Mixed Network Former Effects in Tellurite Glass Systems: Structure

Article pubs.acs.org/JPCC

Mixed Network Former Effects in Tellurite Glass Systems: Structure/ Property Correlations in the System (Na2O)1/3[(2TeO2)x(P2O5)1−x]2/3 Dirk Larink,† Matthias T. Rinke,† and Hellmut Eckert*,†,‡ †

Institut für Physikalische Chemie, WWU Münster, Corrensstrasse 30, D48149 Münster, Germany Instituto de Física Sao Carlos, Universidade de Sao Carlos, CP 369, Sao Carlos, SP Brasil 13560-970

‡

S Supporting Information *

ABSTRACT: Mixed network former glasses in the system (Na2O)1/3[(2TeO2)x(P2O5)1−x]2/3 have been prepared and characterized by DSC, density, and electrical conductivity measurements. Within the compositional region 0 < x < 0.5, the electrical conductivity shows a positive mixed network former effect, i.e., enhanced glass transition temperatures and ionic conductivities in relation to the linearly interpolated values between the corresponding binary sodium tellurite and sodium phosphate systems. The structural origins of this effect have been studied by 31 P, 23Na, and 125Te high-resolution and dipolar solid-state nuclear magnetic resonance (NMR) techniques, O-1s, Na-1s, Te-3d5/2, and P-2p X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. A quantitative structural model has been developed that is consistent with all of the experimental data and provides a detailed description of network modification processes, interactions, and connectivities. For x values 99.8%, Roth), and TeO2 (>99.99%, Alfa Aesar). The educts were dried for at least 48 h at 150 °C, ground together in an argon-filled glovebox, and heated in a silica container. After a few minutes at 600 °C (following decomposition of the Na2CO3), the container was closed with a silica lid, which was weighted down further by a piece of copper to avoid vaporization losses of potentially volatile P2O5. The melt was heated for 30 min at a temperature of 800−900 °C depending on composition and subsequently cast into a steel mold kept at 150 °C. Subsequent to this treatment the glasses were annealed at temperatures about 5 °C below their respective glass transition temperatures for 2 h and thereafter cooled to room temperature at a rate of 0.5 K/min. Evaporation losses were found to be less than 2%, indicating that target and actual compositions are in good agreement; this was confirmed further by XPS analysis. FTIR spectra (obtained on a Nicolet attenuated reflectance spectrometer) indicate the absence of undecomposed residual carbonates and give no evidence for the incorporation of silica. A Netzsch STA409 thermal analyzer was used to determine the glass transition temperatures by differential thermal analysis, using a heating rate of 10 K/min. The density of the glasses was measured at 25 °C with a pycnometer using ethanol as a solvent. Raman spectra were recorded with a Jobin Yvon Horiba Raman spectrometer using a Nd/YAG laser with a wavelength of 532.18 nm. X-ray photoelectron spectra (XPS) were measured using an AxisUltra spectrometer (KRATOS, Manchester, UK) in ultrahigh vacuum (5 × 10−9 mbar). Immediately before the measurement, all glasses were powdered and pressed into small copper plates (in order to get a smooth surface), within an Ar-filled glovebox. This procedure had been shown previously to result in quantitatively reliable information representative of the bulk in other phosphate and borophosphate glasses.37,44 Al−Kα radiation (ℏν = 1486.6 eV) was used with a 12 kV accelerating voltage and 10 mA filament current. Surface charges were neutralized by a corresponding device operated with a filament current of 1.8 A, a charge balance of 2.3 V, and a filament bias of 1.0 V. For narrow scans with high-energy resolution, a pass energy of 20 eV was applied. Further data analysis was performed from the peak shape using the software CasaXPS Version 2.2.0 (by Neal Fairley). The C-1s peak arising from pump oil impurities was used as an internal reference, and the binding energy of this C-1s peak was assumed to be 284.6 eV referenced to its Fermi level. The raw spectral data were subsequently smoothed and fitted using Gaussian line shapes. 17541

DOI: 10.1021/acs.jpcc.5b04074 J. Phys. Chem. C 2015, 119, 17539−17551

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The Journal of Physical Chemistry C lengths on the solid sample used for the dephasing and the recoupled nuclei were 7.5 to 8.0 μs. Static 23Na spin echo decay measurements were carried out on a Bruker-Avance-DSX-400 spectrometer at a magnetic flux density of 9.40 T with a respective resonance frequency of 105.805 MHz, following the methodology developed in ref 54. To eliminate the effect of ionic motion on the data, the experiments were conducted at a temperature of 160 K, a temperature for which it was previously shown that ionic motion does not influence the spin echo decay rates in a wide range of binary sodium oxide-containing glasses.54 Soft pulses with a nutation frequency near 33 kHz on the solid sample, corresponding to selective excitation of the central transition, were applied using repetition times of 1−10 s. Fitting of the echo decay for small evolution times (2τ ≤ 200 μs) to a Gaussian function yields the homonuclear dipolar second moment M2(23Na−23Na).

