The Structure of P2O5-SiO2 Pure Network Former Glasses Studied by

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C: Plasmonics; Optical, Magnetic, and Hybrid Materials 2

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The Structure of PO-SiO Pure Network Former Glasses Studied by Solid State NMR Spectroscopy Marcos de Oliveira, Bruce G. Aitken, and Hellmut Eckert J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b06055 • Publication Date (Web): 08 Aug 2018 Downloaded from http://pubs.acs.org on August 19, 2018

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The Journal of Physical Chemistry

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The Structure of P2O5-SiO2 Pure Network Former Glasses Studied by Solid State NMR Spectroscopy Marcos de Oliveira Jr.,1 Bruce Aitken,2 Hellmut Eckert1,3* 1

Instituto de Física de São Carlos, Universidade de São Paulo, CP 369, 13566-590, São Carlos, SP,

Brasil. 2

Corning Incorporated, Corning, New York, United States.

3

Institut für Physikalische Chemie, Westfälische Wilhelms-Universität Münster, Corrensstraße 30,

D-48149 Münster, Germany.

ABSTRACT The structure of binary (SiO2)100-x-(P2O5)x glasses has been investigated by Raman scattering,

29

Si and

31

P magic angle spinning (MAS) as well as static

31

P NMR

spectroscopy. 29Si chemical shift trends reflect the successive replacement of Si-O-Si by SiO-P linkages as the compositional parameter x is increased. While 31P MAS-NMR does not resolve separate phosphate species, the static

31

P NMR lineshapes were successfully

simulated by considering the effect of uncorrelated distribution functions of the chemical shift tensor components upon the lineshape. Based on these simulations, which were also found to be consistent with the experimental 31P MAS NMR spectra, two distinct sites can be resolved: a dominant site characterized by an axially symmetric chemical shift tensor, assigned to P(3) units, and (only in the case of the x = 25 and 30 glass) a Gaussian component reflecting phosphate species interacting with five- and six-coordinated silicon species. For 0 ≤ x < 25, the decrease in average coordination number may provide the structural explanation for the strong decrease in the glass transition and liquidus temperatures over this composition range, whereas the subsequent increase in Tg at higher P2O5 contents is correlated with the appearance of the higher-coordinated silicon species. While these higher-coordinated silicon species occur within separate microdomains,

31

P

spin echo decay spectroscopy suggests that the majority of P atoms tends to be randomly distributed in space, consistent with a statistical P-O-P, Si-O-P, and Si-O-Si connectivity distribution.

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1. INTRODUCTION The combination of several network formers presents a well-established method of modifying physical properties of technologically relevant glasses. Depending on the components involved and on their concentrations, both incremental and dramatic changes can be effected, which need to be rationalized on the basis of structural concepts. A case in point is the addition of P2O5 to glassy SiO2 for the design of fibers for lasers and optical communications systems.1 Already upon addition of small amounts of P2O5 the liquidus temperature of vitreous SiO2 is lowered drastically and similarly dramatic changes are observed for the viscosity and the glass transition temperatures.2 Structural studies of binary (SiO2)100-x(P2O5)x (0 ≤ x ≤ 35) glasses, prepared from either melts,3–7 sol-gel routes8,9 or by chemical vapor deposition10–12 have been conducted by Raman, EXAFS and solid state NMR. Results have suggested that these glasses are fully polymerized, consisting of randomly interconnected P(3) and Si(4) units. Based on this picture alone, however, the compositional evolution of the thermal and mechanical properties is difficult to understand. Furthermore, Raman and preliminary NMR data acquired in the context of a recent study of the ternary B2O3-SiO2-P2O5 system reveal additional spectroscopic features of the binary glasses requiring clarification. The present contribution reports a detailed characterization of (SiO2)100-x(P2O5)x (0 ≤ x ≤ 30) glasses by

29

Si-,

31

P-NMR and Raman spectroscopies,

focusing on the quantitative aspects of the short- and medium-range order in this glass system. One particular complication encountered in the process of lineshape deconvolutions of static and MAS-NMR spectra of glasses arises from distributions of the values of magnetic shielding tensor components caused by variations in the local environments of the

