Letter pubs.acs.org/JPCL
Terminal Oxygens in Amorphous TeO2 Emma R. Barney,*,† Alex C. Hannon,‡ Diane Holland,§ Norimasa Umesaki,⊥ Masahiro Tatsumisago,# Robin G. Orman,§ and Steve Feller∥ †
Faculty of Engineering, University of Nottingham, Nottingham NG7 2RD, United Kingdom ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot OX11 0QX, United Kingdom § Physics Department, University of Warwick, Coventry CV4 7AL, United Kingdom ⊥ Division of Materials and Manufacturing Science, Osaka University, Suita, Osaka 565-0871, Japan # Department of Applied Chemistry, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan ∥ Physics Department, Coe College, 1220 First Avenue Northeast, Cedar Rapids, Iowa 52402, United States ‡
ABSTRACT: Understanding the structure of single-component glasses is essential for developing structural models of more complex multicomponent glasses. Currently, such models for tellurite systems are purely qualitative. This study presents neutron diffraction and Raman spectroscopy measurements of the structure of pure amorphous TeO2, showing that it is formed from a combination of two-thirds [TeO4] pseudo-trigonal bipyramids and onethird [TeO3] trigonal pyramids with a terminal oxygen. This is in contrast to all crystalline polymorphs of TeO2, which are formed solely from the four-coordinated units. Using this result, a quantitative model has been developed that successfully predicts the average Te−O coordination number, nTeO, for a series of potassium tellurite glasses, xK2O·(100 − x)TeO2. The observed nTeO is constant up to 15 mol % K2O due to the presence of terminal oxygen atoms in the tellurite network. SECTION: Glasses, Colloids, Polymers, and Soft Matter
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TeO2) in the literature (for examples, see refs 7 and 8), but closer inspection of some reportedly “pure” bulk glasses has found small amounts of impurities to be present.9 Such impurities allow an amorphous network to form more readily,10 and it is doubtful that it is possible to form pure a-TeO2 in large quantities by conventional melt quenching. However, tellurite glasses containing modifier oxides (e.g., xK2O·(100 − x)TeO2) are easy to produce in bulk quantities and exhibit a number of properties, such as high refractive indexes, third-order NLO coefficients, near-infrared transmittance, and good chemical durability, that make them promising candidates as components in a range of optical devices. To this end, many diffraction, Raman, and NMR studies7,8,11−20 have been carried out to characterize the structure of a wide range of tellurite glasses, and it is generally accepted that the tellurium coordination number, nTeO, changes with modifier content. This is not uncommon; both boron and germanium increase their coordination number to accommodate modifier oxide in a manner that roughly follows that in the analogous crystal structures.6,21 Therefore, it seems reasonable to use the structure of alkali tellurite crystals as the basis of a model for tellurite glass structures.22 Pure a-TeO2 is usually assumed to have nTeO = 4, analogous to crystalline polymorphs of TeO2,19,23 and the average nTeO decreases as the modifier
rototypical glass-former oxides have a strong tendency to form tetrahedral units. This is true not only in the case of oxides of tetravalent cations, such as SiO2 and GeO2, which form networks of corner-sharing [MO4] tetrahedra, where M is the glass-forming cation, but also for pentavalent glass-forming cations, such as P2O5 (and to some extent V2O5), where only three corners of the [MO4] tetrahedra are shared because the fourth is occupied by a terminal MO bond.1 In this Letter, we report the surprising behavior of the tetravalent cation glassformer oxide, TeO2, where a proportion of the Te−O polyhedra are [TeO3] units with one doubly bonded terminal oxygen, TeO. This is the first known glass former to have a local structure unlike that of its crystalline analogue, and using this unexpected result, we are now able to formulate a quantitative model to successfully predict the previously unexplained behavior of potassium tellurite glasses. Early work by Goldschmidt2 demonstrated that all oxides that readily form glasses have a cation to oxygen radius ratio in the range corresponding to a tetrahedral arrangement, and he posited that these units are necessary for glass formation.3 Zachariasen noted that, in order to minimize the energy of a glass, cation environments are likely to be the same as those in crystals,4 and comparative studies have shown this to be true.5,6 While the environments and behavior of the glass formers discussed above are reasonably well understood, those of tellurium oxide glasses are less clear. TeO2 is usually classed as a conditional network-former, being very difficult to form as a pure glass. There are several reports of amorphous TeO2 (a© 2013 American Chemical Society