■

RESULTS Macroscopic Properties. Table 1 summarizes the compositions and all the bulk property measurement results and the quantities derived from them. Figure 2 reveals a distinctly

Figure 2. Glass transition temperatures in (Na2O)1/3[(2TeO2)x(P2O5)1−x]2/3 glasses as a function of composition.

nonlinear dependence of Tg on x with a clear maximum for x = 0.5. The molar volume passes through a minimum near x = 0.4 (Figure 3a), and correspondingly a maximum is observed near this composition for the cation number density (see Figure 3b). Figure 4a summarizes dc-electrical conductivity data extrapolated from the low-frequency part of the impedance measurements, while the corresponding activation energies are compiled in Figure 4b. The data reveal a clear positive mixed network former effect in the compositional region 0 ≤ x < 0.6, whereas at higher Te contents the values are close to those expected from linear interpolation of the endmember values. The electrical conductivity passes through maximum values for x = 0.2 to 0.3, and a corresponding minimum is observed for the activation energies associated with long-range transport. Toward increasing TeO2 contents, electrical conductivities decrease by several orders of magnitude. Completely analogous trends were observed for the more alkali-rich glasses of the system (Na2O)0.5[(P2O5)1−x(2TeO2)x]0.5 system investigated in ref 17. Overall the extrema observed near x = 0.3 to 0.4 for the glass transition temperature, the molar volume, and the electrical conductivity appear to be related to each other, pointing toward a common structural origin. Raman Scattering. Figure 5 shows Raman spectroscopic data. The spectrum of the pure sodium ultraphosphate glass is

Figure 3. (Top) bulk densities ρ, (middle) molar volumes Vm, (bottom) cation number concentrations NV(Na+) of (Na2O)1/3[(2TeO2)x(P2O5)1−x]2/3 glasses as a function of composition.

in excellent agreement with the data of Hudgens et al.19 It is dominated by three bands near 665, 1160, and 1320 cm−1. The band at 665 cm−1 is assigned to the symmetric P−O−P stretching mode involving bridging oxygen atoms. This band has a shoulder toward higher wave numbers which can be assigned to the corresponding asymmetric stretching mode.19 The band at 1160 cm−1 and its weak high-frequency shoulder near 1210 cm−1 are associated with the symmetric and antisymmetric P−O stretching modes involving the nonbridging oxygen atoms of the P(2) units, whereas the band at 1320 cm−1 must be attributed to the symmetric PO stretching mode of P(3) units. The Raman spectrum of the pure sodium tellurite glass is in excellent agreement with the data published by Heo30 and Sekiya.31 Three bands can be observed: A band near 666 cm−1 may be assigned to asymmetric Te−O stretching modes of Te4 units, while the shoulder near 760 cm−1 is generally attributed to vibrations of nonbridging oxygen atoms attached to anionic three-coordinate Te3 units. The assignment of the third 17542


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The Journal of Physical Chemistry C

