High-temperature structure of K2O–TeO2 glasses

Journal of Non-Crystalline Solids 256&257 (1999) 111±118

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High-temperature structure of K2O±TeO2 glasses R. Akagi a, K. Handa b, N. Ohtori c, A.C. Hannon d, M. Tatsumisago e, N. Umesaki f,* b

a Graduate School of Science and Technology, Kobe University, 1-1, Rokkodai-cho, Nada-ku, Kobe 657-8501, Japan Department of Photonics, Faculty of Science and Engineering, Ritsumeikan University, Kusatsu, Siga 525-8577, Japan c Graduate School of Science and Technology, Niigata University, Igarashi 2-no cho, Niigata 950-2102, Japan d ISIS, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, UK e Department of Applied Material Science, Osaka Prefecture University, Sakai, Osaka 599-8231, Japan f Department of Optical Materials, Osaka National Research Institute (ONRI), AIST, Ikeda, 1-8-31 Midoriga-oka, Osaka 563-8577, Japan

Abstract The structural change of K2 O±TeO2 glasses (xK2 O:TeO4 , x:5, 10, 15, 20, 25 and 30 mol%) from room temperature to a temperature higher than the melting point (Tm ) was studied by the use of Raman spectroscopy, XRD (X-ray diffraction), X-ray RDF (radial distribution function) and XAFS (X-ray absorption ®ne structure) spectroscopy. The Raman results indicated that TeO4 trigonal bipyramid (tbp) units convert to TeO3 trigonal pyramid (tp) units with increasing temperature and by the addition of K2 O to the K2 O±TeO2 glasses. We found that a 10K2 O á 90TeO2 (mol%) glass mainly consists of TeO4 tbp unit, while 20K2 O á 80TeO2 (mol%) and 30K2 O á 70TeO2 (mol%) glasses are composed of the mixture of TeO4 tbp and TeO3 tp units. The high-temperature XRD and X-ray RDF results are in agreement with the results of high-temperature Raman spectroscopy. Furthermore, the XAFS spectroscopy shows that potassium ions have a similar local structure in all of the K2 O±TeO2 glasses studied. Ó 1999 Elsevier Science B.V. All rights reserved.

1. Introduction The local structure of tellurite glasses has attracted interest in connection with their optical and electrical properties. It is well known that binary tellurite easily forms glass compared with pure TeO2 [1,2]. Incorporation of a second component to TeO2 glass is expected to extend the Te± O interatomic distance, which should increase the mobility of the polyhedra and thereby provide a

* Corresponding author. Tel.: +81-727 51 9536; fax: +81-727 51 9631; e-mail: [email protected]

favorable condition for tellurite vitri®cation [3]. The relationship between glass structure and added alkali metal ion concentration has been studied by means of Raman spectroscopy [4]. However, the exact form of the Te±O structural units in potassium tellurite glasses has not yet been established. Therefore, we have studied the structure of K2 O±TeO2 glasses by using Raman spectroscopy, X-ray radial distribution function (RDF) analysis and X-ray absorption ®ne structure (XAFS) measurement. Furthermore, we have extended our study to the structure of the K2 O±TeO2 glasses and their crystallization at high-temperature by

0022-3093/99/$ - see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 3 9 2 - 0

