The Structure of Borophosphosilicate Pure Network Former Glasses

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The Structure of Borophosphosilicate Pure Network Former Glasses Studied by Multinuclear NMR Spectroscopy Tobias Uesbeck, Hellmut Eckert, Randall E Youngman, and Bruce G. Aitken J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10984 • Publication Date (Web): 23 Dec 2016 Downloaded from http://pubs.acs.org on January 3, 2017

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The Journal of Physical Chemistry

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The Structure of Borophosphosilicate Pure Network Former Glasses Studied by Multinuclear NMR Spectroscopy Tobias Uesbeck,1 Hellmut Eckert1,2,* 1

Institut für Physikalische Chemie, WWU Münster, Münster, Germany 2

Institute of Physics in São Carlos, University of São Paulo, Brazil

Randall Youngman,3 Bruce Aitken,3 3

Corning Inc., Corning, N.Y., USA

ABSTRACT New mixed network glasses along the composition line x B2O3- (30-x)P2O5 – 70 SiO2 have been prepared and characterized in terms of their chemical composition, viscosity and characteristic temperatures. The physical properties have been correlated with structural information, obtained from Raman spectroscopy and advanced

11

B,

29

Si, and

31

P single and

double resonance solid state NMR studies. Both the macroscopic and structural properties show non-linear changes as a function of composition, with maximal values of the viscometric glass transition temperature near 12-13 mole% B2O3. The structure of phosphorus-rich glasses is dominated by tetrahedral B(4) units linked to 3-4 phosphorus species and multiple phosphorus environments, including P(2) (metaphosphate), P(3) (branching phosphate) groups linked to silicon and P(4) units forming B-O-P linkages as in boron phosphate, BPO4 (superscripts denote the number of bridging oxygen species). In the boron-rich region, the phosphorus species are exclusively present as P(4) groups and the boron atoms present in excess of a B/P ratio of unity is present in the form of three-coordinated B(3) units forming both B-O-B and B-O-Si linkages. While these results document a strong mutual affinity of the boron oxide and phosphorus oxide components, the species concentrations and numbers of B-O-P linkages fall consistently below those numbers expected from a clustering scenario maximizing the number of such connectivities, indicating the absence of macroscopic phase separation. Important differences relative to the previously studied system x Al2O3- (30-x)P2O5 – 70 SiO2 are discussed.

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INTRODUCTION The large majority of technically relevant ion-conducting glasses are based on more than one network former species. The combination of several network formers usually offers the possibility of fine-tuning physical property combinations to special technological demands, and in certain cases the interaction between the various network former components results in dramatically altered physical properties when compared to the corresponding individual constituent glasses. Such network former mixing effects usually manifest themselves as strongly non-linear changes of physical, chemical, and mechanical properties upon linear changes in composition.1 For optimizing glass formulations for specific applications there is obviously a great need of developing and understanding the relation between glass composition and properties on a structural basis. To this end a detailed structural characterization of the local environment and spatial distribution of the network former ions is of key interest. In this connection one needs to identify and quantify the different coordination polyhedra present (short range order), as well as their respective linkages (connectivities) with each other (intermediate-range order). Nuclear magnetic resonance is a powerful element-selective, inherently quantitative method for structural studies of the solid state. As the spectroscopic information is focused on the local and medium-range structural environments of the nuclear isotopes, the method is uniquely suitable for characterizing glassy materials, which lack long-range order.2,3 While, to the present date, the study of network-former mixing effects has focused on ionically conductive glasses,1 relatively few studies have been carried out on glass systems based purely on network former species, with the exception of the simplest binaries. For example, the B2O3-SiO2 system owing to the possible existence of a metastable region of immiscibility.4-12 High-resolution

