Network structure of M2O–TeO2 (M=Li, Na, Li0.62Na0.38) glasses

Journal of Non-Crystalline Solids 293±295 (2001) 700±704

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Network structure of M2O±TeO2 (M ˆ Li, Na, Li0:62Na0:38) glasses Hideyuki Munemura a,*, Kazuko Mitome a, Masakatsu Misawa b, Kenji Maruyama b a

Graduate School of Science and Technology, Niigata University, 8050 Igarashi 2, Niigata 950-2181, Japan b Department of Chemistry, Niigata University, Niigata 950-2181, Japan

Abstract The network structures of xM2 O±…100 x†TeO2 (M ˆ Li, Na, Li0:62 Na0:38 ) (x ˆ 10, 25, 33 mol%) glasses prepared by melt quenching were studied by X-ray and neutron di€raction. The partial correlation functions for network frameworks have been calculated using the Reverse Monte Carlo (RMC) method. As a result, it was found that Te±Te correlation does not change with increasing alkali content. The RMC result supports recent reported observation that the units of TeO4 trigonal bipyramid decrease by addition of alkali oxide, while the units of TeO3 trigonal pyramid increase. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 61.43.Fs Glasses

1. Introduction It is well known that tellurite glasses form a network structure composed of Te±O. By the melt quenching method, the tellurite (TeO2 ) glass can be easily obtained when a modi®er such as alkali oxide is added to TeO2 . In the glassy state, it is suggested that there are two main structural units of TeO4 trigonal bipyramid (tbp) and TeO3 trigonal pyramid (tp) by diffraction experiments [1±6], spectroscopic and NMR analyses [7±10] and an ab initio molecular orbital calculation [11]. According to these observations, the tbp and tp units have low sym-

* Corresponding author. Tel.: +81-25 262 7758; fax: +81-25 262 6158. E-mail address: [email protected] (H. Munemura).

metry. In the tbp unit, a lone pair electron occupies one of the equatorial sites of tellurium sp3 d hybrid orbitals and oxygen atoms occupy the other two equatorial and axial sites. When alkali oxides like Li2 O are added, the concentration of the tp units in which a lone pair electron occupies one of the tellurium sp3 hybrid orbitals are increased. However the network structure is not clear yet. In order to observe the change of the network structure caused by the addition of network modi®ers, pulsed neutron scattering and X-ray di€raction measurements were carried out for the glasses. For the blended alkali oxide …Li0:62 ±Na0:38 †2 O, Li and Na were mixed at the ratio where the neutron scattering length averaged for alkali cations became null to investigate tellurite network structure. For further analysis of the framework, a Reverse Monte Carlo (RMC) calculation was performed.

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 7 3 - 6

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2. Experiment xM2 O±…100 x†TeO2 (M ˆ Li, Na, Li0:62 Na0:38 ) (x ˆ 10, 25, 33 mol%) glassy samples were prepared by melt quenching. As starting materials Li2 CO3 enriched 7 Li to 99.96 at.%, Na2 CO3 and TeO2 were used. The component oxides were melted in a platinum crucible at 550±600 °C for 30±60 min in air, and then the melt was quenched by putting the bottom of the crucible into the water. Glassy state was con®rmed by the visual observation and X-ray di€raction. The neutron scattering intensities were mea 1 by using the high sured in the Q range 0.3±30 A intensity total scattering spectrometer (HIT-II) installed in High Energy Accelerator Research Organization (KEK), Tsukuba, Japan. This instrument employed pulse neutron so that the time of ¯ight method was performed. The cylindrical cell made of vanadium ®lm of a thickness 0.025 mm was used. The X-ray di€raction measurements were carried out by using h±h re¯ection type X-ray spectrometer, RIGAKU Geiger ¯ex DXG1, with Mo-Ka radiation in 2h ˆ 2±155°. The structure factor S…Q† in Faber±Ziman de®nition [12] were obtained after standard corrections.

Fig. 1. The structure factors S…Q†s for xLi2 O±…100 x†TeO2 glasses obtained by X-day di€raction and x…Li0:62 Na0:38 †2 ± O±…100 x†TeO2 glasses obtained by Neutron scattering: x ˆ 10 (solid curves); 25 (broken curves); 33 (dotted curves).

3. Result Fig. 1 shows the structure factors S…Q†s of …Li0:62 Na0:38 †2 O±TeO2 glasses measured by neutron di€raction, and Li2 O±TeO2 glasses of X-ray di€raction both of which have the smallest contribution of scattering from alkali ions. As the di€erence of neutron scattering length and X-ray atomic form factor, the neutron structure factor shows O±O and Te±O correlations mainly, while the X-ray structure factor shows Te±Te correlation mainly. Therefore the patterns of S…Q† obtained by both experiments are greatly di€erent. By addition of alkali oxide, the shapes of S…Q† seem to not depend on alkali content except for the low-Q region of the S…Q†s, the X-ray ®rst peak and neutron second peak. Fig. 2 shows the pair distribution function g…r† of the same glasses given in Fig. 1,  1 which are the Fourier transform of S…Q† at 30 A

Fig. 2. The pair distribution functions g…r†s for xLi2 O±…100 x†TeO2 glasses obtained by X-day di€raction and x…Li0:62 Na0:38 †2 O±…100 x†TeO2 glasses obtained by neutron scattering: x ˆ 10 (solid curves); 25 (broken curves); 33 (dotted curves).

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Fig. 3. The comparison of experimental structure factor (dotted curves) and composed structure factor (solid curves) for 33…Li0:62 Na0:38 †2 O±67TeO2 .

