Short-Range Structure of TeO2 Glass - American Chemical Society

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On the Short-Range Structure of TeO Glass Mounesha Nagendrachar Garaga, Ulrike Werner-Zwanziger, Josef Wilson Zwanziger, Anthony DeCeanne, Brittney Hauke, Kathryn Bozer, and Steve Feller J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08978 • Publication Date (Web): 28 Nov 2017 Downloaded from http://pubs.acs.org on November 28, 2017

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The Journal of Physical Chemistry

On the Short-Range Structure of TeO2 Glass Mounesha N. Garaga,† U. Werner-Zwanziger,† J. W. Zwanziger,∗,† A. DeCeanne,‡ B. Hauke,‡ K. Bozer,‡ and S. Feller‡ †Department of Chemistry and Institute for Research in Materials, Dalhousie University, PO Box 15000, Halifax, NS B3H 4R2, Canada ‡Department of Physics, Coe College, Cedar Rapids, IA E-mail: [email protected] Phone: +1 902 494 1960. Fax: +1 902 494 1310

Abstract

structure of TeO2 glass has been difficult to determine. 1–3 TeO2 is found in three crystal polymorphs. The more common α and β phases consist of disphenoidal TeO4/2 units (also known as seesaw or trigonal bipyramidal), where the third equatorial position is occupied by a stereochemically active lone pair. 4,5 In the metastable γTeO2 phase, the same motif is found except that one of the axial bonds becomes quite elongated as one of the equatorial bonds contracts. 6 It had been long assumed that TeO2 glass would resemble α-TeO2 in consisting of four-coordinated Te in a disphenoidal geometry, perhaps with a spread of bond lengths as found in the β phase. 1,7,8 Recent experimental and modeling studies, however, have found that the effective Te coordination number is about 3.65, significantly less than four. 2,3 The origin of this low coordination number however is not clear. It may arise from discrete, well-defined tellurite polyhedra carrying terminal (non-bridging) oxygen, 2,9 or by tellurite polyhedra of the TeO3+1 type, that is, a fourth oxygen bond that is long and weak. 3 To study this question we used solid-state NMR in conjunction with first principles calculations, Raman spectroscopy, and x-ray diffraction. Our primary objective was to prepare oxygen-17-enriched TeO2 glass, so that both the tellurium and oxygen sites could be studied by solid-state NMR. As model compounds we prepared α and γ phases of TeO2 , also isotopically

The short-range structure of TeO2 glass was studied by a combined solid-state NMR, Raman, and first-principles calculation approach. Glass samples enriched in oxygen-17 were prepared and both 125 Te and 17 O spectra were recorded. In addition, spectra of α-TeO2 and γ-TeO2 crystal phases were recorded to aid in spectral assignment. The identity of the crystal phases was confirmed by x-ray diffraction and Raman spectroscopy. First-principles computations on the TeO2 crystal phases as well as on several potassium tellurites were carried out for comparison. It was found that the structure of TeO2 glass is best described as a highly disordered form of the γ-TeO2 crystal.

1

Introduction

Because of their attractive optical characteristics and high refractive index, tellurite glasses are important for applications, especially in fiber optic technology. Such glasses contain multiple modifying components. Pure TeO2 glass itself, however, usually described as a conditional glass former and its structure appears to be complicated. Whereas the canonical glass former silica clearly consists of SiO4 tetrahedra, with four equivalent Si-O bonds, linked through corner-sharing vertices, tellurite crystal chemistry is much richer and even the short-range

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labeled, and used their NMR response in combination with first-principles calculations to assign the resonances observed in the TeO2 glass. We found that the glass structure could be best understood in terms of the γ-TeO2 crystal, as suggested by Gulenko et al. 3

2 2.1

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Pt crucible in water. The resulting TeO2 -rich glasses were transferred to a silica crucible, then heated at 400–410◦ C for 24 hrs. γ-TeO2 and residual α-TeO2 crystals were obtained in all batches. The γ-TeO2 content was maximized with 7.5% PbO as verified by x-ray diffraction and Raman spectroscopy.

Methods

2.2

Sample preparation

X-ray Diffraction

Powder X-ray diffraction patterns of all studied tellurite glasses and crystals were collected on a Rigaku diffractometer equipped with CuKα source at a voltage of 45 kV and a current of 40 mA. The XRD patterns were scanned at the rate of 1◦ /min with a step size of 0.05◦ in 2θ from 10◦ or 20◦ to 60◦ at room temperature.