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O MAS-NMR studies of binary glasses have provided clear-cut

peak assignments for bridging oxygen atoms within Si-O-Si linkages, Si-O-B linkages, and B-O-B linkages, indicating that the concentration of the Si-O-B linkages is lower than that expected for statistical mixing.10,11 The data have been modelled in terms of the hypothetical reaction scheme Si-OSi + B-O-B -> 2 Si-O-B yielding good agreement with a positive mixing energy of 8 kJ/mol. In good agreement with these results 11B, 29Si and 29Si{11B} double resonance NMR studies have shown that the maximum number of Si-O-B linkages per Si(4) species is limited to two, suggesting a phase separation tendency at high B2O3 contents.12 Likewise, the structure of binary SiO2-P2O5 glasses, prepared from either melts,13-17 sol-gel routes18,19 or by chemical vapor deposition20-22 has been studied


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in detail, suggesting the absence of phase separation and the formation of P-O-Si linkages in the silicarich region. In contrast to the considerable work devoted to binary pure network former glasses, the only ternary glasses studied so far come from the SiO2-B2O3-P2O5 system. MacDowell demonstrated the existence of a broad field of ternary glass formation, particularly within the SiO2-BPO4-P2O5 subsystem. These glasses could be heat treated to form glass-ceramics consisting of homogeneouslynucleated BPO4 crystallites embedded in a silica-rich glassy matrix, yielding materials with excellent dielectric properties.23 Due to the resistant nature of the encapsulating glassy phase, the chemical durability of some of these glass-ceramics exceeds that of the precursor glasses at both neutral and acidic pH by more than three orders of magnitude. These glasses are of further interest because, when prepared under reducing conditions, they can be converted to hydrogen gas containing microfoams (“gas-ceramics”) having rather uniform pore size distributions. Upon annealing at 1000 °C, glassceramics containing crystalline BPO4 are formed. Preliminary structural studies using vibrational spectroscopy and solid state NMR have portrayed a qualitative picture of segregated BPO4-like domains embedded in a silica-rich matrix.24-26 The present work is devoted to the quantitative aspects of the short- and medium-range order in this glass system. Using a combination of Raman spectroscopy, solid state NMR lineshape analysis and advanced dipolar methods we will develop a quantitative picture of the site speciations and the network connectivities in the glass system 70 SiO2-

30-x P2O5- x B2O3.

EXPERIMENTAL Sample Preparation and Characterization. The xB2O3 – (30-x)P2O5 – 70SiO2 glasses of this study were prepared

from mixtures of previously synthesized 30B2O3-70SiO2 and

30P2O5-70SiO2 glass powders. The borosilicate precursor was made by melting 1000g mixtures of B2O3 and SiO2 with a 30B2O3:70SiO2 nominal composition in a 650cc Pt crucible at 1650oC for ~20h and then cooling in the crucible. The phosphosilicate precursor was made by melting 1000g calcined mixtures of H3PO4 and SiO2 with a 35P2O5:65SiO2 nominal composition in a 650cc Pt crucible at 1550oC for 2h; it was then quenched by pouring onto a steel plate. 500-1000g batches of mixtures of these precursor glasses were melted in 650cc Pt crucibles for 2h at 1500-1550oC. The melts were quenched to glass by pouring onto a steel plate; the glasses were then annealed by heating at 575-650oC. The chemical composition was analyzed by inductively coupled plasma emission spectroscopy (ICP). The viscosity in the 1011-1013 Pa-s range was measured by beam-bending viscometry on 2” x 0.125” x 0.125” bars with an experimental error of ±2oC for the associated temperature. Raman spectra were collected on an Instruments SA T64000 system using a monochromator and holographic ACS Paragon Plus Environment


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notch filter for Rayleigh scatter rejection. Scattering was collected in the 90o transmission configuration using 514 nm radiation from an Ar+ laser source. The spectral resolution of the detector was ±3 cm-1. Solid State NMR.

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B NMR spectra were obtained under MAS conditions (spinning

rate 20.0 kHz) on a Chemagnetics Infinity 500 MHz spectrometer (magnetic flux density 11.7 T) using π/12 pulses of 0.6 µs length and relaxation delays of 2 s. Additional spectra were recorded at 9.4 T, on a Bruker DSX 400 spectrometer, using a rotor-synchronized Hahn spin 29

echo sequence at a spinning frequency of 12.0 kHz.