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use of high-temperature Raman spectroscopy and a high-temperature XRD method. 2. Experimental procedures For the preparation of K2 O±TeO2 glasses, commercial powders of reagent-grade K2 CO3 and TeO2 were used as starting materials. The xK2 O á TeO2 (x ˆ 5, 10, 15, 20, 25 and 30 mol%) glasses were prepared using a roller-quenching technique by a twin-roller apparatus with a thermal-image furnace [5,6]. Mixtures of these materials of appropriate composition were melted at 973±1173 K for 1 h in a platinum crucible. The resultant liquid was poured into rotating twin rollers (3000 rpm) to yield thin ¯akes. The quenching rate was estimated to be between 105 and 106 K/s. Glass transition temperature (Tg ), crystallization temperature (Tc ) and melting temperature (Tm ) of the K2 O±TeO2 glasses were determined by di€erential thermal analysis (DTA). DTA was performed with a thermal analyzer Rigaku (TAS-200) in air at a rate (both heating and cooling) of 10 K/min. Raman spectra were obtained with equipment combining a Raman triple-grating spectrometer (Jobin Yvon T64000) with charge coupled device (CCD) detector and Linkam heating stage for the microscope. Parallel polarized Raman spectra were observed in this study. The Raman scattering for the glass samples was excited with the 488 nm line of an Ar‡ laser, using a power of 500 mW. The scattered radiation was collected in 180° geometry using a long working distance with an objective lens (45´) in a microscope. The sample was placed in a Pt-wound furnace in a microscope heating stage with a silica glass cover, and the temperature was measured with a Pt±PtRh thermocouple. The X-ray di€raction experiments for RDF analysis were carried out using a X-ray di€ractometer (Rigaku) having h/h type re¯ection geometry and a Ge solid-state detector (Canberra GL0210R).  radiation was used with a Mo Ka (k ˆ 0:7107 A) continuous output of 60 kV and 300 mA. Measurements were made using a step-scanning technique, with a ®xed time of 30 s at every Dh ˆ 0:25 in the range 5±20° and 180 s in the range of 15±60°,

ÿ1 to 15.31 A ÿ1 in covering the range from 0.772 A scattering vector, Q ˆ 4p sin h=k. Details of the Xray measurement and data treatment for RDF analysis are given in a previous paper [7]. The reduced intensity function, Q
i…Q†, and radial distribution function, G…r†, were obtained as follows: Q
i…Q† " ˆQ

coh …Q† Ieu

X ÿ fi …Q†2 i

#,

X

!2 fi …Q†

i

…1† and  G…r† ˆ 1 ‡

1 2p2 q0



QZmax



Q
i…Q† sin …Qr†dQ: 0

…2† coh …Q†, fi …Q†, q0 are intensity in electron Here, Ieu units scattering, atomic scattering factor, average number density, respectively. Furthermore, XRD measurements were carried out to determine whether any crystals were included in the glasses and to identify the crystalline phases formed by the heating and cooling of the glasses or melts. XAFS measurements were carried out at the BL4 beamline in Synchrotron Radiation Center of Ritsumeikan University. A photon ¯ux of 108 photons/s was achieved for ring operating conditions of 575 MeV and ring current of 300 mA. Te L3 -edge (4.342 keV) and K K-edge (3.608 keV) XAFS spectra were measured by the transmission method under a helium atmosphere by using  double monochromator Si(2 2 0) [d ˆ 1.92016 A] crystals and two ionization chambers ®lled with N2 gas. The energy resolution for the double crystal monochromator was about 0.23 at 4000 eV. The EXAFS interference function k 3 v…k† was extracted from the absorption spectrum using standard procedures, which have been described in a previous paper [8]. Theoretical phases and amplitude functions of Rehr et al. [9] were employed for the k 3 v…k † ®tting procedure in order to obtain the short range order parameters such as mean distance Riÿj , coordination number Niÿj and root 1=2 2 i for K2 O± mean square displacement hDriÿj TeO2 glasses.

R. Akagi et al. / Journal of Non-Crystalline Solids 256&257 (1999) 111±118

3. Results Fig. 1 shows Raman spectra of the xK2 O á TeO2 (x ˆ 5, 10, 15, 20, 25 and 30 mol%) samples. The Raman spectra obtained for these

Fig. 1. Raman spectra of the xK2 O á TeO2 (x ˆ 5, 10, 15, 20, 25 and 30 mol%) glasses.