Figure 5 reveals important compositional trends for the vibrational modes associated with the phosphate species. The shift of the band from 1320 toward 1180 cm−1 can be attributed to the continuous transformation of P(3)0Te to P(3)mTe units (0 < m < 3). A similar trend is observed for the corresponding P(2) mode which gradually shifts toward 1050 cm−1. Finally, starting with x = 0.6 a new band appears at 960 cm−1 and gains in intensity until it dominates the phosphate-based Raman scattering in the x = 0.9 sample. This band can be assigned to the PO3 stretching modes of P(1)1Te units. The compositional evolution of the tellurium-based vibrational modes is more difficult to follow, because of serious peak overlap. It is clear, however, that the feature near 760 cm−1, which is associated with the nonbridging oxygen atoms of Te3 units, dramatically diminishes in intensity with decreasing Te content, whereas the band at 666 cm−1 associated with Te4 units shows less diminution. In part this may be the consequence of peak overlap with the P−O−P stretching mode in the same wavenumber region. Finally, a number of new bands are observed, which are not present in the spectra of the binary glasses: Samples with compositions 0.4 < x < 0.8 show a welldefined Raman band near 830 cm−1, a feature that had been observed (but not assigned) in our previous study of the (NaPO3)1−x(TeO2)x glass system.44 This band must be assigned to a different type of Te−O stretching mode, presumably one that is associated with nonbridging oxygen atoms. Finally, glasses within the composition range 0.1 < x < 0.8 show a new feature near 580−620 cm−1. We suggest that this band arises from vibrations of bridging oxygen atoms involved in P−O−Te linkages. X-ray Photoelectron Spectroscopy. Figure 6 summarizes the Na-1s, Te-3d, and P-2p X-ray photoelectron spectra. The binding energies extracted from these data are compiled in Table 2. The P-2p binding energies show a monotonic decrease, reflecting a gradual increase in local electron density at the P site, which can be associated with the gradual transformation of P−O−P linkages into P−O−Te linkages and P−O bonds with nonbridging oxygen atoms as x is increased. Likewise, the Te-3d binding energies decrease with increasing x. On the basis of the Raman results, we suggest that the spectroscopic response of the Te atoms in glasses with low x values is dominated by Te4 species involved in Te−O−P linkages. These have lower local electron densities (and hence higher binding energies) than either Te4 species involved in Te−O−Te linkages or anionic Te3 species carrying nonbridging oxygen atoms. As the latter units become more abundant in high-tellurium glasses, the observed spectroscopic trend can be rationalized. Finally, the Na-1s data also show a gradual decrease from 1071.5 to 1070.4 eV, reflecting their increasing interaction with nonbridging oxygen atoms associated with the anionic Te3 units as the tellurium content of the glasses is increased. Figure 7 shows the compositional evolution of the O-1s spectra, giving a rather detailed account of the oxygen speciation in these glasses. The pure sodium ultraphosphate glass shows two resolved signals at 533.6 and 531.6 eV arising from the bridging and the nonbridging oxygen atoms; the area ratio of 46:54 is very close to the theoretically expected one (45:55) at this composition, thereby validating the analysis. For the pure sodium tellurite glass (x = 1.0), only one signal at 529.9 eV can be observed. This finding is good agreement with previous XPS studies of alkali tellurite glasses which have noted the lack of spectroscopic differentiation between the bridging and the

Figure 4. Results from impedance measurements on (Na2O)1/3[(2TeO2)x(P2O5)1−x]2/3 glasses: (a) temperature-dependent electrical conductivities log σ as a function of composition, (b) activation energies EA derived from Arrhenius plots of dc-electrical conductivities as a function of composition.

Figure 5. Raman scattering of (Na2O)1/3[(2TeO2)x(P2O5)1−x]2/3 glasses.

band near 460 cm−1 is less certain but believed to reflect vibrations of bridging oxygen atoms within Te−O−Te linkages. 17543


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Figure 6. Na-1s (left), Te-3d5/2 (middle), and P-2p (right) X-ray photoelectron spectra of (Na2O)1/3[(2TeO2)x(P2O5)1−x]2/3 glasses.

Table 2. Binding Energies (BE) and Line Widths (LB) of the Individual Na-1s, Te-3d, and P-2p XPS Line Shapes Na-1s x x x x x x x x x x x

= = = = = = = = = = =

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Te-3d3/2

Te-3d5/2

P-2p

BE (±0.2)/ eV

LB (±0.2)/eV

BE (±0.2)/eV

LB (±0.2)/eV

BE (±0.2)/eV

LB (±0.2)/eV

BE (±0.2)/eV

LB (±0.2)/eV

1071.5 1071.3 1071.2 1071.2 1071.1 1070.9 1070.9 1070.7 1070.7 1070.5 1070.4

1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.5

587.8 587.4 587.3 587.1 586.9 586.8 586.5 586.4 586.1 586.0

1.7 1.6 1.6 1.6 1.5 1.5 1.5 1.5 1.5 1.5

577.4 577.0 576.9 576.7 576.5 576.4 576.1 576.0 575.8 575.6

1.8 1.6 1.6 1.5 1.5 1.5 1.5 1.5 1.5 1.5

135.0 134.6 134.2 134.0 133.7 133.5 133.3 133.0 132.9 132.7 -

1.9 1.8 1.8 1.8 1.8 1.8 1.8 1.8 1.8 1.8 -

nonbridging oxygen atoms in these systems.55 Thus, only the combined area fraction of NBO(Te) and Te−O−Te species can be determined. With the line-shape parameters of these three components known (Table 3), the position, width, and area fraction of the fourth contribution, assignable to the bridging oxygen atoms involved in P−O−Te linkages, can be extracted by fitting. A more detailed analysis shows that weak, but uniform monotonic trends in binding energies have to be assumed for all four line-shape components. These trends can be understood to be a consequence of the fact that the second coordination spheres of the oxygen atoms involved are subject of a continuous evolution as a function of x, because the connectivities of the P- and Te-based polyhedra change monotonically as a function of composition. Figure 8 summarizes the oxygen atomic speciation derived from XPS in this manner. 31 P MAS NMR. Figure 9 summarizes the 31P MAS NMR results. While there is a clear continuous evolution in the spectra toward higher resonance frequencies with increasing tellurium substitution level, the spectroscopic resolution is generally poor, because of multiple overlapping signal contributions. Fitting was performed on the basis of complementary MAS NMR experiments, obtained by the Refocused INADEQUATE method.46 This technique acts as a double quantum filter based on the existence of indirect magnetic dipole−dipole interactions occurring between phosphorus atoms joined by a P−O−P linkage. Consequently, P(3)3Te, P(2)2Te, P(1)1Te, and P(0) units, which do not possess P−O−P linkages, are not detected