Si MAS-NMR spectra were obtained

on an Agilent DD2 200 MHz spectrometer (magnetic flux density 4.7 T), using π/6 pulses of 2.4 µs length and relaxation delays of 120 s. 31P static spectra were recorded at the same field with a Chemagnetics Infinity spectrometer, using π/6 pulses of 1.7 µs length and relaxation delays of 15 s. Chemical shifts are reported relative to BF3OEt2 solution, tetramethylsilane, and 85% H3PO4, respectively. Lineshapes were simulated using the DMFIT software.27 For representative samples the strength of homonuclear

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P-31P magnetic dipole-dipole

interactions was measured by static Hahn spin echo decay spectroscopy,28 using π-pulses of 8.1 µs length and relaxation delays of 360-480 s.

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B{31P} REDOR experiments were

conducted at 9.4 T with representative glass compositions, using the standard sequence by Gullion and Schaefer.29 Typical experimental conditions were: rotor frequency 12.0 kHz, π pulse lengths 5.5 to 6.1 µs and 7.4 to 8.8 µs) for the

11 (3)

B

and

11

B(4) units, respectively and

10.0-11.1 for 31P, and relaxation delay 20 s. As the three- and four-coordinate boron species show different

11

B nutation behaviors owing to their distinctly different nuclear electric

quadrupolar coupling strengths,30 two separate measurements were carried out, with the

11

B

refocusing pulse lengths optimized for B(4) and B(3) detection. While the relative signal areas were found to be affected by the length of the

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B refocusing pulse (as expected), the

differences in the REDOR results were within experimental error limits. A single-point 29

Si{31P} REDOR measurement with a dipolar evolution time of 2.6 ms was possible on a

low-boron glass with x = 2.5, using the following conditions: spinning frequency 10.0 kHz, relaxation delay 900 s, 31P-π pulse length 9.9 µs.

RESULTS, DATA ANALYSIS, AND INTERPRETATION Physical Properties. Table 1 gives the analyzed chemical composition of the glasses under study, along with the viscometric glass transition temperature Tg, defined as the temperature at which the viscosity is 1012 Pa-s. The results of the ICP analysis show that the ACS Paragon Plus Environment

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actual glass compositions are very close to the nominal values, which was aided by the use of the 30B2O3:70SiO2 and 30P2O5:70SiO2 glassy starting materials. Tg depends strongly on composition, showing a well-defined maximum at x = 12.5 mole % B2O3 (see Figure 1). Although the viscometric Tg typically differs from the calorimetric value as measured by DSC by only ±5 oC, the experimental error associated with the latter is an order of magnitude greater, in part due to the weak endothermic feature that is characteristic of these strong network glasses

750

700

650

600

550

500

xB O : (30-x)P O : 70SiO 2

3

2

5

2

450 0

5

10

15

%B O 2

20

25

30

3

Figure 1: Glass transition temperatures Tg of glasses in the system 70 SiO2- 30-x P2O5- x B2O3..

Table 1: Chemical Composition (Mole %) and Tg of xB2O3-(30-x)P2O5-70SiO2 Glasses. x

B2O3

P2O5

SiO2

Tg(1012Pa-s)/°C

2.5 5

2.5 4.9

27.4 25.4

70.1 69.7

602.1 613.3



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7.5 10 12.5 15 17.5 20 22.5 25 27.5