Received: May 23, 2013 Accepted: June 17, 2013 Published: June 17, 2013 2312
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tellurite network and modifying cations on the basis of reliable measurements of nTeO for both pure a-TeO2 and a series of potassium tellurite glasses, made using modern neutron diffraction instrumentation. A sample of pure a-TeO2 was produced using a large twinroller quencher, with cooling rates in excess of 105 K/s.28 The resultant material was shown to be amorphous using X-ray and neutron diffraction, and X-ray fluorescence was used to confirm that the glass was free of impurities. The GEM diffractometer at the ISIS spallation source29 was used to obtain total neutron scattering data for pure a-TeO2. The method of data collection, normalization, and analysis follows previous work.23,30 Figure 2
content increases through the intermediate formation of [TeO3+1]24 until the tellurium is entirely three-coordinated. The two extremes of the tellurium environment are illustrated in Figure 1a. However, this qualitative description has not
Figure 1. (a) A ring in crystalline K2Te4O9,25 on which connectivity diagrams for (b) amorphous TeO 2 and (c) amorphous 14.3K2O·85.7TeO2 are based. (d) A nonbridging oxygen in K2TeO3 supports three K−O bonds and satisfies the bond valence requirement. (Red spheres denote BOs, and red/gray spheres represent terminal oxygens in a-TeO2 and NBOs in 14.3K2O·85.7TeO2. Yellow spheres represent tellurium atoms, and larger blue spheres represent potassium atoms. Thick black lines represent long Te−O bonds, and thin black lines represent short bonds or NBOs. Double bonds () indicate a terminal oxygen on a [TeO3] unit.).
Figure 2. (a) Total correlation functions measured using neutron diffraction for a series of glasses xK2O·(1 − x)TeO2. (b) T(r) for amorphous TeO2 (black line), shown with the fitted peaks for the Te− O (green dotted) and O···O (gray dashed) distributions. The sum of the fits (black dashed line) and the residual (black dotted line) are also shown.
shows T(r) together with results for a series of potassium tellurite glasses, measured using the LAD diffractometer (ISIS) and formed with more conventional cooling rates. The distribution of bond lengths in the glass is significantly broader than that for α-TeO2,23 and nTeO can be obtained both by fitting a series of peaks and by integration. Following Becker’s analysis of Te−O bond lengths in alkali tellurite crystals,22 a series of three peaks (the minimum number required to adequately fit the asymmetric distribution; ref 30 gives details of the peak fitting method) was used to fit the Te−O region of T(r) for a-TeO2 (Figure 2b), yielding a value of 3.68(4) for nTeO and, correspondingly, 1.84(1) for nOTe. This is a highly unusual result; the short-range structures of the glass and crystalline forms of TeO2 are markedly different, with the glass structure formed from about two-thirds four-coordinated units and one-third three-coordinated units. The presence of Te4+ atoms in a [TeO3] environment is analogous to amorphous P2O5, in which P5+ forms [PO4] units with one terminal oxygen atom bonded to each phosphorus atom (PO).32,33 As each Te atom is bonded to a weighted total of two O atoms, by bonding to either four bridging oxygens (BOs) ([TeO4]) or two BOs and one terminal oxygen ([TeO3]), approximately 16% of oxygen atoms are terminal oxygens (i.e., bonded to only one Te atom in the distance range of 0−2.36 Å). Although a-TeO2 is traditionally considered to be comprised exclusively of [TeO4] units,7,8,11,12,20 a molecular dynamics (MD) simulation33 has yielded values for nTeO and nOTe that are in excellent agreement with those reported here (3.69 and 1.85, respectively, determined with a radial cutoff of 2.36 Å), showing that our result is consistent with this 3-D model of the glass structure. We have drawn the analogy between the presence of