in such experiments. Thus, the contributions of the remaining phosphorus species appear more distinct in these spectra and thus facilitate line-shape deconvolutions (see Figure 9, left part). On the basis of the detailed comparison of the total MAS NMR line shapes with the somewhat simplified line shapes measured with the double quantum filtering (Refocused INADEQUATE) method, it is possible to obtain the individual line-shape parameters and the fractional areas of all the P(n)mTe units contributing to the overall 31P resonance. This information is summarized in Table 4. Note that substituting a P−O−P linkage by a P−O−Te linkage results in a high-frequency shift of about 10 ppm for a given P(n) species, consistent with results found in the literature for the related zinc tellurophosphate glass system.32 As indicated further below, these data contain useful information regarding (1) the competition of the two network formers P2O5 and TeO2 for the network modifier, i.e., the extent to which each of the two network former components is converted into an anionic species, and (2) the connectivity distribution in the network, i.e., the competition between homoatomic (P−O−P and Te−O−Te) linkages versus heteroatomic (P−O−Te) linkages in the network structure. In addition, inferences about the type of anionic tellurium species are possible. 125 Te NMR CPMG-NMR. Figure 10 summarizes the results from 125Te NMR. Shown are static spikelet spectra obtained after Fourier transformation of the full CPMG spin echo trains.47 This acquisition mode was essential for acquiring the 17544


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Table 3. Binding Energies, Line Widths, and Areas (in %) of the Individual O-1s Line-Shape Components Extracted from Least-Squares Fitting of the XPS Signals type of oxygen x = 0.0 P−O−P NBO x = 0.1 P−O−P P−O−Te NBO NBO(Te)/Te−O−Te x = 0.2 P−O−P P−O−Te NBO NBO(Te)/Te−O−Te x = 0.3 P−O−P P−O−Te NBO NBO(Te)/Te−O−Te x = 0.4 P−O−P P−O−Te NBO NBO(Te)/Te−O−Te x = 0.5 P−O−P P−O−Te NBO NBO(Te)/Te−O−Te x = 0.6 P−O−P P−O−Te NBO NBO(Te)/Te−O−Te x = 0.7 P−O−P P−O−Te NBO NBO(Te)/Te−O−Te x = 0.8 P−O−P P−O−Te NBO NBO(Te)/Te−O−Te x = 0.9 P−O−P P−O−Te NBO NBO(Te)/Te−O−Te x = 1.0 NBO(Te)/Te−O−Te

Figure 7. O-1s X-ray photoelectron spectra of (Na2O)1/3[(2TeO2)x(P2O5)1−x]2/3 glasses.

Figure 8. Oxygen speciation of (Na2O)1/3[(2TeO2)x(P2O5)1−x]2/3 glasses extracted from XPS data.

binding energy line width (±0.2)/eV (±0.2)/eV 533.6 531.6 533.3 532.2 531.2 530.5 533.0 532.0 531.0 530.6 533.0 531.8 531.0 530.6 533.0 531.7 530.8 530.5 532.9 531.5 530.7 530.4 532.9 531.5 530.7 530.3 532.8 531.2 530.6 530.1 532.7 531.3 530.5 530.1 532.7 531.5 530.6 529.9 529.9

1.7 1.5 1.6 1.6 1.4 1.6 1.6 1.6 1.4 1.6 1.5 1.6 1.4 1.4 1.6 1.6 1.4 1.4 1.6 1.6 1.4 1.4 1.6 1.6 1.4 1.4 1.6 1.6 1.4 1.4 1.6 1.6 1.4 1.5 1.6 1.5 1.4 1.5 1.5

area (±5) (in %) 46.3 (45.55) 53.7 (55.55) 35.8 12.3 50.2 1.8 28.0 20.2 48.2 3.6 20.7 27.2 42.2 9.9 12.5 32.6 38.0 16.9 8.4 29.1 32.2 30.4 4.5 27.7 29.3 38.6 2.0 21.6 22.8 53.6 3.0 13.5 15.7 67.8 1.8 4.3 11.5 82.4 100.0