7.4 10.3 12.6 15.3 17.7 19.6 21.5 24.5 26.6

22.8 19.9 17.1 14.8 12.4 10.0 7.5 5.0 2.5

69.8 69.9 70.3 69.9 69.9 70.4 71.1 70.5 71.0

647.0 691.6 698.4 678.0 653.0 609.7 578.8 527.1 494.0

Raman Spectra. Figure 2 shows the Raman spectroscopic results. In the phosphorusrich glasses, prominent bands are visible near 1350, 1190, 1100, 920, 820, 730, 620, 580 and 450 cm-1. The highest frequency band is assigned to the P=O stretching vibration of terminal P(3) units.31 The relative intensity of this band decreases with increasing boron content, indicating that this unit reacts with the boron species added to the glass, forming a new type of unit. The intense band at 1190 cm-1 is assigned to vibrations of the terminal oxygen atoms of the P(2) units. We further attribute the band at 1100 cm-1, which persists throughout the glass compositions to vibrations of the bridging O atoms within Si-O-Si linkages. Based on the spectra of other borophosphate glasses and that of BPO4, we further attribute the band at 920 cm-1 to vibrations of the bridging oxygen atoms involved in P-O-B linkages. The assignment of the band near 820 cm-1 is not entirely clear at the present time. From the concentration dependence of its intensity it also appears to be a due to the vibration of a Pbased species, possibly reflecting the vibration of a bridging oxygen atom within Si-O-P units. At the high-boron end of the composition region, a clear sharp signal is observed near 808 cm1

typical of the symmetric B-O stretching vibrations of the boroxol rings.32 The other band

observed predominantly at high boron contents is the one near 730 cm-1 which can be assigned to the B-O- stretching vibration associated with four-coordinated boron linked to other borate species. Finally, the band near 580 cm-1, whose contribution decreases with decreasing phosphorus content, is assigned to vibrations of the bridging oxygen atoms within P-O-P linkages. Finally, the broad band in the region 400-450 cm-1 can be assigned to Si-O-Si bending modes as in vitreous silica, and the lowest-frequency band near 25 cm-1 is attributed to the boson peak.


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70%SiO2-xB2O3-yP2O5

27.5

2.5

25

5

22.5

7.5

20

10

15

15

10

20

7.5

200

400

600

800

1000

1200

1400

1600

22.5

5

25

2.5

27.5

1800

2000

Raman Shift (cm-1)

File # 4 = 17252

Figure 2: Raman spectra of glasses in the system 70 SiO2- 30-x P2O5- x B2O3. 29

Si MAS-NMR and REDOR. Figure 3a summarizes the

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Si MAS-NMR data.

Relatively narrow single resonances are observed, whose isotropic chemical shift changes monotonically as a function of composition (see Figure 3b). No chemical shift resolution into distinct sites can be observed. At low B2O3 contents, the 29Si chemical shift values are found at distinctly lower frequencies than silica glass (-112 ppm). Consistent with chemical shift trends found in other types of phosphosilicate glasses,33 this finding indicates a substantial extent of Si-O-P linking. With increasing B2O3 (decreasing P2O5) content, the resonance moves continuously towards higher frequencies, indicating the expected diminution in Si-O-P linking with decreasing phosphorus content. As indicated further by Figure 4, small amounts of five-and six-coordinated silicon can be observed in the high-phosphorus glasses. For glasses with B2O3 contents exceeding 15 mole %, Si-O-P linkages are no longer detectable, and the

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Si chemical shifts tend to be less dependent on composition. This finding is

consistent with trends observed in binary SiO2-B2O3 glasses and other borosilicate glasses, where Si-O-B(3) connectivities were found to have little influence on the 29Si chemical shifts.8 In principle it would be highly desirable to quantify the presence of Si-O-P and Si-O-B linkages by 29Si{31P} and/or 29Si{11B} REDOR experiments. Unfortunately, such experiments are severely restricted by the very long

29

Si spin-lattice relaxation times. It was possible,

however, to obtain qualitative evidence for Si-O-P linkages by a single-point comparisons of



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S and S0 obtained at a fixed dipolar evolution time of 2.6 ms. Figure 5 compares the magnitude of the difference signal with that expected for a