29

Si and

31

P nuclei. Here, we introduce a non-rigorous but very

practical approach of fitting experimental static and MAS-NMR spectra consistently by considering such distributions explicitly. We also examine the possibility of phosphate clustering based on the measurement of 31P-31P magnetic dipole-dipole interactions. Using a combination of Raman spectroscopy, solid state NMR lineshape modelling and advanced

b


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dipolar methods we will develop a comprehensive quantitative model of site speciations and network connectivities in this system.

2. EXPERIMENTAL 2.1. Glass preparation and characterization The 30 % P2O5 glass used in this study was made by melting 1000 g calcined mixtures of H3PO4 and SiO2 with a nominal molar composition of 35P2O5:65SiO2 in 650 cm3 Pt crucibles at 1550oC for 2 h; the melt was then quenched to glass by pouring onto a steel plate. After reserving a portion of the resultant glass to serve as the most P-rich sample of this study, the remaining glass was powdered to serve as the P source for the other study samples. Glasses containing 2.5 to 15% P2O5 were prepared by melting 500 g mixtures of the latter phosphosilicate glass and SiO2 in 650 cm3 Pt/Rh crucibles at 17501790 oC for 6 h. As these melts were extremely viscous, they were quenched by plunging the crucibles into water; a cylinder of glass was subsequently extracted from the crucible by core drilling. Glasses containing more than 15% P2O5 were prepared in a similar fashion, although in this case the melts were sufficiently fluid that they could be quenched to glass by pouring onto a steel plate; these glasses were then annealed at 575 oC. The chemical composition was subsequently determined by inductively coupled plasma optical emission spectrometry. Density was measured by He pycnometry for the glasses containing 2.5 to 15% P2O5; otherwise it was determined by the Archimedes method Glass transition temperatures were measured on a Netzsch 404F1 differential scanning calorimeter using a heating rate of 10°C/min. Raman spectra were collected from 30 to 1775 cm-1 in backscatter configuration with a 10x objective on a Horiba LabRam HR Evolution system under excitation by 532 nm radiation.

2.3 Solid state NMR spectroscopy Solid state NMR experiments were performed on an Agilent DD2 spectrometer operating at 5.64 T (1H frequency at 240 MHz) equipped with a triple-channel 4 mm NMR probe.

31

P MAS- and static-NMR spectra were recorded using a Hahn spin echo sequence

with π/2 and π pulses of 2.5 µs and 5 µs length respectively and an interpulse delay of 50

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µs. Rotor-synchronized MAS spectra were acquired at a spinning speed of 10.0 kHz. Additional Hahn spin echo spectra were measured at 14.1 T on a Bruker Avance NEO 600 NMR spectrometer, using π/2 and π pulses of 1.8 µs and 3.6 µs length respectively, and an interpulse delay of 50 µs. By using a small dwell time of 1.6 µs, the top of the echo was located as exactly as possible (to within ±2.0 µs), Left shifting beyond the echo top by more than 2.0 µs led to significant lineshape alteration. For all the 31P NMR experiments a recycle delay of 50 s was used and up to 320 scans were added up for noise averaging. Longer recycle delays did not result in any increases of signal-to-noise ratios. 31P chemical shifts were referenced against 85% H3PO4.

29

Si MAS-NMR spectra were measured in a 7

mm Agilent probe using a single π/2 pulse of 5 µs length, spin rate of 5.0 kHz, recycle delay of 1200 s for up to 60 scans. 29Si chemical shifts were referenced using kaolinite as a secondary standard (-91.5 ppm relative to tetramethylsilane)13. Static Hahn spin-echo decay experiments were used to measure homonuclear 31P-31P dipole-dipole interactions, utilizing π pulses of 5 µs length and a relaxation delay of 50 s. Following the approach previously discussed in the literature, second moments characterizing the strength of these homonuclear interactions were extracted from spin echo intensities measured over the range of dipolar evolution times 60 µs to 1.2 ms, using the expression:14,15