frequencies much lower than that of the Te30 units in Na2TeO3. Part of the compositional dependence seen in Figure 12 may be due to a continuous decrease in the average Te coordination number with increasing Te content. Second, it can be expected that the continuous trend in the 125Te chemical shift reflects the gradual replacement of Te−O−P connectivity (at low x values) by Te−O−Te connectivity (at high x values), and additionally there may be a compositional dependence of the balance between neutral and anionic tellurium species. This topic will be discussed in more detail below. 23 Na MAS NMR Spectra and 23Na{31P} REDOR. Figure S-3 (Supporting Information) shows the 23Na MAS NMR spectra, indicating the typical asymmetric line shapes reflecting secondorder quadrupolar effects in the presence of electric field gradient distributions. As revealed by Table 5, the centers of gravity of these signals remain more or less constant at low Te content, whereas above x = 0.4 there is a continuous shift toward higher resonance frequencies with increasing tellurium content. As this

necessary signal-to-noise ratio in these studies. Included in Figure 10 are further spectra of Na2TeO3 and TeO2, which represent three-coordinate Te03 units and four-coordinate Te44 groups, respectively. The envelope of the spikelet pattern forms the static anisotropic powder pattern, whose form is dominated by the magnetic shielding anisotropy. From the center of gravity, the isotropic chemical shift is determined as 2590 ± 20 ppm and 2900 ± 20 ppm vs CdTe for TeO2 and Na2TeO3, respectively. For the glasses, the average isotropic chemical shift increases continuously with increasing tellurium content, however, Figure 11 clearly illustrates the existence of two distinct regimes: a steep change in chemical shift within the composition range 0 ≤ x ≤ 0.5, and a more gradual increase toward higher Te contents. These trends can be associated with various effects, which are most likely superimposed upon each other: first of all, the model compound results indicate that the 125Te chemical shift depends on coordination number: the Te44 units in TeO2 resonate at 17545


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Table 4. Deconvolution of the 31P MAS-NMR Spectra in (Na2O)1/3[(2TeO2)x(P2O5)1−x]2/3 Glassesa

x = 0.0 x = 0.1

x = 0.2

x = 0.3

x = 0.4

Figure 9. 160.5 MHz 1D 31P refocused INADEQUATE- (left) and 31P MAS NMR-spectra (right) of (Na2O)1/3[(2TeO2)x(P2O5)1−x]2/3 glasses. Dashed curves indicate individual fitting components and red curves are the simulated total line shapes based on these components.

x = 0.5

compositional dependence could either reflect a trend in isotropic chemical shifts or nuclear electric quadrupole coupling constant, the technique of triple-quantum excitation was used to separate these parameters on selected samples. The results, which are included in Table 5, clearly illustrate that the compositional trends are dominated by the 23Na isotropic chemical shift: up to x = 0.4, δCSiso(23Na) remains approximately constant, whereas at higher x values there is a monotonic increase in this parameter. The latter trend presumably reflects the increasing participation of oxygen atoms bound to anionic tellurium species in the first coordination sphere of the sodium ions. These results are fully reinforced by the 23Na{31P} REDOR results shown in Figure 12: up to x = 0.4, the second moment characterizing the magnetic dipolar coupling strength between these two nuclei do not depend on composition, and its absolute values suggest that the first coordination sphere of sodium is largely dominated by phosphate ions. For compositions beyond x = 0.4, however, a nearly linear decrease of M2(23Na{31P}) can be observed, suggesting significant participation of oxygen atoms bound to anionic tellurium species in the first coordination sphere of the sodium ions. Table 5 also summarizes the M2(23Na−23Na) values determined from spin echo decay spectroscopy at 160 K. As illustrated in Figure S-4 (Supporting Information), the weak dependence of this parameter on x can be rationalized to originate from variations in the cationic density of these glasses. No systematic trends are observed, indicating that there are no pronounced tendencies toward cation clustering in these glasses.

x = 0.6

x = 0.7

x = 0.8

x = 0.9

unit

δiso (±0.5)/ ppm

G/L

fwhm (±0.5)/ ppm

area (±2) (%)

P30Te P20Te P30Te P31Te P20Te P32Te P31Te P20Te P32Te P21Te P31Te P20Te P32Te P21Te P20Te P32Te P21Te P33Te* P22Te* P32Te P21Te P33Te* P22Te* P32Te P21Te P33Te* P22Te* P10Te P11Te* P32Te P21Te P33Te* P22Te* P10Te P11Te* P32Te P21Te P33Te* P22Te* P10Te P11Te* P21Te P33Te* P22Te* P10Te P11Te*

