTe nav nav Te

2.90 1.93 2.42 2.0

2.86 2.49 2.78 2.59

2.83 3.03 3.10 3.20

2.83 3.60 3.42 3.80

of Ag2 Te into AsTe glass is not accompanied by any signi®cant change in the local co-ordination environment of both As and Ag atoms, but it may a€ect the MRO of the system. On the other hand, the co-ordination number of Te atoms nTe increases from 1.93 to 3.60 with Ag content. Formal valence shell (FVS) model for chalcogenide glasses developed by Liu and Taylor [17] predicts that when noble metals are alloyed with binary As-chalcogenide glasses, the average coordination of chalcogen atoms can be continuously increased from 2 to 4, where noble metal and As atoms are always fourfold and threefold coordinated, respectively. According to this model, the average co-ordination number of Te atoms in …Ag2 Te†x …AsTe†1 x glasses can be described as: nav cAg †, where cAg is the atomic Te ˆ 2 ‡ 12cAg =…2 fraction of Ag, relating to the mole fraction of x as cAg ˆ 2x=…2 ‡ x†. As seen in Table 3, resultant values of nav Te agree well with nTe obtained by the present least-squares analysis. The upper limit of cAg for the above derivation is determined by nav Te 6 4 [17], which yields cAg 6 2=7 or x 6 1=3. When x is less than 1/3, the FVS model requires that Te atoms can supply enough formal valence electrons to Ag atom and form the Ag±Te bonds with nAg±Te ˆ 4. Therefore, the structural characteristics of …Ag2 Te†x …AsTe†1 x glasses obtained by the present study is completely consistent with the prediction of the FVS model.

x

glasses

The increase in nTe leads to an abrupt increase of the total average co-ordination number of the system nav with Ag content (see Table 3), suggesting that the rigidity of the glassy network, commonly associated with Tg , may also increase. Zotov et al. [18] have pointed out by their neutron di€raction study for Cu±As±Te glasses that there is a linear correlation between nav and Tg , and the thermal stability of the glasses increases with Cu content. However, observed Tg in the present …Ag2 Te†x …AsTe†1 x glasses changes little and the thermal stability …DT † of the glasses decreases with increasing x. The di€erence in their bonding nature between Cu±Te and Ag±Te bonds can probably a€ect the rigidity of the glassy network, that is, in the Cu±As±Te glasses the covalent Cu±Te bonds may interact e€ectively with the parent As±Te network and attempt to form the optimum rigid glassy network with them, whereas, since Ag±Te bonds in the Ag±As±Te glasses are expected to have a more ionic character, the rigidity of the glasses may not increase with increasing Ag content, although both the Cu±As±Te and Ag±As±Te glasses possess qualitatively similar local co-ordination environments. Moreover, the low co-ordination of Te atoms …nTe ˆ 2† in AsTe glass results in non-bonding p-orbitals on Te atoms forming the highest-lying states in the valence band. With increasing Ag content, the density of localised states near the valence band edge may increase due

T. Usuki et al. / Journal of Non-Crystalline Solids 293±295 (2001) 799±805

to the formation of Ag±Te bonds. This is supported by the result that the conductivity pre-exponential factor r0 , which is related to the density of localised states [19], decreases with increasing Ag content in the present glasses. The changes in the density and location of the localised states account for the monotonic decreases in both DE and r0 and are, therefore, responsible for the increase in r. After all, the thermal stability and electrical properties of the present glasses may be strongly associated with the local structure and bonding nature of Ag±Te bonds. 5. Conclusions The present results of DSC, dc electrical conductivity, X-ray di€raction and EXAFS measurements in …Ag2 Te†x …AsTe†1 x glasses with x ˆ 0 to 0.3 can be concluded as follows: 1. The glass structure becomes more thermally unstable (decreasing DT ) with increasing x. 2. The incorporation of Ag2 Te into AsTe glass is responsible for a pronounced increase in the electrical conductivity and corresponding decrease in the electrical activation energy. 3. The structure of the present glasses can be described as the pseudo binary mixture of As2 Te4=2 pyramidal network units and percolated AgTe4 tetrahedra, where the incorporation of Ag2 Te is not accompanied by any signi®cant change in the local co-ordination environment of both As and Ag atoms. 4. The co-ordination number of Te atoms increases with increasing Ag content, as suggested

805

by the FVS model proposed for chalcogenide glasses. References [1] Z.U. Borisova, Glassy Semiconductors, Plenum, New York, 1981. [2] C.J. Benmore, P.S. Salmon, Phys. Rev. Lett. 73 (1994) 264. [3] V. Mastelaro, S. Benazeth, H. Dexpert, J. Non-Cryst. Solids 185 (1995) 274. [4] N. Zotov, F. Bellido, R. Jimenez-Garay, J. Non-Cryst. Solids 209 (1997) 149. [5] Y. Nagata, S. Kokai, O. Uemura, Y. Kameda, J. NonCryst. Solids 169 (1994) 104. [6] S.S.K. Titus, S. Asokan, E.S.R. Gopal, Solid State Commun. 83 (1992) 745. [7] A. Giridhar, S. Mahadevan, J. Non-Cryst. Solids 238 (1998) 225. [8] N. Zotov, C. Wagner, F. Bellido, L.M. Rodriguez, R. Jimenez-Garay, Thermochim. Acta 296 (1997) 23. [9] S. Mahadevan, A. Giridhar, J. Non-Cryst. Solids 197 (1996) 219. [10] S. Mahadevan, A. Giridhar, J. Non-Cryst. Solids 197 (1996) 228. [11] T. Usuki, O. Uemura, S. Iwabuchi, Y. Kameda, M. Sakurai, J. Non-Cryst. Solids 232±234 (1998) 688. [12] Q. Ma, D. Raoux, S. Benazeth, J. Non-Cryst. Solids 150 (1992) 366. [13] S. Hosokawa, K. Tamura, M. Inui, H. Endo, J. Non-Cryst. Solids 156 (1993) 712. [14] D.A. Keen, S. Hull, J. Phys.: Condens. Matter 10 (1998) 8217. [15] M. Kobayashi, K. Ishikawa, F. Tachibana, H. Okazaki, Phys. Rev. B 38 (1988) 3050. [16] J. Liu, P.S. Salmon, Europhys. Lett. 39 (1997) 521. [17] J.Z. Liu, P.C. Taylor, J. Non-Cryst. Solids 114 (1989) 25. [18] N. Zotov, F. Bellido, M. Dominguez, R. Jimenez-Garay, A.C. Hannon, R. Sonntag, J. Phys. Chem. Solids 58 (1997) 1625. [19] N.F. Mott, E.A. Davis, Electronic Properties in NonCrystalline Materials, Clarendon, Oxford, 1971.