Tellurium dioxide (≥ 99.9%), potassium carbonate (anhydrous, ACS reagent, ≥ 99%), lead (II) oxide (99.9%, metals basis) and niobium pentoxide (99.9%, metals basis) were purchased from Sigma-Aldrich. Tellurium (IV) isopropoxide (99.9%, metal basis) and tellurium (IV) ethoxide (technical grade 85%, remainder ethanol) were purchased from Alfa Aesar. 70% and 40% 17 O-enriched H2 O were purchased from Cortecnet. The 17 O-enriched TeO2 was synthesized as described by Hart et al. 10 17 O-enriched crystalline TeO2 was prepared by adding two equivalents of 17 O-enriched H2 O drop-wise to a solution of absolute ethanol and tellurium isopropoxide or tellurium ethoxide under vacuum, with continuous stirring on an ice bath until precipitation of TeO2 was complete. The vacuum-dried precipitate was then melted at 750 ◦ C for about 10 min and cooled to ambient temperature at 2 ◦ C per minute. Formation of α-TeO2 was verified through x-ray diffraction (XRD). TeO2 glass was made by the following procedures. For the isotopically unenriched samples, crystalline α-TeO2 was melted at 780–800◦ C for 10–20 min in a platinum crucible, while for 17 Olabelled TeO2 , crystals were melted at 900◦ C for only 3 min to minimize exchange with atmospheric oxygen. All melts were quenched by quickly and repeatedly immersing the bottom of the Pt crucible in water. 11 Crystalline γ-TeO2 was synthesized by seeding the melt with Nb2 O5 or PbO (< 7.5%), as follows. 6 Crystalline α-TeO2 was mixed in an agate mortar with the seed crystals, then melted in a Pt crucible at 850◦ C for 5–10 min. Again, the melts were quenched by dipping the

2.3

Raman spectroscopy

Raman spectra were collected using a Nicolet NXR 9650FT-Raman spectrometer equipped with Nd:YVO4 1064 nm laser. The spectra were accumulated over 128 scans of 10 s each with a resolution of 2 cm−1 .

2.4

Solid-state NMR spectroscopy

Solid-state one-dimensional (1D) 125 Te and 17 O NMR experiments were performed on a 400 MHz Bruker Avance DSX and a 700 MHz Bruker Avance NMR spectrometer operating at magnetic fields of 9.4 T (125 Te and 17 O Larmor frequencies of 126.275 MHz and 54.258 MHz respectively) and 16.4 T (17 O Larmor frequency 94.932 MHz). 1D 125 Te NMR spectra of αTeO2 , γ-TeO2 , and TeO2 glass were collected at a magnetic field of 9.4 T on a wide line NMR probe at static condition by using the Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence. 12 The spectra were collected by moving the offset frequency in steps of 40 kHz (the VOCS method). 13,14 The signal of each spectrum was accumulated over 50 ms of 200 spin echoes accumulating 8 scans with 400 s recycling delay. The final spectrum combines all VOCS slices in a skyline projection. Chemical shifts were externally referenced to 1M telluric


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Holzwarth set 18 when used with both Quantum Espresso and Abinit, on geometry optimization. A planewave cut-off energy of 600 eV was used, together with automatic k-point generation using a minimum mesh step of 1/40 of reciprocal cell vector lengths. These parameters were sufficient to converge the chemical shielding to the sub-ppm level. The experimental structures were used without optimization. Electric field gradients and chemical shieldings were computed using the PAW and GIPAW approaches, respectively. 19–21 For the electric field gradients, observable quadrupole couplings CQ where generated using the nuclear quadrupole moment for 17 O found in Pyykkö. 22 The electron density computed by Vasp was analyzed using the Quantum Theory of Atoms in Molecules approach (QTAIM), in which bonding interactions between atoms are assessed based on the internuclear electron density. 23 The software Critic2 was used to carry out the QTAIM analysis. 24

acid, Te(OH)6 , calibrated at 713 ppm with respect to a primary reference Te(CH3 )2 . On the 400 MHz spectrometer, 17 O NMR spectra of crystalline TeO2 were accumulated from 4096 transients following single-pulse excitation (pulse duration 0.83 µs), and 5 s recycling delay in 4 mm rotors spinning at 13 kHz. 17 O Hahn echo NMR spectra of TeO2 glass, at 13 kHz MAS and at static condition, were acquired using 2048–4096 transients with 2 s recycling delay and 523 µs echo delay. On the 700 MHz spectrometer, 17 O Hahn echo NMR spectra of TeO2 crystals and glasses were collected using 500 µs echo delays over 16 and 16–256 transients, with 500 s and 15 s recycling delays, respectively. Two-dimensional (2D) 17 O MQMAS NMR spectra of α-TeO2 and γ-TeO2 crystals were collected at 12.5 and 10 kHz MAS, respectively. The triple quantum excitation and reconversion pulses were optimized to 6.0 µs and 1.5 µs, respectively, while selective 180◦ pulse was set to 22 µs. The MQMAS spectra of α-TeO2 were collected over 192 transients and 36 slices in F1 dimension with 25 s recycling delay, while for γ-TeO2 , 768 transients and 56 slices in F1 dimension with 8 s recycling delay were accumulated. 2D MQMAS spectrum of TeO2 glass was collected over 2304 transients for each of 10 slices in F1 dimension with 3 s recycling delay. 17 O chemical shifts were externally referenced to water at 0 ppm. The indirect dimension of the MQMAS spectra was referenced using Dmfit. 15 All NMR experiments were performed at room temperature. All line shape simulations of the NMR spectra were performed with the program Dmfit (dm2015vs/release #201500521 15 ).