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samples had three Raman bands at around 770, 670 and 460 cmÿ1 at room temperature. These bands are ascribed to the stretching mode of the TeO3 trigonal pyramid (tp) units containing terminal Te±O bonds such as Te@O and Te±Oÿ with non-bridging oxygen atoms (NBO), the stretching mode of TeO4 trigonal bipyramid (tbp) units with bridging oxygen atoms (BO) and the bending mode of Te±O±Te or O±Te±O linkages, respectively [9,10]. The amplitude of the 770 cmÿ1 band becomes larger, and the amplitudes of the 670 and 470 cmÿ1 bands become smaller as the K2 O content increases. We suggest that these results show the addition of K2 O, brings about the conversion of the TeO4 tbp units with BO into the TeO3 tp units with NBO in the xK2 O á TeO2 samples. Table 1 indicates the structural parameters for the K±O and Te±O correlations in the xK2 O á TeO2 (x ˆ 10, 20 and 30 mol%) samples which were obtained from the EXAFS and X-ray RDF analyses. In the 20K2 O á 80TeO2 sample, the coordination number Niÿj of the Te±O correlation

Table 1  coordination number Niÿj (atoms) and root mean square disStructure parameters of short range order, mean distance Riÿj (A), 2  for the xK2 O á TeO2 (x ˆ 10, 20 and 30 mol%) glasses and melts i1=2 (A) placement hDriÿj 2   Riÿj /A hDriÿj i1=2 /A Method Sample iÿj Niÿj (atoms) 10K2 O á 90TeO2 glass

K±O Te±O Te±O Te±O Te±O

5.9 3.7 1.41 1.92 0.95

2.71 1.89 1.80 2.04 2.38

0.077 0.044 0.073 0.091 0.115

EXAFS EXAFS X-ray RDF X-ray RDF X-ray RDF

10K2 O á 90TeO2 melt

Te±O Te±O Te±O

1.58 2.01 0.96

1.78 2.02 2.40

0.063 0.084 0.118

X-ray RDF X-ray RDF X-ray RDF

20K2 O á 80TeO2 glass

K±O Te±O Te±O Te±O Te±O

6.0 3.7 1.32 1.84 1.05

2.71 1.89 1.78 2.04 2.41

0.084 0.046 0.111 0.115 0.128

EXAFS EXAFS X-ray RDF X-ray RDF X-ray RDF

20K2 O á 80TeO2 melt

Te±O Te±O Te±O

1.44 1.85 1.06

1.75 2.01 2.42

0.110 0.111 0.129

X-ray RDF X-ray RDF X-ray RDF

30K2 O á 70TeO2 glass

K±O Te±O

6.3 3.4

2.74 1.89

0.086 0.050

EXAFS EXAFS

 Niÿj  0:2  0:3 (atoms); hDr2 i1=2  0:01 (A).  Riÿj  0:01  0:02 (A); iÿj

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obtained from the EXAFS results is 3.7. This Niÿj corresponds to the middle of the TeO3 tp unit …Niÿj ˆ 3† and the TeO4 tbp unit …Niÿj ˆ 4†. On the other hand, the Niÿj s of the Te±O bond at 1.78,  determined from Debye's equa2.04 and 2.41 A tion ®tting [11] are 1.3, 1.8 and 1.1, respectively. The minimum bond length of these is assumed to be the Te±NBO bond distance [12]. The di€erence of Te±O correlations obtained from the X-ray RDF results and those from the EXAFS results arises from the fact that the former shows the distance of each Te±O bond, while the latter shows the average distance of various Te±O bonds. In addition, the pulsed neutron di€raction carried out to obtain Te±O correlations in the K2 O±TeO2 glasses [13] gave the result that the well-de®ned r component of the Te±O peak in the neutron RDF become smaller r and its width decreased with the addition of K2 O. This result corresponds to the Xray RDF result. The details of the K±O correlation in the xK2 O á TeO2 samples were studied by the XAFS measurement. The structural parameters for the K±O correlation in the xK2 O á TeO2 samples have been obtained from a best-®t routine, as summarized in Table 1. The EXAFS results indicate that the K±O distances are almost equal to the sum of  and that the ionic radii K‡ and O2ÿ ( ˆ 2.73 A), total coordination numbers of potassium surrounded with oxygen atoms are six. Experimental k 3 v…k† curves of the xK2 O á TeO2 samples are compared with calculated k 3 v…k† curves of K2 TeO3 [14] and K2 Te4 O12 [15] crystals in Fig. 2. These k 3 v…k† spectra from the experimental data resemble more the k 3 v…k† spectrum calculated using the ab-initio XAFS computer code, FEFF [9] of K2 TeO3 crystal [14] than that of K2 Te4 O12 crystal [15]. We assume that the local structure of potassium in the xK2 O á TeO2 (x ˆ 5, 10, 15, 20, 25 and 30 mol%) glasses is similar to that in the K2 TeO3 crystal in which the K atoms are octahedrally coordinated to six oxygen atoms with  [14]. distances ranging from 2.728 to 3.050 A Fig. 3(a) shows the Raman spectra for the 20K2 O á 80TeO2 glass during heating and Fig. 3(b) during cooling. The amplitude ratio of 770, 670 and 470 cmÿ1 bands became larger with increasing temperature. And then, the glass melted

