Tellurite Base Glasses – A Structural Study

In contrast to the tetrahedral [ZnO4] structure observed in ZnO(Shimizugawa et al., 1957) [12], the zinc atoms in glasses are occupied in the [ZnO6] and [ZnO5] structures, like those seen in Zn2Te3O8 and ZnTe3, respectively. 2.2 TeO2-Na2O. Sodium's monovalent nature implies it dissociates from oxygen-based bonds in networks. Depolymerization of TeO2 glasses will occur to a far greater extent with addition of Na2O, a network modifier, than upon addition of ZnO. Utilizing X-ray photoelectron spectroscopy, Himei et al. 1997[13] studied the make-up and structure of alkali (Li, Na, K, Rb, and Cs) tellurite glasses (XPS). Valence band spectra changed from having the [TeO4] tbp structure to having the [TeO3] tp structure upon the introduction of alkalis (from 0 to 30 eV). The [TeO4] tbp contains a single electron pair. The [TeO3] sp3 hybrid orbital and the tellurium sp3d hybrid orbital both have electrons in their equatorial regions. Zwanziger et al. 1997 used spinecho nuclear magnetic resonance to study the sodium distribution in TeO2-Na2O glasses [14]. Twenty mole percent Na2O has been demonstrated to be the sweet spot for stability. Sodium was found to be located at different positions in the glass than it is in crystalline Na2Te4O9, and the spacing between sodium atoms in the glass was found to be much shorter than what would be expected from a uniform distribution centering on 20 mol percent of the element. This structural difference at optimal glass stability may provide a high enough energy barrier to halt structural rearrangement and, by extension, crytallization. [15]. The various structural units of the glasses and the possible entrance locations for M2O into the glass network are shown in Figure 3. (a) (b)

It was shown that the composition of the glass altered when alkali was added; the non-bridging oxygens (NBOs) in the [TeO4] units were replaced by NBOs in the [TeO3] units (Sakida et al. 1999). This was shown by the increased abundance of [TeO4] and [TeO3] units with NBOs in the glass. McLaughlin et al. conducted experiments with neutron diffraction, X-ray diffraction, and nuclear magnetic resonance to confirm that TeO2-Na2O glasses contain all five polyhedra depicted in Figure 4.

When Na2O is present, the TeO2 network is constantly cleaved (McLaughlin et al., 2000) [16], leading to an equilibrium population and type of polyhedra that are compositionally dependent. It has been revealed that crystalline sodium tellurites include dimers linked together by four-member rings. However, in TeO2-Na2O glasses, only a small percentage of the tellurium atoms adopt a ring shape (around 3 percent). This finding may explain why glasses are so tough to break and hints that devitrification requires a major structural reorganisation. There was also found to be a high concentration of sodium cations close to the eutectic composition, which included 20 mole percent of Na2O (From) towards the Glass. ((McLaughlin et al., 2001) [17] 2.3 TeO2-ZnF2. Tetragonal ZnF2 (space group P42/mnm) has a structure comparable to rutile (TiO2). Zinc cations are organised such that they are surrounded by fluorine anions at octahedral sites. Since each fluorine ion is surrounded by three zinc ions, there are a total of six zinc ions around each zinc ion. This points to the incorporation of [ZnF6] units into the tellurite glass matrix, and it is possible that the sequence [ZnF6-xOx], where x might be 1, 2, 3, 4, 5, or 6, has many structural units. Zinc may coordinate six times with both F- and O-2 anions, allowing for this substitution to occur. This is due to the similar ionic radii of F- and O-2. Reducing the amount of ZnO in tellurite glasses by adding ZnF2 may provide a composition with stability Sidebottom et al. 1997 [18], who discovered some intriguing findings. However, the Raman bands at 650-750 cm-1, which result from the symmetric stretching modes of TeO2 structural units ([TeO4], [TeO3+1], and [TeO3]), were unaffected by the incorporation of fluoride. The relevant structural units were [TeO4], [TeO3+1], and [Te3]. As ZnF2 concentration increased, however, the bending mode strength at 420 cm-1 increased. The results indicated that fluorine could easily substitute for oxygen in the glassy network without triggering further depolymerization. The strength of the bending mode at 420 cm-1 rises with increasing fluoride concentration in this glass because more [TeO3] units are converted to [Te(O,F)3+1]. Although it was shown that ZnO would lead to depolymerization when added does not provide the same results as Na2O. To put it another way, it just wouldn't have the same effect [18]. 3. CONCLUSION Studies of tellurite base glass have been done. By adding modifiers as ZnO, ZnF2 and Na2O structure of the system get changed and hence properties.