=

(1)

The homonuclear second moments M2(P-P) can be compared to distance information using the van Vleck equation given by16

= + 1 ℏ ∑

(2)

Here, γS denotes the gyromagnetic ratio of the observed nuclei having spin quantum number S and rSS are the P-P interatomic distances as viewed from one central P atom. Equation (2) assumes a weak dipolar coupling limit, i.e. for the large majority of the

31

P nuclei the

strength of the dipole coupling between neighboring 31P nuclei (in kHz) is weaker than the resonance frequency difference between these nuclei. This assumption has been shown to be generally valid in glasses, where disordering and anisotropy effects cause a wide resonance frequency dispersion.15

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Double-quantum filtered spectra were obtained in a Bruker Avance NEO spectrometer operating at 14.1 T (600 MHz) using the 1-D refocused INADEQUATE method.17 This experiment results in the selective detection of only those 31P nuclei that are involved in PO-P linkages and therefore allow excitation of a double quantum coherence through indirect 31P-31P spin-spin coupling. In contrast, the signals of isolated P units are suppressed by appropriate receiver phase cycling. Experimental conditions were: spinning speed 15.0 kHz, π/2 pulse length 1.6 µs, relaxation delay 50 s. The mixing time for DQ coherence creation was 16.6 ms, corresponding to a value of the indirect coupling constant 2J(31P-31P) of 30 Hz.

3. RESULTS AND DISCUSSION 3.1.

Glass Properties Figure 1 shows the compositional dependence of density and molar volume for the

glasses investigated in this study (cf. Table 1). The latter properties change nearly linearly with rising P2O5 content. However, between 25 and 30% P2O5, both trends show a discontinuity in slope, with the molar volume of the 30% P2O5 glass being less than the value expected upon extrapolating the lower P2O5 sample data, suggesting a more efficiently packed structure for the 30% P2O5 glass. Somewhat similar results were reported by Takahashi et al., although the slope change of their molar volume data occurs at about 20% P2O5.18 Tg results are plotted in Figure 2, along with data from other studies of glass formation in this binary system.18-21 Tg decreases strongly with increasing x within the range of 2.5 to 25% P2O5. However, there is a distinct rise in Tg as the P2O5 content increases from 25 to 30%, corresponding to the above anomaly in the density and/or molar volume trends and suggestive of a major change in glass network connectivity in this compositional range.

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Table 1 – P2O5 concentrations as measured by Inductively Coupled Plasma - Atomic Emission Spectrometry (ICP), glass transition temperatures (Tg), densities (ρ) and molar volumes (VM) of x(P2O5) – (100-x)SiO2 glasses.

2.5

[P2O5] (± 0.05mol %) 2.26

Tg (°C) (± 2°C) 882

5

4.77

874

7.5

7.16

10

ρ (±0.001g.cm-3) 2.229 2.235

VM (cm3) 27.78 28.63

785

2.250

29.31

10.2

701

2.268

30.15

12.5

12.3

681

2.281

30.74

15

15.0

659

2.294

31.56

17.5

17.0

598

2.309

32.04

20

18.1

588

2.317

32.33

25

19.6

594

2.324

32.77

30

25.0

572

2.351

34.26

35

29.3

643

2.44

34.46

x

b


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2.45

36

2.4

34

V 32

2.35

ρ 2.3

30

2.25

28

2.2

26 0

5

10

15

20

25

30

%P O 2

5

Figure 1 – Density (ρ, diamonds) and molar volume (V, circles) of (SiO2)100-x(P2O5)x glasses as a function of analyzed P2O5 concentration.

900 850 800 750 700 650 600 550 500 0

5

10

15

20

25

30

35

%P O 2

5

Figure 2 – Tg of (SiO2)1-x(P2O5)x glasses as a function of P2O5 concentration with results from this study (circles) along with data from references 10, 18-21 (triangles).