2.5

3 3.1

Results α-TeO2

The x-ray diffraction results for α-TeO2 and TeO2 glass are shown in Fig. 1. The pattern for α-TeO2 matches that for the published structure. 4 These results demonstrate the successful synthesis of crystalline α-TeO2 enriched in 17 O, and its successful conversion into glass. The NMR spectra for α-TeO2 are shown in Fig. 2. The 125 Te spectra (Fig. 2(a) and (b)) were acquired with a CPMG sequence, while the 17 O NMR spectra were acquired with a Hahn echo (for one-dimensional spectra) and MQMAS (two-dimensional) (Fig. 2(c) and (d)) sequence. The α-TeO2 structure has a single crystallographic Te site and O site each, so the spectra should consist of single components, as observed. 4 The geometry around the Te site is disphenoidal, with two shorter equatorial bonds (1.90 ˚ A) and two longer axial bonds (2.08 ˚ A). Each oxygen thus bridges two Te atoms, with bond lengths 1.90 and 2.08 ˚ A. The 125 Te NMR parameters were extracted from the

First principles calculations

First principles calculations were performed using the Vasp code (www.vasp.at). This code performs density functional theory calculations, using the projector-augmented wave (PAW) formalism and a planewave basis. 16 The Vasp version 5.4 PAW atomic data sets were used, with the PBE exchange and correlation functional. 17 These atomic data sets gave quite similar results in Vasp to the Jollet-Torrent-


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various spectra using the Dmfit program. Results are shown in Table 1 for 125 Te and Table 2 for 17 ONMR. The 125 Te NMR shift parameters determined for α-TeO2 are in reasonable agreement with those published by Sakida et al. 25 and by Holland et al. 26 The 17 O NMR parameters agree with those previously reported by Hart and Zwanziger. 10

3.2

γ-TeO2

The x-ray diffraction and Raman spectrum of γTeO2 are shown in Fig. 3. The results shown are for the sample crystallized using PbO as a seed, and are in excellent agreement with the structure described by Champarnaud-Mesjard et al. 6 The γ-TeO2 phase contains a single Te site and Figure 1: X-ray diffraction patterns for (a) αTeO2 , (b) α-TeO2 enriched in 17 O, and (c) TeO2 glass enriched in 17 O.

Figure 3: X-ray diffraction (a) and Raman spectrum (b) of γ-TeO2 , enriched in 17 O and prepared by seeding a melt of TeO2 with PbO. Peaks are identified by γ in accord with the results of Champarnaud-Mesjard et al.; 6 it is apparent that a small amount of residual α-TeO2 remains.

Figure 2: (a) and (b) show the 125 Te static NMR spectra of α-TeO2 , both unenriched and 17 O enriched. (c) shows the one-dimensional MAS 17 O spectrum of α-TeO2 , and (d) the MQMAS spectrum.

two O sites. 6 The Te site is again disphenoidal, but now the two short equatorial bonds have different lengths (1.86 and 1.95 ˚ A), as do the two long axial bonds (2.02 and 2.20 ˚ A). The oxygen sites each bridge to two tellurium atoms; one site has a long and a short bond (1.86 and


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Table 1: 125 Te NMR parameters extracted from fitting spectra of various compounds as listed, using Dmfit. 15 Shifts, in ppm, are referenced to Te(OH)6 at 713 ppm. Sample α-TeO2 17 O-enriched α-TeO2 PbO-seeded α-TeO2 PbO-seeded γ-TeO2 PbO-seeded, 17 O-enriched γ-TeO2 TeO2 glasse

δiso a ∆δ b 1469(2) -738(4) 1470(2) -731(4) 1470(5) -732(4) 1509(10) -938(8) 1500(2) -941(4) 1570(10) -840(30)