provided a quantitative understanding of the mechanism that drives the coordination change. Furthermore, the results of diffraction studies of vanadium tellurite glasses are not consistent with this model, with coordination numbers considerably less than 4 for glasses modified by only 425 and 5 mol % V2O5.26 Experimental testing of any model of tellurite glasses by quantitative measurements of nTeO has proved difficult due to the relatively complex nature of the bonding between tellurium and oxygen. The active lone pair of electrons (E) on the tellurium atoms induces a pseudo-trigonal bipyramid structure ([TeO4E]) in α-TeO2, where E occupies an equatorial position.27 Therefore, unlike a typical tetrahedral environment, α-TeO2 is comprised of asymmetric [TeO4] units with two short (1.882 Å) equatorial and two long (2.117 Å) axial Te−O bonds (Figure 1a).23 Diffraction studies show that tellurite glasses have an even wider distribution of Te−O bond lengths, which is both broad and asymmetric.11−20 As a result, determining the exact extent of Te−O bonds is problematic, and coordination numbers (especially those derived from experiments with poor real-space resolution, such as EXAFS and early diffraction measurements) are inexact. However, with the advent of high-resolution time-of-flight neutron diffraction instruments, total scattering measurements can produce correlation functions, T(r), which yield accurate coordination numbers; for example, our recent study of α-TeO2 gives a value for nTeO of 3.95(3) (nOTe = 1.98(1)),23 while a measurement of amorphous GeO2 gave nGeO = 4.02(3).6 In this Letter, we develop a coherent description of the interaction between the 2313
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network is charge-balanced. The length of each Te−NBO bond (∼1.84 Å35,36) corresponds to a bond valence of ∼1.5,37 and the NBO can therefore further support up to three K−O bonds, as illustrated in Figure 1d, on the assumption that K is six-coordinated. For every K2O unit added to the glass, 12 K− O bonds are required, but the two additional NBOs created by breaking bridges only provide 6. In current understanding, it is this shortage of coordinating oxygens that drives the tellurium coordination change; the structural rearrangement of the tellurite network to produce a [TeO3] unit results in the formation of an additional NBO. However, the presence of [TeO3] units in the connectivity model for a-TeO2 (Figure 1b) creates one terminal oxygen for every three Te atoms present in the TeO2 base glass and leads to a surplus of potential K−O bonds in glasses with low modifier contents. This surplus allows K2O to be accommodated in the tellurite glass network by the formation of NBO atoms alone, without the conversion of [TeO4] to [TeO3], until a composition x = 100/7 (i.e., 14.3 mol % K2O), at which the number of potential K−O bonds provided by the glass network equals the number required by the potassium modifier (Figure 1c). The formation of NBOs is analogous to the formation of [TeO3+1] units discussed in the literature (see, e.g., ref 18). Once all of the potential K−O bonds on the terminal oxygen atoms are saturated, the network can no longer support the addition of K2O without greater structural rearrangement. For glasses containing more than 14.3 mol % K2O, the deficit in K−O bonds is overcome by the formation of [TeO3] units and the formation of potassium bonds to bridging oxygens (BOs). The excellent agreement with experimental results (Figure 3), including an accurate prediction of the composition at which the formation of the [TeO3+1] units gives way to [TeO3] units, demonstrates the success of the model and confirms our initial result; nTeO in pure amorphous TeO2 is significantly less than 4. With adjustments for both the valence and coordination number of the modifier, this model can be successfully applied to other binary tellurite glasses. For example, nTeO for vanadium tellurites (nVO = 638) is predicted to exhibit the same behavior as that for potassium tellurites, and this is confirmed by the experimental data shown in Figure 3.25,39 In addition to explaining the change in nTeO with composition, this model also gives an interpretation of the physical property variations reported in the literature for lithium40 and sodium41 tellurite glasses. The conductivity of lithium tellurite glass with 10 mol % Li2O is markedly lower than that for glasses with 15 mol % Li 2O or greater,40 and our model provides a simple interpretation of this result in which Li+ ions that are bonded to BOs are more mobile than Li+ ions that are not. Analysis of the TeO2 Raman spectra from amorphous and crystalline tellurites indicates that symmetric Te−O−Te bridges vibrate in the region of 400−500 cm−1, while asymmetric Te−O−Te bridges, or nonbridging Te−O bonds, give rise to vibrations between 650 and 800 cm−1.19 Previous interpretations of the Raman spectra of tellurite glasses have been based on the premise that pure a-TeO2 is composed solely of [TeO4] units (with no terminal oxygens) and the region at ∼550−700 cm−1, resembling the Raman spectrum of crystalline α-TeO2,42 is associated with the asymmetric Te−O−Te bridge between [TeO4] units. The two fitted peaks at ∼700−800 cm−1 have been assumed to arise from a continuous network of [TeO4] units, even though the Raman spectrum of α-TeO2 does not exhibit a corresponding feature. However, this region of the Raman spectrum corresponds to the vibrational