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Si-31P two-spin system on the

basis of SIMPSON simulations for different effective Si-O-P distances. The closest agreement with the experimental data is found for a distance of 3.20 ± 0.10 Å. While this distance agrees reasonably well with the range of values (3.05 to 3.15 Å) found in the various polymorphs of SiP2O7 and Si5O(PO4)6,34-36 one has to remember that this REDOR effect is the net result obtained for a collection of Si atoms, some of which have zero, some of which have one and some of which have two Si-O-P linkages. The REDOR result merely suggests that the average number of Si-O-P linkages is close to one. Finally, Figure 5, bottom, indicates (despite the admittedly poor signal-to-noise ratio) a clear chemical shift differentiation between the S0, S, and ∆S signals in the 29Si{31P} REDOR experiment. 29Si species involved in Si-O-P linkages (which are emphasized in the ∆S signal) are characterized by a distinctly more negative chemical shift (-118.5 ppm) than those exclusively involved in Si-O-Si linkages (-112.6 ppm, S signal). Analogous 29Si{11B} REDOR experiments did not show such effects, indicating that Si-O-B(III) linkages do not cause significant resonance displacements relative to Si-O-Si linkages.


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Figure 3: Top: 29Si MAS-NMR spectra, bottom: average 29Si chemical shift of 70 SiO2- 30-x P2O5 - x B2O3 glasses.



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Figure 4: 29Si MAS-NMR spectra of glasses in the system 70 SiO2- 30-x P2O5- x B2O3. Right: Additional features observed in the glasses with x = 0 and x = 2.5 indicating five- and sixcoordinated Si species.

Figure 5: Top: Single-point 29Si{31P} REDOR measurement on the sample with x = 0.05, at a dipolar evolution time of 2.6 ms. The data point is compared with two-spin simulations based on different Si-O-P distances. Bottom: Fourier Transforms of S0, S, and ∆S are shown: Single-point dipolar dephasing experiments on glasses in the system 70 SiO2- 30-x P2O5- x B2O3. Left: 29Si{11B} REAPDOR of glass with x = 22.5; dipolar mixing time 1.6 ms. Right: 29 Si{31P} REDOR of glass with x = 2.5; dipolar mixing time 2.6 ms. ACS Paragon Plus Environment

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B MAS-NMR Spectra. Figure 6 shows the

11

B MAS-NMR spectra. At low boron

contents, the borate species are exclusively four-coordinated, and the chemical shifts of these B(4) units (-4.4 ppm) indicate dominant connectivity with the P-atoms. As the B2O3 content increases for glasses beyond x = 7.5, the spectra reveal systematic changes: a minor second B(4) lineshape near -2.0 ppm, and the appearance of three-coordinated boron species (B(3) units), which become the dominant boron sites at x ≥ 22.5. In the very high-B glasses there is clear evidence for a second B(3) component near 17 ppm. As there are multiple ways of fitting these spectra, we constrained the parameters of this latter B(3) component to be close to those of boroxol rings, consistent with the observation of these units in the Raman spectra of glasses with x > 20% B2O3. In addition, only those fits were accepted that led to correct simulations of the spectra at two different magnetic field strengths (11.7 and 9.4 T; see Figure S1 for an example). Table 2 summarizes all of the parameters obtained. Some representative simulations are shown in Figure 7.

Figure 6: 11B MAS-NMR spectra of glasses in the system 70 SiO2- 30-x P2O5- x B2O3.



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Figure 7: Representative examples for the deconvolution of the 11B MAS-NMR spectra (top) and species concentrations (bottom) of glasses in the system 70 SiO2- 30-x P2O5- x B2O3 according to the parameters given in Table 2. Solid lines are guides to the eye.


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Table 2: 11B Interaction Parameters for the Simulation of the MAS-NMR spectra. Estimated errors of δiso are ±0.2 ppm and ±0.5 ppm for B(4) and B(3) units, respectively. For CQ the Estimated Errors are ±0.1 MHz unless Noted Otherwise. x[% B2O3]

δiso [ppm]

CQ [MHz]