ηc δ11 d δ22 d δ33 d 0.66(2) 2080(3) 1585(3) 735(3) 0.66(2) 2081(3) 1590(3) 745(3) 0.66(2) 2077(3) 1594(3) 738(3) 0.45(2) 2189(3) 1767(3) 570(3) 0.45(2) 2180(3) 1759(3) 557(3) 0.5(1) 2200(10) 1770(10) 730(10)

a

δiso = (δ11 + δ22 + δ33 )/3 ∆δ = δ33 − δiso c η = (δ22 − δ11 )/∆δ d |δ33 − δiso | ≥ |δ11 − δiso | ≥ |δ22 − δiso | e Fits include 3200 Hz Gaussian broadening (25 ppm at 9.4 T)

b

Table 2: 17 O isotropic chemical shift, quadrupolar coupling, quadrupole asymmetry, p 2 and quadrupole coupling product PQ = CQ 1 + η /3 extracted from fitting spectra of various compounds as listed, using the Dmfit software. 15 Shifts, in ppm, referenced to H2 O at 0 ppm. Sample α-TeO2

PbO-seeded γ-TeO2 TeO2 glass

Conditions δiso , ppm CQ , MHz η PQ , MHz 9.4 T, MAS 180(2) 7.39(5) 0.43(1) 7.62(5) 16.4 T, MAS 179(2) 7.39(5) 0.42(1) 7.61(5) 16.4 T, MQMAS 178(1) 7.35(5) 0.41(1) 7.55(5) 16.4 T, MAS 189(3) 6.8(3) 0.46(2) 7.0(3) 16.4 T, MQMAS 187(3) 6.9(3) 0.44(3) 7.1(3) 9.4 T, MAS 216(10)a 6.7(4) 0.4(1) 6.9(4) b 9.4 T, static 220(3) 6.2(4) 0.46(5) 6.4(4) c 16.4 T, MAS Single Gaussian 16.4 T, MQMASd Slice 272 ppm 247(8) 8.0(6) Slice 259 ppm 237(8) 7.4(6) Slice 246 ppm 223(5) 6.9 0.5 7.6(5) Slice 233 ppm 211(5) 7.0 0.4 7.4(4) Slice 220 ppm 198(5) 6.9 0.4 7.4(4) Slice 207 ppm 184(5) 6.9 0.4 7.6(5)

a

Fits include 2400 Hz broadening (44 ppm at 9.4 T) Fits include 6000 Hz broadening (110 ppm at 9.4 T) c At high field the 17 O MAS spectrum could not be decomposed into shift and quadrupole contributions. d Slice shift values of the isotropic dimension correspond to the convention used in the Dmfit software. 15 Where possible, the quadrupole product has been decomposed into quadrupole coupling and asymmetry. b


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2.20 ˚ A) while the other has more equal bonds of 1.95 and 2.02 ˚ A. The shift parameters of the Te site were determined and are reported in Table 1. The 17 O NMR spectra of γ-TeO2 are shown in Fig. 4. While the peak of the pro-

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The amount of residual α-TeO2 is too small to be detected in the MQMAS spectrum.

3.3

TeO2 glass

The x-ray diffraction pattern of TeO2 glass was shown above in Fig. 1(c). No Bragg peaks are visible. The 125 Te and 17 O NMR spectra are presented below in Figs. 5 and 6. The Raman

Figure 5: Static 125 Te NMR spectrum of TeO2 glass. The spikelet pattern is due to the echo sequence used to collect the wide powder pattern. Shown also is the fit to a single site, using 25 ppm broadening.

Figure 4: (a) Hahn echo MAS spectrum (spinning sidebands denoted by asterisks) and (b) MQMAS spectrum of 17 O in γ-TeO2 , prepared by seeding the melt with PbO. jected isotropic MQMAS dimension is approximately twice as wide as that of the α-TeO2 (see Fig. 10), we are not able to resolve two distinct oxygen sites, even in MQMAS. This may reflect the fact that the γ-TeO2 phase is not especially well crystallized (note peak widths in Fig. 3). On the other hand, calculating the expected peak separation of the two sites in the isotropic MQMAS dimension based on the First Principles calculation (see Table 3 below) reveals that the peaks are predicted to differ by only about the full width at half height (FWHH) of the experimental projection ( Dmfit convention: calculated difference 4.0 ppm, FWHH 3.7 ppm). Most likely, the signals of both sites merge in the observed peak and the parameters extracted from the 17 O spectra of γ-TeO2 should be understood as characterizing the average oxygen environment in this material. Table 2 shows that the site is deshielded compared to α-TeO2 , with a slightly smaller quadrupolar interaction in agreement with first principles calculations.

Figure 6: 17 O NMR of TeO2 glass. (a) Magic angle spinning spectrum; (b) static spectrum; (c) MQMAS spectrum and slices along the MAS dimension. spectrum of TeO2 glass has been published previously, 9 and the Raman spectrum of our sample is quite similar (Fig. 7). Our glass sample


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3.4

First principles Calculations

Calculations of NMR observables are summarized in Table 3 for the crystal model compounds studied.