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3.2.Raman spectroscopy Figure 3 shows Raman spectra obtained for the studied glasses. For the phosphorusrich glasses, prominent bands are visible near 1330, 1240, 1170, 1020, 808, 720, 615, 535 and 400 cm-1. The bands at 720 cm-1 and 535 cm-1 can be attributed to bending vibration of O=P-O and O-P-O units respectively.4 The broad band around 400 cm-1 is associated with the transverse optical mode and the longitudinal optical mode of Si-O-Si bond bending vibrations.22 The band around 615 cm-1 can be associated with the electronic defect structure represented as Si+...O—Si groups.23 In order to interpret the higher frequency Raman bands and to understand their relation to the glass structure we have deconvoluted the bands observed in the 900–1500 cm-1 frequency range into various Gaussian components, see Figure 4. The band at around 1330 cm-1 can be attributed to the terminal P=O groups of P(3) units.23 As shown in Figure 4a, for the phosphate rich glasses this peak can be deconvoluted into two components at around 1345 cm-1 and 1327 cm-1. Based on quantum chemical modeling performed for phosphosilicate systems,24 these bands are attributed respectively to P=O bond stretching in double O=P-O-P=O centers and in single P(3) centers (not bound to other P units). For decreasing P2O5 content the relative intensity of the band related to double P(3) centers decreases while the band related to single centers increases (see Figure 4c), indicates the continuous replacement of P-O-P by P-O-Si linkages. The same conclusion is obtained from 31P NMR results shown below. The band at around 1250 cm-1 can be attributed to anti-symmetric stretching vibrations of Si-O bonds in Si-O-Si linkages.4,24 The decrease in the relative intensity of this band for increasing P2O5 content confirms this attribution. The band at around 1140-1170 cm-1 can be assigned to the summed contribution of anti-symmetric stretching vibrations in P-O-P and P-O-Si linkages,4,24 which cannot be unequivocally resolved. Contradictory attributions are found for the band at around 1050 cm-1. Shibata et al.4 attribute this band to P-O-P linkages based on data for vapor-deposited binary phosphosilicate glasses11 and vitreous P2O5. On the other hand, Plotnichenko et al. attribute this band to P-O-Si and Si-O-Si linkages, based on quantum chemical modeling and intensity variations as a function of phosphosilicate glassy compositions.24 Figure 4c shows a decreasing relative intensity for increasing P2O5 content, suggesting that the second attribution is more likely.

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Figure 3 – Raman spectra measured for selected samples of the glass system (100-x)SiO2xP2O5.

Figure 4 – (a) Deconvolutions of the reduced Raman spectra of selected samples of the studied glasses into Gaussian curves. (b) Peak positions as a function of composition for the individual Gaussian components. (c) Relative areas of the deconvoluted curves considering only the spectral region displayed in (a).

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10

3.3 29Si MAS-NMR Figure 5 shows

29

Si MAS NMR spectra for the set of glasses. The isotropic

chemical shifts (δiso) and full-widths at half-maximum (FWHM) for these spectra are shown in Table 2. For the glasses with x = 2.5 to x = 25 the spectra are composed of a single line with decreasing isotropic chemical shift for increasing P content. This signal can be attributed to fully polymerized Si(4) units (the superscript denotes the number of bridging O atoms) involved in Si-O-Si and Si-O-P linkages. In agreement with previously published 29

Si chemical shift trends in phosphosilicate glasses,25-27 the monotonic chemical shift trend

shows that the fraction of Si-O-P linkages increases with increasing x, as predicted on the basis of compositions. No chemical shift resolution into distinct Si(4)nP sites can be observed (the subscript denotes the number of next-nearest P atoms). On the other hand, for the x = 30 glass two additional low-intensity peaks near -170 (3%) and -215 ppm (13%) signify the formation of higher-coordinated Si(5) and Si(6) species, respectively.7,25-27 Such species, which are well-known in the crystal chemistry of silicon phosphates (SiP2O7, Si5O(PO4)6, or Si3(PO4)4) have been previously detected also in binary (SiO2)100-x(P2O5)x glasses (x = 30 and 356,7,26,27 and related to the crystallization of these compounds.6,7,26-28