ηQ

G/L

A [%] ±2

±0.05

M2(11B{31P}) [106 rad² s-2] ∆S/S0 ≤ 0.2; ±10%

2.5

B(4)-I B(4)-II B(3)-I

-4.3 -2.2 7.3

2.9±0.3

0.20

0.85 0.85 -

94 1 5

-

5.0

B(4)-I B(4)-II B(3)-I

-4.3 -2.1 7.3

2.4±0.3

0.20

0.85 0.85 -

92 2 6

14.6 -

7.5

B(4)-I B(4)-II B(3)-I

-4.3 -2.0 11.1

2.9±0.3

0.20

0.85 0.85 -

91 2 7

-

10.0

B(4)-I B(4)-II B(3)-I

-4.3 -2.2 12.2

2.7±0.3

0.30

0.85 0.85 -

81 4 15

14.1 7.5

15

B(4)-I B(4)-II B(3)-I

-4.3 -2.3 -12.3

2.7

0.10

0.85 0.85 -

71 6 23

14.3 3.6

20.0

B(4)-I B(4)-II B(3)-I B(3)-II

-4.5 -2.3 12.5 17.0

2.5 2.5

0.15 0.16

0.85 0.85 -

31 9 54 6

14.8 9.0 1.3 1.4

22.5

B(4)-I B(4)-II B(3)-I B(3)-II

-4.0 -1.6 13.0 17.2

2.6 2.6

0.15 0.16

0.85 0.85 -

29 1 63 6

15.2 11.6 0.6 0.7

25.0

B(4)-I B(4)-II B(3)-I B(3)-II

-4.2 -2.9 13.0 17.3

2.6 2.6

0.15 0.16

0.85 0.85 -

13 2 73 11

-

27.5

B(4)-I B(4)-II B(3)-I B(3)-II

-4.2 -1.9 13.1 17.4

2.6 2.6

0.15 0.16

0.85 0.85 -

6 1 76 17

-

-

11

B{31P} REDOR Experiments. For further quantification of the extent of

phosphorus-boron connectivity, detailed

11

B{31P} REDOR experiments were conducted on

representative samples. Regarding the analysis of these data we proceeded as follows: the signal amplitudes S and S0 (with and without dipolar dephasing) were obtained by lineshape fitting using the DMFIT software, evaluating the separate contributions of the two abovediscussed distinct B(3) and B(4) sites (BO3-I and BO3-II as well as BO4 –I and BO4 –II, ACS Paragon Plus Environment


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respectively). For each resolved component the initial parts of the REDOR curves were then fitted to the parabolic function37

∆S/S0 = (4/3π2) M2 (NTr)2

(1)

where M2 is the van Vleck dipolar second moment characterizing the average strength of the dipolar field created by the

31

P nuclei at the site of the

11

B observe nuclei.38 As the

parabolic approximation is only valid at short dipolar mixing times, the data range was restricted to ∆S/S0 ≤ 0.2 for the B(4) sites. For the B(3) sites, some deviations from the parabolic shape were clearly evident for the points taken over this data range, and therefore, the data range for analysis was further restricted to ∆S/S0 ≤ 0.1 (see further discussion below). The method was calibrated with the model compound BPO4 (four B-O-P linkages at a distance of 2.73 Å.39 By analyzing the REDOR data in terms of eq. (1) we measured M2 = 15.5 × 106 rad2s-2, to be compared with the theoretical value of M2 = 20.0 × 106 rad2 s-2, resulting in a calibration factor of 0.77. All the experimental data obtained for the glasses were corrected according to this calibration factor. This factor accounts for experimental imperfections such as finite pulse widths during the rotor period, mis-set of the 31P 180° recoupling pulse angles, resonance offset effects caused by the