−40.5 −25.3 −37.5 −32.1 −24.1 −20.3 −30.0 −23.3 −19.3 −13.0 −29.5 −22.5 −18.3 −12.3 −22.0 −18.1 −11.8 −9.4 −5.5 −17.0 −10.7 −8.7 −4.5 −15.8 −9.7 −8.0 −3.5 −2.0 4.0 −14.8 −8.4 −7.0 −2.5 −1.5 4.0 −13.0 −7.6 −6.0 −2.0 −1.2 4.0 −6.6 −5.0 −1.5 −1.0 4.5

0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9

13.6 10.4 13.5 11.5 10.0 9.0 12.5 10.0 9.0 8.0 12.5 10.0 9.5 8.5 10.0 9.5 8.5 8.0 8.0 9.5 8.5 8.0 8.0 9.5 8.5 8.0 8.0 8.0 8.0 9.5 8.5 8.0 8.0 8.0 8.0 9.5 8.5 8.0 8.0 8.0 8.0 8.5 8.0 8.0 8.0 8.0

50.6 (50.0) 49.4 (50.0) 16.1 26.1 52.9 4.9 24.2 46.7 24.1 5.1 6.1 31.9 42.4 19.5 7.1 38.4 41.7 5.1 7.7 26.0 49.0 7.4 17.6 10.4 40.1 14.6 32.8 1.3 0.9 3.2 26.3 19.8 45.7 2.7 2.2 2.4 7.5 22.5 40.4 22.5 4.9 8.0 5.1 22.7 51.5 12.7

The positions δiso and peak widths (FWHM) of those components observed in the 1D 31P refocused INADEQUATE experiments are also used for fitting the standard 31P MAS-NMR spectra. G/L: Gaussian fraction of the Gauss/Lorentz peak shape used for the deconvolution. Asterisks mark species not observed in refocused INADEQUATE experiments. a

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DISCUSSION Network Modification. On the basis of the above-described analyses, we can provide a comprehensive atomic-level description of (Na2O)1/3[(2TeO2)x(P2O5)1−x]2/3 glasses and develop structural rationales for the compositional dependences of the physical properties noted in Figures 2−4. The 31P MAS NMR data indicate the presence of neutral P(3) species and anionic P(2) and P(1) species, and all of the spectroscopic evidence presented here indicates that tellurium also occurs to some extent in an

anionic form, called Te(a) hereafter. Indeed, this extent can be deduced from the charge balance constraint: [Na +]total = [P(2)] + 2[P(1)] + [Te(a)]

The total sum of positive charges equals the total sum of negative charges. In this analysis we assume for the sake of simplicity that no doubly charged anionic tellurium species (TeO32− ions) are 17546


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Figure 10. Static 125 Te-CPMG-NMR spectra of (Na 2 O) 1/3 [(2TeO2)x(P2O5)1−x]2/3 glasses and of crystalline Na2TeO3 and α-TeO2. Figure 12. 23 Na{ 31 P} REDOR NMR results of (Na 2 O) 1/3 [(2TeO2)x(P2O5)1−x]2/3 glasses. Top: representative REDOR curves for samples with x = 0.2 and 0.8, including parabolic fits to the initial data range (ΔS/S0 ≤ 0.2). Bottom: Dependence of M2(23Na{31P}) on composition.

assumption that all the neutral species are represented by Te44 units, and all the anionic species are represented by Te13 units. This model can be again illustrated with the example x = 0.5: at this composition, the 31P MAS NMR analysis indicates 13 P(3)2Te, 4 P(3)3Te, 24 P(2)1Te, and 9 P(2)2Te units, which comprise oxygen environments in the following proportions: 83 NBO(P), 19 P−O−P, 80 P−O−Te. With regard to the tellurium species, the 33 neutral species (all assumed to be Te44 units) and the 17 anionic species (all assumed to be Te13 units) bind a total of 34 NBO(Te) and (149−80)/2 = 35 Te−O−Te species. On this assumption, the oxygen speciation deduced from the 31P MAS NMR data is shown in Figure 14a. As noted in part b of this figure, this speciation (summing up again the NBO(Te) and Te−O−Te species, which are indistinguishable by XPS) is found to be in excellent agreement with the oxygen speciation deduced from O-1s XPS data. Figure 13 consistently indicates a greater preference of the network former P2O5 over the network former TeO2 to be modified by alkali ions. This preference is enhanced in particular in the region of high TeO2 contents, where a significant fraction of dianionic P(1) units are being formed. Network Former Connectivity Preferences. Finally, Figure 15 compares the concentrations of homo- (P−O−P and Te−O−Te) and heteroatomic (P−O−Te) connectivities with those predicted from a random linkage model based on the quantitative network former speciations of Figure 13, assuming no bonding preferences. Clearly, two distinct compositional regions can be noted: for x < 0.5, there is a clear preference for heteroatomic bond formation, whereas for x ≥ 0.5 the connectivity distribution is well approximated by a