4

Discussion

The structure of TeO2 glass will be considered in light of the above NMR results on both crystalline models and the glass itself, as well as a simple model of the TeO2 glass-forming ability.

4.1

Figure 7: Raman spectrum of TeO2 glass as prepared in this study.

Glass-forming Ability

The fact that TeO2 is a conditional glass former, requiring quite rapid quenching to form a bulk glass sample, could be given a structural explanation based on the Gupta-Cooper constraint model. 29,30 In this model, the degrees of freedom of a network of polyhedra are computed based on the average number of polyhedral vertices V and their connectivity C. The degrees of freedom f are computed as 1 δ(δ + 1) , (1) f =d−C δ− 2V

thus appear quite compareable to those in previous works. The 125 Te NMR spectrum of TeO2 glass may be fit adequately by a single site with a moderate amount of broadening, with parameters given in Table 1. The fit is of course (visually) improved if two sites are used, but the parameters of the two-site fits are ill-defined. Equally good fits can be obtained with the minor site intensity ranging from 1–40%, asymmetry ranging from 0.0–1.0, anisotropy ranging from 300–850 ppm, and isotropic shift ranging from 1390–1890 ppm. The 17 O MAS NMR spectrum (Fig. 6(a)) shows a single broad resonance. The MQMAS spectrum reveals a broad resonance that stretches parallel to the diagonal line, the latter indicating frequency correlations where sites with zero quadrupole interaction would resonate. The shifts of the center of gravity of each slice can be related to isotropic chemical shifts and quadrupolar products, PQ , using formulae summarized by Millot and Man. 27 The results are given in Table 2. The fitted quadrupolar parameters CQ , and η are consistent with these results and optimized with the line shapes of each MQMAS slice.

where δ = 3 for three-dimensional polyhedra and d = 3 for three-dimensional space. Silica, for instance, is described via tetrahedra with each corner doubly connected, thus V = 4 and C = 2, leading to zero degrees of freedom and hence an optimal network for condensation, without requiring long range order. In the case of TeO2 , if the traditional view of the structure is taken, V = 5 and C = 9/5, because the TeO4/2 polyhedra include a stereochemically active lone pair and hence effectively 5 vertices, with four doubly-connected and one singly-connected. The number of degrees of freedom is thus −6/25, suggesting that the glass is over-constrained and should be hard to form, as it indeed is. 30 If terminal oxygen are present in TeO2 , as posited by Barney et al., 2 they would likely enter the structure through a Te(=O)O2/2 unit, that is, Te bound to a terminal doubly-bonded oxygen and two bridging oxygen. Such a struc-


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Table 3: First-principles calculations of NMR observables, performed using the PAW and GIPAW formalisms. Input crystal structures were used without further relaxation. For tellurium sites the isotropic chemical shift, anisotropy, and asymmetry are reported, while for the oxygen sites, the isotropic chemical shift, quadrupole coupling, and quadrupole asymmetry are reported. Shifts, in ppm, are referenced to the computed α-TeO2 values, with anisotropy as defined in Table 1. CQ values in MHz, for 17 O with quadrupole moment -25.58 mb. 22 Compound Site δiso ∆δ ηCS CQ ηQ 4 α-TeO2 Te 0 -464 0.65 O 0 -7.64 0.48 γ-TeO2 6 Te 51 -767 0.63 O1 22 -7.41 0.51 O2 35 -6.80 0.56 K2 Te4 O9 28 Te1 -1 -449 0.48 Te2 280 -500 0.36 Te3 238 -772 0.48 Te4 280 -506 0.77 O1 82 7.43 0.14 O2 -6 6.84 0.67 O3 37 5.49 0.89 O4 32 6.69 0.54 O5 6 6.48 0.63 O6 19 6.44 0.57 O7 -1 7.11 0.19 O8 52 6.13 0.60 O9 -13 7.37 0.01


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ture maintains the Te valence of +4 and is formally neutral. This unit still has a lone pair and thus would be described by V = 4 and C = 3/2, for an effective f = +3/4. With both the TeO4/2 and Te(=O)O2/2 units present, the effective f is an average of the two units (the averaging procedure is described in Narayanan and Zwanziger 30 ). We find by this procedure that f = 0 when the structure consists of approximately 25% Te(=O)O2/2 units and 75% TeO4/2 units, leading to a Te coordination number of 3.75, similar to, though a bit higher than, that found experimentally and computationally. 2,3 The above argument suggests the plausibility of lower coordinate units in TeO2 glass, but does not provide evidence for their existence. First of all, any unit with coordination less than four, not only Te(=O)O2/2 , will contribute a positive f and thus relieve the over-coordination of the TeO4/2 unit. More importantly, if indeed the Te(=O)O2/2 units are present, thus relieving the over-coordination, TeO2 should be a good glass former, but experimentally it is not. The suggestion of Te(=O)O2/2 as the reason for the low coordination number in TeO2 glass thus seems to be too simplistic.