Fig. 2. Experimental interference function k 3 v…k† spectra of the xK2 O á TeO2 (x ˆ 10, 20 and 30 mol%) glasses and k 3 v…k† spectra calculated from K2 TeO3 crystal [14] and K2 Te4 O12 [15] crystal.

via a crystallization process. As shown in Fig. 3, the intensity ratio of the 770, 670 and 470 cmÿ1 bands reversed between the glass and the liquid. This Raman result shows that the Te±O bond has been broken and a part of TeO4 tbp units have changed to TeO3 tp units with increasing temperature. Table 2 indicates Tg , Tc and Tm temperatures of the K2 O±TeO2 glasses determined by DTA. The Raman pro®le of 20K2 O á 80TeO2 glass at 600 K, which is above glass transition temperature (Tg ˆ 503 K), is similar to that above the melting temperature (Tm ˆ 737 K). At a temperature above the crystallization temperature (Tc ˆ 597 K), bands were observed at around 720 cmÿ1 , and the positions of these bands are similar to those of the bands at a temperature above Tm . This result shows that a large portion of the crystal structure consists of TeO3 tp units. As the main

R. Akagi et al. / Journal of Non-Crystalline Solids 256&257 (1999) 111±118

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Fig. 3. (a) High-temperature Raman spectra for the 20K2 O á 80TeO2 (mol%) during heating. (b) High-temperature Raman spectra for the 20K2 O á 80TeO2 (mol%) during cooling.

Table 2 Compositions and thermal properties of xK2 O á TeO2 (x ˆ 5, 10, 15, 20, 25 and 30 mol%) glasses Heating process xˆ5 x ˆ 10 x ˆ 15 x ˆ 20 x ˆ 25 x ˆ 30

Cooling process

Tg (K)

Tc (K)

Tm (K)

562 547 522 503 484 471

644 672 580 507 541 513

708 907 859 715 737 729 702 750

677 660 653 624 535 579

556 531 513 668 720

(Tg ) (Tg ) (Tg ) (Tc ) (Tc )

Tg ± Glass transition temperature; Tc ± Crystallization temperaure; Tm ± Melting temperature. Tg ± ‹5 K; Tc ± ‹5 K; Tm ± ‹5 K.

band is 770 cmÿ1 at 790 K, the network structure in the 20K2 O á 80TeO2 liquid is mainly made up of TeO3 tp units. In the cooling process in Fig. 3(b), the liquid spectrum at 870 K continuously changes to the glass spectrum at 300 K, which indicates that the liquid has cooled through the undercooled

liquid to the glassy state without crystallization. The relative amplitude of the 770 cmÿ1 band relative to the 670 cmÿ1 band decreased and the amplitude of the 460 cmÿ1 band increased continuously with decreasing temperature. The Raman spectrum at 300 K for the sample after melt-cooling is similar to the spectrum of the sample before heating. The Raman spectra of the 10K2 O á 90TeO2 glass in the process of heating are shown in Fig. 4. With an increasing temperature up to 630 K, which is above the Tg , the band around 770 cmÿ1 due to the TeO3 tp units tend to increase compared to the 670 cmÿ1 band due to the TeO4 tbp units. This property of the 10K2 O á 90TeO2 sample is the same as that of the 20K2 O á 80TeO2 sample. At 770 K, which is between Tc and Tm , bands were observed at 650 and 400 cmÿ1 . These observed bands correspond to the frequency bands of a±TeO2 crystal [16]. These crystals of the 10K2 O á 90TeO2 sample are di€erent from those of the 20K2 O á 80TeO2 sample.