[TeO3] units and the well-understood structure of P2O5, but while the terminal PO bond is easily observable and quantifiable by diffraction, NMR, and Raman spectroscopy,31 the tellurium environment is far more difficult to probe experimentally. This, coupled with the common expectation that tellurium should be four-coordinated, as in α-TeO2, has thus far hindered the development of models to explain the changes in structure of tellurite glasses with composition. Our accurate measurement of nTeO for a-TeO2 leads to the development of a simple connectivity model for the glass, shown in Figure 1b, that can be conveniently illustrated using the ring structure in crystalline K2Te4O922 (Figure 1a). While there is no direct evidence for such rings in a-TeO2, a similar structure is observed in the low-density crystalline phase, γTeO2, a phase that has been observed to form prior to α-TeO2 when TeO2 glass is heated, suggesting that the γ-phase is most closely related to that found in the glass. Partial polymerization of the (TeO2)6 molecule, derived in the ab initio study of Noguera et al.,34 also produces the structure shown in Figure 1a. The ring is comprised of [TeO3] and [TeO4] units in a ratio such that the average coordination number is 3.667 (identical, within error, to the measured value for the glass), although the [TeO3] units in K2Te4O9 are formed as a result of the addition of a modifier and are not intrinsic to the tellurite network. It should be noted that this illustration contains only two Q species18 and is much simplified compared to the range of Te environments observed by MD.33 The values of nTeO measured by neutron diffraction for potassium tellurite glasses (Figure 3) have a composition
Figure 3. Coordination numbers for tellurite glasses measured using a range of methods. The dashed lines represent the predicted coordination numbers, as given by the structural model described in the text. ● = potassium tellurite coordination numbers measured using neutron diffraction (this study). Additional values for potassium tellurite coordination numbers, measured using Raman, are taken from ref 7 (△). □ = Vanadium tellurite coordination numbers measured using neutron diffraction.25
dependence with two different regions. For glasses containing less than ∼15 mol % K2O, nTeO is the same, within error, as that measured for pure a-TeO2. However, for samples containing more than 15 mol % K2O, there is a linear decrease in nTeO, which is consistent with the formation of one additional [TeO3] unit for each additional K2O unit added to the glass. By using the connectivity models (Figure 1), it is possible to accurately predict the composition dependence of nTeO for the first time. In potassium tellurite glasses, xK2O·(100 − x)TeO2, the addition of a K2O unit introduces one additional oxygen that may break a Te−shortOlong−Te bridge to produce two nonbridging oxygens (NBOs), as shown in Figure 1c. These NBOs bond with the modifying K+ cations so that the glass 2314
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(4) Zachariasen, W. H. The Atomic Arrangment in Glass. J. Am. Chem. Soc. 1932, 54, 3841−3851. (5) Lower, N. P.; McRae, J. L.; Feller, H. A.; Betzen, A. R.; Kapoor, S.; Affatigato, M.; Feller, S. A. Physical Properties of Alkaline-Earth and Alkali Borate Glasses Prepared over an Extended Range of Compositions. J. Non-Cryst. Solids 2001, 293−295, 669−675. (6) Hannon, A. C.; Di Martino, D.; Santos, L. F.; Almeida, R. M. Ge−O Coordination in Cesium Germanate Glasses. J. Phys. Chem. B 2007, 111, 3342−3354. (7) Sekiya, T.; Mochida, N.; Ohtsuka, A.; Tonokawa, M. RamanSpectra of MO1/2−TeO2 (M = Li, Na, K, Rb, Cs and Tl) Glasses. J. Non-Cryst. Solids 1992, 144, 128−144. (8) Sakida, S.; Hayakawa, S.; Yoko, T. Part 2.125Te NMR Study of M2O−TeO2 (M = Li, Na, K, Rb and Cs) Glasses. J. Non-Cryst. Solids 1999, 243, 13−25. (9) Lambson, E. F.; Saunders, G. A.; Bridge, B.; El-Mallawany, R. A. The Elastic Behaviour of TeO2 Glass under Uniaxial and Hydrostatic Pressure. J. Non-Cryst. Solids 1984, 69, 117−133. (10) Bednarik, J. F.; Neely, J. A. Thermal-Properties of a Single Component Antimony Oxide Glass. Phys. Chem. Glasses 1982, 23, 204−205. (11) Neov, S.; Kozhukharov, A. V.; Gerasimova, A. I.; Krezhov, A. K.; Sidzhimov, A. B. A