4.2

Figure 8: Comparison of 125 Te NMR spectra of α, γ and glassy TeO2 samples shown in spikelet presentations and corresponding skyline projections. Asterisks (*) indicate non-negligible signal from artifacts near the carrier frequency.

Tellurium sites in TeO2 glass

The 125 Te NMR spectra of three α, γ and glassy TeO2 samples are shown in Fig. 8 as spikelet and skyline projections. The line shape fits in Table 1 confirm the visual impression, namely that the spectra cover similar frequency ranges with α-TeO2 having the smallest anisotropy, (∆δ ≈ -734 ppm), the γ-TeO2 crystal has the largest (≈ -940 ppm), while the glass places in between (-840 ± 30 ppm, disregarding the 25 ppm applied Gaussian broadening). A detailed analysis of the chemical shift tensor components (Fig. 9) reveals that the two low field values, δ11 , and δ22 , of the γ-TeO2 crystal, and the glass are remarkably similar (δ11 ≈ 2185 ppm versus 2200 ppm, δ22 ≈ 1764 ppm versus 1770 ppm, for γ-TeO2 crystal versus TeO2 glass, respectively). The high field chemical shift component, δ33 , of the glass on the other hand is remarkably similar to that of α-TeO2

Figure 9: Comparison of the 125 Te NMR spectra of α, γ and glassy TeO2 skyline projections with the single line fit of the glass spectrum: The fit is overlayed with the glass spectrum (top) and repeated at the lower trace. Overlayed to it is the same chemical shift pattern but without the applied Gaussian broadening. The positions of the three shift principal axes are marked by the lines for easy comparison to the experimental data.


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culated to be at 765 cm−1 in a different quantum chemical study, 32 a spectral region with other, overlapping, strong features. Both quantum chemical studies used small cluster models, and relatively simple methods. We do not find the evidence from Raman spectra for this structure to be definitive, and turn to NMR for additional tests of this hypothesis. Comparison of the 17 O MQMAS spectra of the glassy, α-TeO2 and γ-TeO2 spectra and their isotropic (vertical) and MAS (horizontal) projections (Fig. 10) shows a broad 17 O resonances of the TeO2 glass spreading parallel to the diagonal line with roughly constant widths. The diagonal line represents frequency corre-

with δ33 ≈ 740 ppm versus 730 ppm, for α-TeO2 crystal versus TeO2 glass, respectively. Consequently, the isotropic chemical shifts follow the trend with δiso of the glass being the most deshielded (1570 ppm) compared to those of the γ-TeO2 crystals (δiso ≈1504 ppm) and the αTeO2 crystals (δiso ≈1470 ppm). In analyzing the 125 Te NMR spectra of various crystalline compounds with different Te-coordinations, we have found (paper submitted) that the chemical shift tensor component δ33 lies roughly along the axis through the Te lone electron pair, and depends linearly on the O-Te-O axial bond angle. δ11 is aligned roughly along the axial oxygens, and δ22 lies orthogonal to both in the equatorial plane, both linearly depending on the O-Te-O equatorial bond angle. The axial tellurium environment of α-TeO2 is comprised of two equal Te-O bonds with an open axial O-Te-O bond angle (2.08 ˚ A, and 168.5◦ ), 4 in contrast to γ-TeO2 (2.20 ˚ A and 2.02 ˚ A, and ◦ 6 153.6 ). The similarity of the δ33 chemical shift component of the glass and α-TeO2 suggests, that the axial oxygens in TeO2 glass open up to form a more nearly linear arrangement. The chemical shift components δ22 and δ33 of the glass are more similar to those of γ-TeO2 , the latter having two unequal bonds (1.86 ˚ A and ˚ 1.95 A), with a smaller angle between them (99.1◦ ), compared to α-TeO2 with two 1.90 ˚ A ◦ bonds and a 102 equatorial O-Te-O angle. It appears that in the glass structure the larger axial bond angle favors a smaller bond angle of the equatorial oxygens. Quite likely, these geometries have consequences for the Te-O bond length. Of course, in the glass, there is a high degree of disorder around these crystal values, leading to a broad distribution of sites, likely including bond angles and distances.

4.3

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Figure 10: Comparison of the 17 O MQMAS NMR spectra of α, γ and glassy TeO2 together with projections of the horizontal (MAS) dimension and the vertical, isotropic dimension. The diagonal line indicates frequency pairs, where sites without quadrupole interactions would resonate. The offset from that line is a measure of the quadrupole coupling strengths (i.e., quadrupolar product), any shift parallel to the line indicates chemical shift distributions.