Figure 11. Isotropic 125Te chemical shifts (vs CdTe) determined from the center of gravity as a function of composition in (Na2O)1/3[(2TeO2)x(P2O5)1−x]2/3 glasses.

present. (Their occurrence indeed appears unlikely in view of the relatively low Na concentrations). Based on these constraints, the quantitative network former speciations of phosphorus and tellurium can be deduced at all compositions, and this result is shown in Figure 13. It is best to illustrate this analysis with an example for a glass with x = 0.5, containing 50 phosphorus and 50 tellurium network former atoms as well as 50 Na+ ions. On the basis of the 31P MAS NMR analysis, we find that 17 of these 50 phosphorus species are neutral P(3) units while 33 of them are anionic P(2) units. There are no P(1) units at this glass composition. Consequently, charge balance dictates that 17 of the 50 tellurium species must be anionic and hence 33 are neutral. Whether these anionic species are Te34 or Te13 units and whether the remaining neutral tellurium species are Te44 or Te23 species cannot be resolved at the present stage. However, as discussed below, a model using the complementary strengths of 31 P MAS NMR and XPS data can be developed based on the 17547


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Table 5. 23Na NMR Parameters of (Na2O)1/3[(2TeO2)x(P2O5)1−x]2/3 Glasses: Center of Gravity of the MAS-NMR Signal δCG, Isotropic Chemical Shift δisoCS, Second-Order Quadrupolar Effect SOQE, and Second Moments Characterizing the Strengths of the 23Na−31P and 23Na−23Na Magnetic Dipole−Dipole Interactions x

δCG(23Na) (±0.5)/ ppm

δisoCS(23Na) (±0.5)/ ppm

SOQE(23Na) (±0.2)/ MHz

M2(23Na{31P})/106 rad2·s−2 (±10%)

M2(23Na{23Na})/106 rad2·s−2 (±10%)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

−11.2 −10.6 −11.4 −10.7 −9.8 −8.5 −6.4 −4.8 −3.3 −2.0 0.0

−7.0 −7.1 −7.3 +2.0 +6.7

1.6 1.5 1.5 1.7 1.6

3.3 3.0 3.0 3.0 3.0 2.5 2.1 1.7 1.2 0.7 -

4.1 4.1 4.6 3.9 3.9 3.6 3.2 3.4 3.0 3.2 3.4

Figure 13. Network former speciation in (Na2O)1/3[(2TeO2)x(P2O5)1−x]2/3 glasses. A representative error bar is shown on the top right.

random linkage model. Comparing this figure with Figure 13, we note that the presence of P(3) units in the glasses may be the key factor for a preferred association of phosphorus with tellurium. A strong interaction between P(3) and Te is also noted in the substantial shift of the PO stretching mode from 1320 cm−1 (P(3)0Te) to 1180 cm−1 (P(3)3Te) at the highest tellurium contents. Relation to Physical Properties. Finally, we will attempt to rationalize the trends denoted in Figures 2−4 in terms of the structural information discussed previously. Contrary to the situation in other mixed network former systems, in the present system, a correlation with the average number of bridging oxygen atoms per network former unit cannot be established, as the coordination numbers of the anionic and neutral tellurium species are not known with certainty. We note, however, that in the composition range in which the clearest preference for heteroatomic P−O−Te linkages is found, the glass transition temperature increases steeply as a function of x and a minimum in molar volume is observed. Figure 16 indicates that Tg is strongly correlated with the fraction of P−O−Te linkages in the bridging oxygen inventory. Thus, we can envision the Te species to cross-link the phosphate chains, resulting in a higher packing density. In addition, the electronegativity difference between P and Te implies a polar character of P−O−Te linkages, making them stronger than homoatomic P−O−P and Te−O−Te linkages. A similar correlation of Tg with the fractional contribution of B−O−P linkages to the bridging oxygen inventory has been previously observed by us in alkali-modified borophosphate glasses.31

Figure 14. Oxygen speciation in (Na2O)1/3[(2TeO2)x(P2O5)1−x]2/3 glasses: (a) speciation deduced from the 31P MAS NMR data summarized in Table 4. (b) Consistency of speciations deduced from 31 P MAS NMR and XPS assuming that neutral Te44 and anionic Te13 are the only tellurium species present.