Model for Structural Recombination in Tellurite Glasses. J. Phys. C: Solid State Phys 1979, 13, 2475−2485. (12) Himei, Y.; Osaka, A.; Nanba, T.; Miura, Y. Coordination Change of Te Atoms in Binary Tellurite Glasses. J. Non-Cryst. Solids 1994, 177, 164−169. (13) Shimizugawa, Y.; Maeseto, T.; Suehara, S.; Inoue, S.; Nukui, A. EXAFS and RDF Studies of TeO2−Li2O Glasses. J. Mater. Res. 1995, 10, 405−410. (14) Zwanziger, J. W.; McLaughlin, J. C.; Tagg, S. L. Sodium Distribution in Sodium Tellurite Glasses Probed with Spin−Echo NMR. Phys. Rev. B 1997, 56, 5243−5249. (15) Akagi, R.; Handa, K.; Ohtori, N.; Hannon, A. C.; Tatsumisago, M.; Umesaki, N. Structure of K2O−TeO2 Glasses. Jpn. J. Appl. Phys., Part 1 1999, 38, 160−163. (16) McLaughlin, J. C.; Tagg, S. L.; Zwanziger, J. W.; Haeffner, D. R.; Shastri, S. D. Section 1: Glass Structure 1. The Structure of Tellurite Glass: A Combined NMR, Neutron Diffraction, and X-ray Diffraction Study. J. Non-Cryst. Solids 2000, 274, 1−8. (17) Iwadate, Y.; et al. X-ray Diffraction Study on the Short-Range Structure of K2O−TeO2 Glasses and Melts. J. Alloys Compd. 2000, 311, 153−158. (18) McLaughlin, J. C.; Tagg, S. L.; Zwanziger, J. W. The Structure of Alkali Tellurite Glasses. J. Phys. Chem. B 2001, 105, 67−75. (19) Noguera, O.; Merle-Mejean, T.; Mirgorodsky, A. P.; Smirnov, M. B.; Thomas, P.; Champarnaud, J. C. Vibrational and Structural Properties of Glass and Crystalline Phases of TeO2. J. Non-Cryst. Solids 2003, 330, 50−60. (20) Hoppe, U.; Gugov, I.; Burger, H.; Jovari, P.; Hannon, A. C. Structure of Tellurite GlassesEffects of K2O or P2O5 Additions Studied by Diffraction. J. Phys.: Condens. Matter 2005, 17, 2365−2386. (21) Hannon, A. C.; Holland, D. A Parameterisation for the Composition Dependence of N-4 in Borate Glasses. Phys. Chem. Glasses: Eur. J. Glass Sci. Technol., Part B 2006, 47, 449−454. (22) Becker, C. R.; Tagg, S. L.; Huffman, J. C.; Zwanziger, J. W. Crystal Structures of Pottassium Tetratellurite, K2Te4O9, and Pottasium Ditellurite, K2Te2O5, and Structural trends in Solid Alkali Tellurites. Inorg. Chem. 1997, 36, 5559−5564. (23) Barney, E. R.; Hannon, A. C.; Holland, D. Short-Range Order and Dynamics in Crystalline α-TeO2. J. Phys. Chem. C 2012, 116, 3707−3718. (24) [TeO3+1] indicates a [TeO4] unit with three short Te−O bonds and one long Te−O bond, as illustrated in Figure 1c. (25) Hoppe, U.; Yousef, E.; Russel, C.; Neuefeind, J.; Hannon, A. C. Structure of Vanadium Tellurite Glasses Studied by Neutron and X-ray Diffraction. Solid State Commun. 2002, 123, 273−278.
frequency for isolated [TeO3] units (nonbridging Te−O bonds) in crystalline K2TeO3.7 By using this alternative assignment (and neglecting possible variation in the Raman matrix element), Kalampounias et al. have estimated the relative numbers of [TeO3] and [TeO4] units from the intensities of the two regions of the Raman spectrum.43 The fit to the Raman spectrum measured for the diffraction sample, shown in Figure 4, yields nTeO = 3.64(4), in good agreement with the neutron
Figure 4. A Raman spectrum for amorphous TeO2. The strongest vibrations observed from crystalline α-TeO2 (entirely connected network comprised of pseudo-trigonal bipyramid [TeO4E] units)42 and K2TeO3 (a crystal comprised of isolated pseudo-tetrahedral [TeO3E] units) are also marked.7 The five peaks fit to the data are shown in gray. The summed fit is shown as a black dashed line, and the residual is a blacked dotted line.
diffraction result. Furthermore, published fits to Raman spectra for potassium tellurite glasses7 lead to nTeO values shown in Figure 3. The correspondence with the neutron diffraction results suggests that Raman spectroscopy may have the potential to determine nTeO.43 In conclusion, we have demonstrated the presence of terminal oxygens in amorphous TeO2 and, using this result, have, for the first time, developed a quantitative model that can be used to predict the coordination number of tellurium in binary tellurite glasses over a range of compositions. The strong links established between NLO properties and the tellurium environment44,45 make this a key result in developing tellurite glasses for optical applications such as switches, laser hosts, and supercontinuum generation. Furthermore, this study has yielded a greater understanding of the important role that terminal oxygens play in the glass network.
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AUTHOR INFORMATION
Corresponding Author
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[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
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