 1 for X-ray S…Q†. The for neutron S…Q† and 17 A ®rst peak poison of g…r† that corresponds to Te±O bond length decreases a little bit from 1.90 to 1.86  with increasing alkali content. A Fig. 3 compares the experimental S…Q† of 33…Li0:62 Na0:38 †2 O±67TeO2 glasses and the composed S…Q† of the same glass; The composed S…Q† is the weighted sum of the S…Q† of 33Li2 O±67TeO2 glass and 33Na2 O±67TeO2 glass, that is, 0:5712SXLi …O† ‡ 0:4288SXNa (Q) and 0:4722SNLi …Q† ‡ 0:5278SNNa (Q), so that the contributions of Li and Na cancelled each other. The composed S…Q† correspond well to the experimental S…Q†. Though the correlation of the alkali ions is not taken into consideration, this value is about 2% of the total intensity and may be ignored here. This result strongly suggests that the network structure is almost independent of alkali species as far as alkaline concentration is small.

formed. The neutron g…r† has high resolution due to the high Qmax value of the Fourier transform of  1 , while the X-ray g…r† does not have S…Q† at 30 A so high resolution due to Qmax value at 17 1 . Therefore RMC ®ttings were done with neutron experimental pair distribution function g…r† over  and X-ray structure factor the r range of 0±12 A  1 . It was inS…Q† over the Q range of 0.5±17 A tended that by using both of the X-ray S…Q† and the neutron g…r† we could reproduce better real structure from short range to long range. The detail of the RMC method has been described in the report by McGreevy [13]. The size of cells was the  and about four thousand cube of one side 40 A, particles were treated. All atomic pairs except for  Te±Te pair were restricted to be longer than 1.5 A based on the fact that the starting position of the  ®rst peak of the experimental g…r† is beyond 1.5 A. The closest distance of Te±Te pair was limited at  in consideration that the covalent bond of 2.5 A  the Te atom in the four-fold structure is 1.32 A [14]. The results of RMC ®ts of S…Q† and g…r† for 10Na2 O±90TeO2 glass are shown in Figs. 4 and 5. The RMC model reproduces experimental data very well. Figs. 6 and 7 show the partial correlations for gij …r† and Sij …Q† obtained from the RMC model for 10Na2 O±90TeO2 and 10Li2 O±90TeO2 glasses, respectively. The relation between the partial correlations and the total correlation is written in the following equations:

4. Discussion Here we consider the structure of network frame in alkali-tellurite glasses. In order to analyze partial structures the RMC calculation was per-

Fig. 4. S…Q† obtained by RMC ®ts for 10Na2 O±90TeO2 glass: experiment data (solid curves); RMC ®ts (dot).

H. Munemura et al. / Journal of Non-Crystalline Solids 293±295 (2001) 700±704

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Fig. 5. g…r† obtained by RMC ®ts for 10Na2 O±90TeO2 glass: experiment data (solid curves); RMC ®ts (dot).

Fig. 7. Partial pair structure factor with the weight of concentration ci cj Sij …Q† from the RMC model for 10M2 O±90TeO2 glasses: M ˆ Na (solid curve); Li (dotted curve).

Fig. 6. Partial pair distribution function with the weight of concentration ci cj gij …r† from the RMC model for 10M2 O±90TeO2 glasses: M ˆ Na (solid curve); Li (dotted curve).

P S…Q† ˆ P g…r† ˆ

ci cj bi bj Sij …Q† hbi

2

ci cj bi bj gij …r† hbi

2

;

;

…1† …2†

where ci and bi are the concentration and neutron scattering length or X-ray atomic form factor of atom i, respectively. We were able to separate the seasonable distribution of each pair. The correlations in the network frame, Te±O, O±O and Te±O seem not to depend on alkali species. However, some peaks which are not realistic appear around  of gij …r† in O±O and M±Te correlations. r ˆ 1:9 A Therefore we examine qualitatively the change in the network mainly. And the FSDP at about  1 in S…Q†s come from O±O and Te±O cor1:5 A relations for the most part. The changes in the structure in real-space with increasing Na content are shown in Fig. 8. It seems that Te±Te correlation does not change with an increase of Na concentration, but the correlation  of Te±O changes. The shoulder located at 2.20 A in gTeO …r† decreases in height with increasing Na content. This decrease is not a€ected by the ghost of gOO …r† mentioned above because the height of

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RMC calculation, it was con®rmed that alkalitellurite glasses change Te±O network by changing their unit from TeO4 tbp to TeO3 tp with increasing alkali content. The Te±O correlations at  gradually weaken with increasaround 3.5±4.5 A ing alkali content. While the Te±Te correlation does not change. The short-range correlation about alkali is still not known. A more detailed RMC simulation will be conducted in future studies. Acknowledgements The authors are grateful to Dr H. Ebata and Mr S. Suzuki (Niigata University, Japan) for the support of the RMC programming and analysis. Fig. 8. The partial pair distribution function gij …Q† of network components for xNa2 O±…100 x†TeO2 glasses: x ˆ 10 (solid curves); 25 (dotted curves); 33 (broken curves).

 is not large and does not the ghost at around 2.2 A depend on alkali content. This result supports the widely accepted assumption [2±11] that in the TeO2 -based glass, some TeO4 tbp units gradually become TeO3 tp units by the addition of alkali content. The broad second peak of the Te±O correlation  for from chain shifts its position from r ˆ 4:54 A   x ˆ 10±3.74 A for x ˆ 25 and 3.52 A of x ˆ 33.  for these glasses are The gij …r† beyond r ˆ 6 A almost independent of the alkali concentrations. This may be due to the low resolution of the RMC partial structure in these r regions. It is dicult to argue this point more at present. The accurate three-dimensional structure of the M2 O±TeO2 glass will be made by improving the RMC method of putting a plausible limitation in a near future. 5. Conclusion The X-ray and neutron di€raction experiments were performed on M2 O±TeO2 glasses. By the

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