In a similar vein, the preferential formation of P(3)−O−Te linkages can be related to the positive mixed network former effect on the electrical conductivities. In this context it is significant to note two apparently conflicting results of the present study: while Figure 13 indicates that a significant amount of tellurium species are present in an anionic form (either as Te34 or Te13 units). Figure 12, bottom, indicates nearly constant values of M2(23Na{31P}) within the composition region 0 ≤ x ≤ 0.5. This observation is at first sight surprising, as anionic Te units are expected to contribute to the 17548


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dispersal and hence shallower Coulomb traps, lowering activation energies of ionic hopping, in analogy to what was observed in borophosphate glasses.38 On the basis of these considerations, we attribute the positive mixed network former effect on the ionic conductivities to the additional charge dispersal mechanisms made possible by the formation of Te34− O−P(3) and Te13−O−P(3) linkages. Table 4 reveals that these linkages make particularly important contributions to the medium-range order of glasses with 0 ≤ x ≤ 0.5, where the strongest conductivity enhancement is observed. In contrast they only play a minor role in glasses with x > 0.5, where the electrical conductivities reveal no positive mixed network former effect.

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CONCLUSIONS In summary, the results of the present study provide detailed information about the mixed network former effect in (Na2O)1/3[(2TeO2)x(P2O5)1−x]2/3 glasses. The network former phosphorus oxide preferentially attracts the network modifier over the network former tellurium oxide, and this preference can be quantified. Nevertheless, tellurium oxide is also modified to some extent, particularly in the compositional region of low x values. In this particular region, preferred heteroatomic connectivity (formation of P−O−Te linkages) can be noted, which appears to be linked to the presence of P(3) species. The formation of Te34−O−P(3) and Te13−O−P(3) linkages may also serve as a mechanism for more effective anionic charge dispersal in the network, resulting in shallower Coulomb traps and thus enhanced ionic conductivities. We propose this particular feature of intermediate range order as a structural basis for the positive mixed network former effect observed in this glass system. For x ≥ 0.5, the connectivity distribution is found to be close to random and no mixed network former effect on the ionic conductivities is observed. The results of the present study may be in partial conflict with our interpretation of a previous study on (NaPO3)y(TeO2)1−y glasses.44 In that earlier study we had concluded from combined NMR, Raman, and XPS results that phosphate exclusively attracts the network modifier species Na2O and TeO2 and is not modified at all by the sodium ions; furthermore, no P−O−Te connectivity preference was detected along that composition line. At that time, no Refocused INADEQUATE data were available, and the deconvolution was performed, without considering the possible formation of P(3) units. Given the present results, a reanalysis of the (NaPO3)y(TeO2)1−y glass structure is warranted.

Figure 15. Comparison of the bridging oxygen speciations in (Na2O)1/3[(2TeO2)x(P2O5)1−x]2/3 glasses with predictions made by a random connectivity model (solid curves).

Figure 16. Correlation of the glass transition temperature with the fraction of heteroatomic linkages in the bridging oxygen inventory of (Na2O)1/3[(2TeO2)x(P2O5)1−x]2/3 glasses.

ligation of Na+ and thus, they should lead to a corresponding reduction in M2. However, as previously discussed, heteroatomic linkages offer the redistribution of anionic charges if the bond valences of the participating network former species are sufficiently different. Thus, it was shown in ref 38 that the negative charge formally located on B(4) units involved in B(4)−O−P(3) linkages can be effectively dispersed toward the nonbridging oxygen atoms located on the P(3) unit, making this species partially anionic and interacting more strongly with the Na+ species than the B(4) species do. A similar effect may be operative in the present system. As the Te34 or Te13 units have bond valences different from that of the P(3) units to which they are bonded, one can envision that the anionic charge of these species is partially redistributed toward the terminal oxygen atoms of the P(3) units. This idea is supported by the Raman spectroscopic data, which reveal a significant reduction in the stretching frequency associated with the PO terminal bond with increasing x, in a way similar to that observed for the borophosphates.38 Second, this mechanism may explain the relatively constant value of M2(23Na{31P}) for glasses with x ≤ 0.5 (Figure 12). If the charges on Te34 or Te13 units migrate toward the terminal oxygen atoms of the P(3) species to which they are bonded, they will make a lesser contribution to the Na+ ion coordination sphere, which will then continue to be dominated by phosphorus. In addition one can envision that this charge redistribution results in a more extended charge

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ASSOCIATED CONTENT

S Supporting Information *

Pulse sequences used in the present study, 23Na MAS NMR data and M2(23Na−23Na) values versus concentration of sodium ions, FTIR spectroscopic results. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b04074.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 17549


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ACKNOWLEDGMENTS We acknowledge funding by FAPESP, grant number 2013/ 07793-6 (CERTEV − Center for Research, Technology and Education in Vitreous Materials). The experimental work was partially funded by the DFG-Sonderforschungsbereich SFB 458.

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