Oxygen sites in TeO2 glass

lations, where sites without quadrupole interactions would resonate, while the horizontal spread of signals and their offset from the diagonal line are measures of the quadrupole coupling constant. While the α-TeO2 and γ-TeO2 MQMAS spectra overlap at the edges of the glass spectrum, most of MQMAS spectrum of TeO2 glass is deshielded. For example, the in-

As noted, the Raman spectrum for our TeO2 glass samples (Fig. 7) is quite similar to previously published work. 9 The very weak and broad shoulder centered near 883 cm−1 has been assigned to the Te=O stretch of neutral threecoordinate Te, 9 based on an earlier quantum chemical study. 31 The same structure was cal-


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tensity maximum of the glassy TeO2 MQMAS spectrum (slice at 233 ppm isotropic shift), is deshielded by 30 ppm from α-TeO2 (see Table 2), which itself is offset by about 10 ppm from the γ-TeO2 17 O chemical shift. The approximately constant horizontal width of the 17 O MQMAS spectrum of the glass indicates not much variation in the quadrupole coupling interaction, similar to those of the αTeO2 and γ-TeO2 crystals. The results of Table 2 place the quadrupolar products, PQ , of the glass to about 7.5 ± 0.5, in between those of the α-TeO2 (7.6 MHz) and γ-TeO2 crystals (7.0 MHz). While the center-of-gravity based analysis of the MQMAS spectrum cannot provide information about the asymmetry, η, simulations of the slices are only consistent with values around η = 0.4 ± 0.2 again similar to those of the α-TeO2 and γ-TeO2 crystals. These determinations of the quadrupole coupling constants indicate that the structure of the oxygen environment in the glass is not fundamentally different from those of the crystals; the distribution within the glass structure seems to mostly influence the chemical shift. To aid in the interpretation of 17 O chemical shifts we compare to computational results on K2 Te4 O9 , which shows a similar trend (Table 3): The oxygen in this compound are for the most part deshielded compared to αTeO2 , with slightly reduced quadrupole couplings. The comparison is complicated by the presence of potassium ions. Nevertheless, the most deshielded sites (O1 and O8) also exhibit a short Te-O bond of about 1.85–1.88 ˚ A and a ˚ longer Te-O bond (2.25–2.49 A). Experimentally, in crystalline P2 O5 , which quite clearly has terminal P=O bonds and bridging P-O-P linkages, it is found that the terminal oxygen are both considerably shielded, by about 45 ppm, and with a quadrupole coupling about 50% reduced from the bridging sites and very small quadrupole asymmetries, < 0.1. 33 A similar pattern was observed in sodium tellurites, where nonbridging oxygen for the most part show very small quadrupole asymmetries. 34 The oxygen in TeO2 glass show no such patterns. If the oxygen in TeO2 glass are similar to

those in γ-TeO2 and K2 Te4 O9 , in having typically one short and one long, perhaps very long, bond, one may ask at what point the long bond is so long that it should no longer be considered a bond at all. Indeed, in K2 Te4 O9 , Becker et al. identified two Te(=O)O2/2 units, with the caveat that the additional O-Te contact made by the nominally doubly-bonded oxygen is a secondary interaction that might be disregarded. 28 The QTAIM analysis performed here on the electron density computed for the K2 Te4 O9 indicates that these secondary interactions are in fact bonding. In QTAIM, the shape of the electron density is analyzed and if the density has a minimum (but does not vanish) between two nuclei, the minimum is termed a bond critical point. The electron density at a bond critical point is a rough measure of bond strength, as higher density indicates stronger bonding. The bond critical points found in K2 Te4 O9 indicate that the Te-O interactions at 2.40–2.50 ˚ A are in fact bonding, with strength about 25% that of the shortest Te-O bonds. The electron density at the critical points is plotted as a function of bond length in Fig. 11, relative to the intensity of the shortest bond. There is a noticeable gap in lengths at about Relative critical point electron density

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

0.1

0.01

1.5

2

2.5

3

3.5

4

Bond length, Å

Figure 11: Electron density at bond critical points in K2 Te4 O9 , as determined from the electron density computed in Vasp and analyzed with the Critic2 software. The density valued as scaled to the value in the shortest and strongest Te-O bond. 2.50 ˚ A at which point the K-O interactions begin to appear. All the shorter bonds are Te-O bonds, and as noted even the longest are 25% strength of the shortest. We would thus argue


11


that such secondary interactions still are bonding, and no oxygen in K2 Te4 O9 can be cleanly identified as terminal.

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Orman, R. G.; Feller, S. Terminal Oxygens in Amorphous TeO2 . J. Phys. Chem. Lett. 2013, 4, 2312–2316. (3) Gulenko, A.; Masson, O.; Berghout, A.; Hamani, D.; Thomas, P. Atomistic Simulations of TeO2 -Based Glasses: Interatomic Potentials and Molecular Dynamics. Phys. Chem. Chem. Phys. 2014, 16, 14150–14160.