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Fig. 4. High-temperature Raman spectra 10K2 O á 90TeO2 (mol%) during heating.

for

the

Fig. 5 shows the Raman spectra of the 30K2 O á 70TeO2 sample during heating. Above Tc , many bands of 30K2 O á 70TeO2 crystals were observed around 770 cmÿ1 . This result shows that the network structure of these crystals and liquid mainly consist of TeO3 tp units. Fig. 6 shows XRD pro®les of the 20K2 O á 80TeO2 sample. In the temperature region from room temperature to 530 K there were no peaks due to crystal phases. Many crystallization peaks were observed at 590 K. Di€raction peak patterns completely changed at 660 K. At 690 K the di€raction pro®le manifested the fusion state. These crystallizations correlate with the DTA and Raman results. The reduced intensity functions, Q
i…Q†, and the RDF, G…r†, for xK2 O á TeO2 samples and liquids (x ˆ 10 and 20 mol%) are shown in Figs. 7 and 8, respectively. As can be seen in Fig. 7, there are di€erences in the smaller Q range between the glass and the corresponding liquid. We assume

Fig. 5. High-temperature Raman spectra 30K2 O á 70TeO2 (mol%) during heating.

for

the

Fig. 6. High-temperature XRD pro®les of the 20K2 O á 80TeO2 (mol%) glass.

that a medium- and long-range arrangement of the network structure changes from glass to liquid.  in Fig. 8 is attributed to the The ®rst peak at 1.8 A nearest-neighbor Te±O pair with NBO for TeO3 tp

R. Akagi et al. / Journal of Non-Crystalline Solids 256&257 (1999) 111±118

Fig. 7. Experimental reduced intensity Q
i…Q† for the xK2 O á TeO2 (x ˆ 10 and 20 mol%) glasses and melts.

Fig. 8. Radial distribution function G…r† for the xK2 O á TeO2 (x ˆ 10 and 20 mol%) glasses and melts.

units. The intensity of this nearest-neighbor Te±O peak is stronger in the liquid than in the glass. We assume that the cleavage of Te±O bonds generates the Te±O bonds with NBO, which is in agreement with the Raman results.

117

mixture of TeO4 tbp and TeO3 tp units. In the K2 O±TeO2 samples, TeO4 tbp units convert to TeO3 tp units with an increase of temperature and with the addition of K2 O. This structural phenomenon arises because the TeO4 tbp units can be changed into TeO3 tp units without a change of electrical neutrality. Tatsumisago et al. [6] have already reported the above properties of 20Li2 O á 80TeO2 glasses by using Raman spectroscopy. It is possible to explain the structural change on heating by the following schematic model [6] as shown in Fig. 9. Since the increase in the TeO3 tp units with increasing temperature must be accompanied by an increase in the amounts of Te±O terminal bonds such as Te@O and Te±Oÿ with NBO as shown in this model, the structural change can be regarded as an increase of discrete structural units with NBO at higher temperature. However, in the 20Li2 O á 80TeO2 glass, the only observed crystallization peak was a±TeO2 which is solely composed of the TeO4 tbp units [6], while in the xK2 O á TeO2 glass, the a±TeO2 peak was observed in x ˆ 10 mol%, but not in x ˆ 20 mol%. We assumed that the e€ect of K‡ ions as a network modi®er is greater than that of Li‡ ions in the alkali metal tellurite glasses. This di€erence is because the K‡ ion is e€ective for extending the Te±O interatomic distance as the ionic radius of potassium is larger than that of lithium. In the present study, we have obtained structural information of Te±O correlation in the K2 O±TeO2 glasses by the use of X-ray and neutron [8] RDF studies. The amount of Te±O terminal bonds such as Te@O and Te±Oÿ with NBO increases with increasing temperature. Fig. 8 shows that the peak  is due to the Te±O terposition at around 1.8 A minal bonds with NBO, and that the density of the

4. Discussion The 10K2 O á 90TeO2 sample mainly consists of TeO4 tbp units because crystallization is assigned to a±TeO2 which is solely composed of the TeO4 tbp units [17]. On the other hand, samples with additional amounts of K2 O are composed of the

Fig. 9. Possible model for the structural change of the K2 O± TeO2 glasses on heating [6].