Figure 5 – 29Si MAS NMR spectra of selected samples of the studied glass system. b


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Table 2 – 29Si NMR isotropic chemical shifts (obtained from the centers of gravity of the 29

Si MAS NMR spectra) and full-widths at half-maximum (FWHM). bx(P2O5)

δiso

FWHM

(± 0.1 ppm)

(± 0.5 ppm)

2.5

-110.2

12.1

5

-112.4

12.5

10

-113.3

11.9

15

-115.3

12.1

20

-117.7

11.7

25

-118.9

13.3

30

-118.8 -170.5 (Si(5)) -215.4 (Si(6))

14.3 (84%) 6.8 (3%) 9.2 (13%)

3.4. 31P Static and MAS-NMR Spectra and their Simulations Figure 6 shows 31P NMR spectra measured under static (a) and MAS (b) conditions. Most of the glasses show single symmetric 31P MAS-NMR lines; only in the x = 30 sample a low-frequency shoulder can be discerned. The static spectra of all the samples with x < 25 show a characteristic powder pattern reflecting an approximately axial magnetic shielding tensor. The features of this characteristic lineshape are significantly broadened, however, which can be understood in terms of a distribution of local environments as is characteristic for the glassy state. Figure 7 illustrates for a representative sample that the lineshape cannot be reproduced correctly by simply convoluting the powder pattern with a Gaussian broadening function. Rather, the distribution of magnetic shielding values parallel to the principal axis (i.e. parallel to the P=O bond direction) is significantly wider than that perpendicular to the principal axis. To simulate this effect, Vasconcelos et al. have suggested an extended Czjzek approach accounting for such distributions in a quantitative fashion.29 Here we have used a less rigorous method, which also serves to simulate the spectra well, but uses fewer assumptions. Our approach is based on summing up static 31P powder patterns with the amplitudes weighted by Gaussian distributions of the three

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12

principal values of the chemical shift tensor. In this distribution, the amplitude of each powder pattern with sets of principal components !"##$ % is given by the probability &!"##$ % = ∏#

√)*++

/

, . . - ++ ++ 3 01.++ 2

,

(3)

where i = x, y, z, δii are the principal values in the center of the distribution and χ(δii) are the widths (standard deviations) of the principal value distributions. This approach makes the intrinsic assumption that the three magnetic shielding components characterizing the environment of phosphorus in a given site are uncorrelated. In this case the lineshape arises from a statistically weighted superposition of powder patterns constructed from all the possible permutations of δii values found within their respective distribution functions. Finally, the simulated lineshapes are convoluted with Gaussian functions in order to reproduce dipolar broadening effects. Figure S1 (Supporting Materials) confirms that it is possible to simulate the static NMR data measured at two different magnetic field strengths (5.7 and 14.1 T) with the same set of simulation parameters. All of the above arguments confirm the validity of our simulation approach. The idea of uncorrelated δii values may, at first sight, seem mathematically counterintuitive in view of ab-initio calculations, which generally predict that a specific local geometric distortion produces highly correlated changes of δxx, δyy, and δzz. We believe that the workability of our approach stems from the fact that the magnetic shielding changes in glasses do not arise from modifications of a single geometric parameter (such as a bond length or a bond angle) but rather stem from multiple effects operative simultaneously, de facto producing a loss in the correlation between the δii values characterizing a given site.