Conclusions

From the above discussion it appears that TeO2 glass has a structure best described (in traditional terms) as made up of TeO3+1 units, that is, units with four oxygen near each tellurium but for which 3 bonds are relatively short (1.85– 2.10 ˚ A) and the fourth is longer, say 2.2– ˚ 2.4 A. A strict division into well-defined TeO4/2 and Te(=O)O2/2 units is probably an oversimplification. Such units would include terminal oxygen, for which the NMR shift should be distinctly shielded relative to α-TeO2 and with very small quadrupole asymmetries, but this was not observed. In contrast, the oxygen sites are deshielded, indicating sharing of electron density with several neighboring tellurium. We note that this view of the TeO2 glass structure is broadly consistent with the picture put forward from modeling studies by Gulenko et al., where the glass is described as resembling a disordered γ-TeO2 crystal. 3 They showed as well that this model leads to a coordination number less than four, as observed experimentally. 2 Our findings are consistent with this model in that both the Te and O sites more closely resemble γ-TeO2 sites than other likely candidates.

(4) Lindqvist, O. Refinement of the Structure of α-TeO2 . Acta Chem. Scand. 1968, 22, 977–982. (5) Beyer, H. Verfeinerung der Kristallstruktur von Tellurit, dem rhombischen TeO2 . Z. Kristall. 1967, 124, 228–237. (6) Champarnaud-Mesjard, J.; Blanchandin, S.; Thomas, P.; Mirgorodsky, A.; Merle-Mejean, T.; Frit, B. Crystal Structure, Raman Spectrum and Lattice Dynamics of a new Metastable Form of Tellurium Dioxide: γ-TeO2 . J Phys Chem Solids 2000, 61, 1499–1507. (7) McLaughlin, J.; Tagg, S.; Zwanziger, J.; Haeffner, D.; Shastri, S. The Structure of Tellurite Glass: A Combined NMR, Neutron Diffraction, and X-Ray Diffraction Study. J Non-Cryst Solids 2000, 274, 1–8. (8) McLaughlin, J. C.; Tagg, S.; Zwanziger, J. The Structure of Alkali Tellurite Glasses. J. Phys. Chem. B 2001, 105, 67–75.

Acknowledgement We would like to acknowledge funding and resources from NSERC Grant RGPIN 261987 and NSF Grant DMR1407404, and Compute Canada for computational resoucres. We thank Prof. Alex Speed for the help in synthesizing 17 O-enriched TeO2 crystals.

(9) Kalampounias, A.; Tsilomelekis, G.; Boghosian, S. Glass-Forming Ability of TeO2 and Temperature Induced Changes on the Structure of the Glassy, Supercooled, and Molten States. J. Chem. Phys. 2015, 142, 154503. (10) Hart, R. T.; Zwanziger, J. W. 17 O NMR Spectroscopy of α-TeO2 and Na2 TeO3 . J. Am. Ceram. Soc. 2005, 88, 2325–2327.

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(29) Gupta, P.; Cooper, A. Topologically Disordered Networks of Rigid Polytopes. J. Non-Cryst. Solids 1990, 123, 14–21. (30) Narayanan, R. A.; Zwanziger, J. W. The Glass Forming Ability of Tellurites: A Rigid Polytope Approach. J Non-Cryst Solids 2003, 316, 273–280. (31) Uchino, T.; Yoko, T. Ab initio Cluster Model Calculations on the Vibrational Frequencies of TeO2 Glass. J. Non-Cryst. Solids 1996, 204, 243 – 252. (32) Sokolov, V. O.; Plotnichenko, V. G.; Koltashev, V. V.; Dianov, E. M. On the Structure of Tungstate–Tellurite Glasses. J. Non-Cryst. Solids 2006, 352, 5618– 5632. (33) Cherry, B. R.; Alam, T. M.; Click, C.; Brow, R. K.; Gan, Z. Combined ab initio Computational and Solid-State 17 O MAS NMR Studies of Crystalline P2 O5 . J. Phys. Chem. B 2003, 107, 4894–4903. (34) Hart, R.; Zwanziger, J.; WernerZwanziger, U.; Yates, J. On the Spectral Similarity of Bridging and Nonbridging Oxygen in Tellurites. J. Phys. Chem. A 2005, 109, 7636–7641.


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Graphical TOC Entry

125 Te

and 17 O solid-state nuclear magnetic resonance spectroscopy are used to study the short-range structure of TeO2 glass. It is found that the structure is best described as a distorted form of the metastable γ-TeO2 phase, in which Te on average have three covalently bound oxygen and the fourth much more weakly bound.


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Short-Range Structure of TeO2 Glass - American Chemical Society

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