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R. Akagi et al. / Journal of Non-Crystalline Solids 256&257 (1999) 111±118

terminal Te±O bonds increases in the liquid compared to the corresponding glass. On the other hand, the variation of the local structure of the modi®er ion such as K‡ is not a€ected by the composition in the glassy state. This lack of an a€ect indicates that there is a degree of steric freedom in the Te±O network structure in the K2 O±TeO2 glasses.

5. Conclusions The Raman spectroscopy, the XRD and the Xray RDF indicate that the TeO4 tbp units convert to the TeO3 tp units with increasing temperature and the addition of K2 O in the K2 O±TeO2 glasses. XAFS spectroscopy has indicated that potassium ions are in similar con®gurations in all of the studied K2 O±TeO2 glasses. References [1] T. Yoko, K. Kamiya, H. Yamada, K. Tanaka, J. Am. Ceram. Soc. 71 (1988) C±70. [2] H. Burger, W. Vogel, V. Kozhukharov, M. Marinov, J. Mater. Sci. 19 (1984) 403. [3] S. Neov, I. Gerassimova, K. Krezhov, B. Sydzhimov, V. Kozhukharov, Phys. Status Solidi A 47 (1978) 743.

[4] T. Sekiya, N. Mochida, A. Ohtsuka, M. Tonokawa, J. Non-Cryst. Solids 144 (1992) 128. [5] M. Tatsumisago, T. Minami, M. Tanaka, J. Am. Ceram. Soc. 64 (1981) C±97. [6] M. Tatsumisago, T. Minami, Y. Kowada, H. Adachi, Phys. Chem. Glasses 35 (1994) 89. [7] H. Ohno, K. Igarashi, N. Umesaki, K. Furukawa, X-ray Di€raction Analysis of Ionic Liquids, Molten Salt Forum, vol. 3, TransTech Publications, Aedermannsdorf, Switzerland, 1994, p. 1. [8] N. Umesaki, D.A.H. Cunnigham, K. Handa, Y. Iwadate, in: A.C. Wright, S.A. Feller, A.C. Hannon (Eds.), Borate Glasses, Crystal, Melts, Soc. Glass Technol., Sheeld, 1997, p. 99. [9] J.J. Rehr, J. Mustre de Leon, S.I. Zabinsky, R.C. Albers, J. Am. Chem. Soc. 113 (1991) 5136. [10] J. Heo, D. Lam, G.H. Sigel Jr., E.A. Mendoza, D.A. Hensley, J. Am. Ceram. Soc. 75 (1992) 277. [11] H.A. Levy, M.D. Danford, A.H. Narten, Data collection and evaluation with an X-ray di€ractometer for the study of liquid structure, Oak Ridge National Laboratory, TN, ORNL-3960, July, 1966. [12] C.R. Becker, S.L. Tagg, J.C. Hu€man, J.W. Zwanziger, Inorg. Chem. 36 (1997) 5559. [13] N. Umesaki, A.C. Hannon, D.A. Cunningham, Y. Shimizugawa, N. Kitamura, ISIS Experimental Report, No. 9151, Rutherford Appleton Laboratory, Chilton, 1998. [14] L. Andersen, V. Langer, A. Str omberg, D. Str omberg, Acta Crystallogr. B 45 (1989) 344. [15] F. Daniel, J. Moret, M. Maurin, E. Philippot, Acta Crystallogr. B 34 (1978) 1782. [16] T. Sekiya, N. Mochida, A. Ohtsuka, M. Tonokawa, J. Ceram. Soc. Japan 97 (1989) 1435. [17] O. Lindqvist, Acta Chem. Scand. 22 (1968) 977.