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Figure 6 – Left: 31P static NMR spectra for selected (SiO2)100-x(P2O5)x glasses. Dashed blue lines are simulated spectral components and red dashed curves show the total simulated spectrum. Right: 31P MAS-NMR spectra and their simulations based on the simulation parameters of the static spectra. Spinning sidebands are marked with asterisks.

b



14

Figure 7 – Attempts to fit the 31P static NMR spectrum for the x =15 sample based on a fixed set of chemical shift tensor values, with distribution effects modeled by a uniform Gaussian convolution function: a) optimizing the match with the low-frequency component, b) optimizing the match with the high-frequency component, c) our approach, d) distribution funtions in δxx/yy and δzz used for the optimized fit. 3.5. Phosphorus Speciations Table 3 summarizes the relative areas of the two spectral components as a function of composition. The anisotropic component corresponds to the P(3) sites, previously identified in phosphosilicate glasses.12 The isotropic component is only present for the x = 30 and x = 25 samples. As the 29Si NMR spectrum of the x = 30 glass shows clear evidence of Si(5) and Si(6) species we attribute the isotropic

31

P NMR lineshape component to

orthophosphate units interacting with these higher-coordinated silicon atoms. This assignment is nicely consistent with the following quantitative considerations, assuming the formation of small amorphous Si5O(PO4)6- like clusters, in which 3/5 of the Si atoms are

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six-coordinated, linking to six P(0) units, while 2/5 of the Si atoms are four-coordinated, forming bitetrahedral Si2O7 units. Each of the P(0) units is surrounded by three Si(6) species and one bitetrahedral Si2O7 unit.30 For the x = 30 glass (analytical composition x = 29.3), 16 (±2) % i.e. 11.3 out of 70.7 Si atoms, are five-or six-coordinated. Thus, 1.5×11.3 = 17 Si atoms can be accounted for as being part of an Si5O(PO4)6- like cluster. Consequently, 6/5 × 17 = 20.4 (out of 58.6) phosphate species are required for coordination, which corresponds to 35 (±3) % of all the phosphorus species present. This value is in excellent agreement with the fractional area of the isotropic species observed in the static 31P NMR spectrum of this sample (37±2%).

Figure 8- isotropic chemical shift values (δiso) obtained from the simulated 31P static spectra (circles), and MAS spectral peak position (triangles) as a function of the P2O5 concentration in the glasses. Error bars represent the FWHM of the MAS spectra.

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Table 3 – Parameters used in the 31P static spectral simulations shown in Figure 6 and their attributions. δxx, δyy and δzz are the chemical shift tensor principal values, δiso is the isotropic chemical shift, lb is the Gaussian line broadening parameter, χδxx, χδyy and χδzz are the widths of the distributions for the shift tensor principal values. All values are in ppm. I is the relative intensity-area for the P(3) site. P(3)

P(0)

MAS

x δxx/yy

δzz

χδxx/yy

χδzz

δisoa

lb

I

δiso

lb

δiso

FWHM

(±0.5)

(±5)

(±2)

(±10)

(±2)

(±1)

(± 5%)

(±1)

(±2)

(±0.5)

(±0.5)

2.5

38.5

-175

17

130

-33

8

100

-

-

-37.1

-12.9

5

38.5

-175

17

140

-33

8

100

-

-

-37.1

-12.7

10

37

-170

18

160

-32

7

100

-

-

-38.5

12.8

15

36.8

-170

19

120

-32

7

100

-

-

-39.4

-12.9

20

36.8

-180

19

120

-35

6

100

-

-

-41.5

-13.3

25

35

-190

22

80

-40

6

92

-36

30

-41.9

-14.3

30

34

-189

22

75

-40

9

63

-50

30

-43.9*

*

(P2O5)

*slightly asymmetric peak, center of gravity specified, no linewidth specified.

3.6. P-O-P vs. P-O-Si Connectivities and Spatial Phosphate Distributions. In order to probe P-O-P linkages, and improve the assignments of the resonances,

31

31

P

P refocused-INADEQUATE NMR experiments were performed for the x =

2.5, 10 and 30 glasses (see Figure 9). This experiment results in the selective detection of only those 31P nuclei that are involved in a P-O-P linkages by means of the excitation of a double quantum coherence through indirect 31P-31P spin-spin coupling. For the sample with x = 2.5 no DQ-filtered signal was observed, hence no P-O-P linkages were detectable. This finding is consistent with the assignment of the Raman scattering frequency of 1345 cm-1, which is absent in glasses with x ≤ 5, to the P=O stretching mode of a P(3) unit involved in P-O-P bonding. P-O-P linkages are also not detected in the x = 5 sample, whereas they clearly appear in the Raman spectrum of the x = 10 sample. As shown in Figure 9, they are also detected in the Refocused INADEQUATE spectrum of this sample, which thus b