REFERENCES

[1]. W. Vogel, Glass chemistry, 2nd ed. New York: Springer-Verlag, 1994. [2]. J. Berzelius, Annalen der Physik und Chemie, vol. 32, pp. 577, 1834. [3]. V. Lenher and E. Wolesensky, "A study of the metallic tellurites," Journal of the American Chemical Society, vol. 35, pp. 718-733, 1913. [4]. J. E. Stanworth, "Tellurite glasses," Nature, vol. 169, pp. 581-582, 1952a. [5]. J. Stanworth, "Tellurite Glasses," Journal of the Society of Glass Technology, vol. 36, pp. 217-241, 1952b. [6]. H. Rawson, Inorganic glass-forming systems, vol. 2, 1st ed. London: Academic Press, 1967. [7]. N. N. Greenwood and A. Earnshaw, Chemistry of the elements. Oxford: Butterworth-Heinemann, 1995. [8]. M. Redman and J. Chen, "Zinc tellurite glasses," Journal of the American Ceramic Society, vol. 50, pp. 523-525, 1967. [9]. A. P. Mirgorodsky, T. Merle-Mejean, J. C. Champarnaud, P. Thomas, and B. Frit, "Dynamics and structure of TeO2 polymorphs: model treatment of paratellurite and tellurite; Raman scattering evidence for new gamma- and delta-phases," Journal of Physics and Chemistry of Solids, vol. 61, pp. 501-509, 2000. [10]. J. A. K. Tareen and T. R. N. Kutty, A basic course in crystallography. Bangalore: Universities Press, 2001.

777, 1986.

[12]. Y. Shimizugawa, T. Maeseto, S. Inoue, and A. Nukui, "Structure of TeO2.ZnO glasses by RDF and Te, Zn K EXAFS," Physics and Chemistry of Glasses, vol. 38, pp. 201-205, 1997. [13]. Y. Himei, Y. Miura, T. Nanba, and A. Osaka, "X-ray photoelectron spectroscopy of alkali tellurite glasses," Journal of Non-Crystalline Solids, vol. 211, pp. 64-71, 1997. [14]. J. W. Zwanziger, J. C. McLaughlin, and S. L. Tagg, "Sodium distribution in sodium tellurite glasses probed with spin-echo NMR," Physical Review B, vol. 56, pp. 5243-5249, 1997. [15]. S. Sakida, S. Hayakawa, and T. Yoko, "Part 2. Te-125 NMR study of M2O-TeO2 (M = Li, Na, K, Rb and Cs) glasses," Journal of Non-Crystalline Solids, vol. 243, pp. 13-25, 1999. [16]. J. C. McLaughlin, S. L. Tagg, J. W. Zwanziger, D. R. Haeffner, and S. D. Shastri, "The structure of tellurite glass: a combined NMR, neutron diffraction, and X-ray diffraction study," Journal of Non-Crystalline Solids, vol. 274, pp. 1-8, 2000. [17]. J. C. McLaughlin, S. L. Tagg, and J. W. Zwanziger, "The structure of alkali tellurite glasses," Journal of Physical Chemistry B, vol. 105, pp. 67-75, 2001. [18]. D. L. Sidebottom, M. A. Hruschka, B. G. Potter, and R. K. Brow, "Increased radiative lifetime of rare earth-doped zinc oxyhalide tellurite glasses," Applied Physics Letters, vol. 71, pp. 1963-1965, 1997.

Department of Physics, Madhyanchal Professional University, Bhopal