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comprises different P(3)nSi (n ≤ 3) units. While these cannot be spectroscopically resolved by 31

P MAS NMR it is worth noting that the center of gravity of the refocused INADEQUATE

spectrum is shifted by 2 ppm towards lower frequency relative to the single-pulse spectrum, suggesting that P(3)3Si units resonate at higher frequencies than P(3)nSi (n < 3) units. Based on the detailed comparison between the standard MAS spectra and the Refocused INADEQUATE spectra we can identify those phosphate species not involved in P-O-P linking, i.e. the P(3)3Si units). Based on this analysis these units contribute 87% for the sample with x = 10. For the sample with x = 30, the comparison between the single-pulse and the INADEQUATE spectrum is more complex, owing to the additional contribution of the isotropic component manifesting itself as a Gaussian in the static spectra. In Figure 9, top right, we deconvolute the single-pulse spectrum into a majority P(3) component comprising all P(3)(3-n)Si,nP (n ≠ 0) units (69%, including spinning sidebands)

and the

minority P(0) component corresponding to the isotropic contribution (31%, reasonably close to 37% obtained from the static spectrum). The signal from P(3)3Si units, which is also missing in the Refocused INADEQUATE spectrum, has a very low intensity here.

Figure 9 – Experimental (black solid curves) 31P MAS NMR spectra (central peaks only) obtained by single pulse (top) and refocused-INADEQUATE (bottom) techniques for the glasses under study. Dashed curves denote the deconvolutions into individual Gaussian components, including spinning sideband contributions (data shown in the Supporting Information Section – Figure S2).

b



Page 18 of 27

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An alternative method of quantifying P-O-P linkages in these glasses is to probe the strength of homonuclear

31

P-31P magnetic dipole-dipole couplings using spin echo decay

spectroscopy.12,14,15 Figure 10 summarizes the results obtained for the studied glasses. The second moments (M2(P-P)) determined by the Gaussian fitting of the initial decay regime using Equation 1 are summarized in Table 4. Figure 11 shows that up to x = 20 these M2(P-P) values are linearly correlated with the number of P atoms per unit volume (NV), as expected for a random distribution in space. Evidently, the P units are homogeneously dispersed in the glass structure, i.e., there is neither phase segregation nor any kind of preferential formation of P-O-P linkages. This result agrees with a previous study of vapor-deposited silica doped with 1.12 mole% P2O5.12 An alternative method of interpreting these M2(P-P) values is based on the assumption that the major contribution arises from P-O-P linkages, neglecting the dipolar contributions of

31

P spins at longer distances. We consider this

approximation a good one particularly in the samples more dilute in phosphorus. The P….P distance in a P-O-P linkage in the crystalline model compound SiP2O7 is 3.02 Å.31 Considering this distance, the homonuclear 31P dipolar second moment for the limiting case of an isolated P-O-P linkage (not considering further distant neighbors) can be calculated from equation (2), yielding M2(P-P),theor. = 4.04 ×106 rad2s-2. Based on this premise, an average number of P-O-P linkages per phosphorus species, 〈56 7 6 〉9:; , can be estimated

according to

〈56 7 6 〉9:; = M2(P-P)exp/M2(P-P)theor

(4)

These values, which must be considered upper limits due to the neglect of the contributions of the more remote 31P nuclei to M2(P-P)exp are listed in Table 4. We can compare them with those predicted from a random linkage model. Considering that all the P atoms are linked to three bridging oxygen atoms and all the Si atoms are linked to four bridging oxygen atoms, the statistical weights of the four distinct phosphate species are given by the following expressions: =

The Structure of P2O5-SiO2 Pure Network Former Glasses Studied by

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