31

P chemical shift anisotropy, as well as the intrinsic

under-estimation of M2 arising from the parabolic analysis.37 All the data are included in Table 2. Figure 8 shows a typical data set measured for the sample with x =20 %. It is clearly evident that the two distinct B(4) sites are not only distinguished by a 2 ppm difference in isotropic chemical shifts, but also show different magnitudes of their 11B-31P magnetic dipoledipole coupling strengths. As previously shown in extensive work done on other borophosphate glass systems and crystalline model compounds, the experimental M2(11B{31P}) values can be related to the average number of B-O-P linkages of the boron unit investigated, each B-O-P linkage contributing

about 4-5

x 106 rad2s-2 to the second

moment.40 Based on this comparison we can attribute the majority signal near -4.4 ppm to a boron species involved in 3-4 B-O-P linkages, whereas the minority signal near -2.2 ppm can be assigned to a boron site involved in an average of about two B-O-P linkages. Furthermore, it can be noted from the REDOR data that the B(3) units interact significantly more weakly with the 31P nuclei than the B(4) units, suggesting that they are involved in much fewer B-O-P linkages. Nevertheless, for the x = 10 and 15 samples the M2 values evidence a significant extent of B-O-P linking (corresponding to an average of about 1.5 and 1.0 B-O-P linkages per boron atom). Such levels are higher than those previously reported in alkali borophosphate glasses.41 On the other hand, for the glasses with higher boron content, our REDOR results ACS Paragon Plus Environment

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suggest much weaker dephasing of the 11B(3) spins, suggesting a much smaller extent of B-OP linking. In these glasses it is also conceivable that the dephasing arises exclusively from longer-range dipolar interactions beyond the second coordination sphere. The extent of dephasing for both types of B(3) units is comparable, indicating that the boroxol species near 17 ppm are not part of pure glassy B2O3-like nanodomains.

Figure 8: 11B{31P} REDOR curves measured for the individual three- and four-coordinated sites in the glass with x = 20. Parabolic fits to the initial regions are indicated. The top part of the figure shows the applied fitting model. ACS Paragon Plus Environment


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Static static

31

31

P NMR. Previous work on the SiO2-Al2O3-P2O5 system42 had shown that

P NMR turns out to be superior to MAS NMR owing to the resolution of the

spectroscopic features caused by the chemical shift anisotropy of various phosphate species; for this reason the characterization of the glasses by static 31P NMR lineshapes was preferred. Figure 9 shows the set of spectra, revealing a consistent and systematic compositional evolution. The spectra of the glasses with the highest P-contents (x-values below 10) are dominated by a characteristic powder pattern reflecting an axially symmetric magnetic shielding tensor, typical of P(3) units. The widely stretched shoulder attributable to the parallel component of the chemical shift tensor suggests a wide distribution of the magnetic shielding parallel to the P=O bond, which is quite characteristic for the glassy state. An approximate simulation of this situation can be obtained by assuming three distinct axially symmetric P(3) units having somewhat different shielding tensor values. In addition, two further species are observed: a non-axially symmetric species, tentatively attributed to a P(x) unit, and a Gaussian component, not showing any resolvable chemical shift anisotropy assigned to a tetrahedral P(4) unit. This species is thought to be structurally analogous to the (formally cationic) PO4 units in BPO4, and its concentration keeps increasing with increasing borate content. At x ≥ 22.5 % this P(4) species is the sole remaining phosphorus unit. As the individual lineshapes of all of these species are strongly overlapping there are obviously multiple ways for fitting such spectra. Therefore, fitting was done manually, attempting to minimize sample-to-sample variations of the shift tensor components for a given type of phosphorus site. One important constraint is the observation of just single broad MAS-NMR lines in the frequency range -30 to -45 ppm, with no indication of any peak resolution or shoulders (see Figure S2, Supporting Materials Section). An additional important constraint arises from the

11

B NMR results:

Considering that formally anionic B(4) units are present, requiring appropriate charge compensation, the concentration of the formally cationic P(4) species must be close (within experimental error limits) to the concentration of the B(4) units. Representative fitting results are given in Figure 10, which also summarizes the complete phosphate speciation for all the samples (see Table 3). Evidently, the P(4) concentration increases with increasing boron oxide content, while the concentrations of P(3) and P(x) units tend to decrease. Overall, the compositional evolution of the phosphate speciation is found to be rather similar to that observed in the previously studied SiO2-P2O5-Al2O3 system.42 A remaining uncertainty concerns the interpretation of the non-axially symmetric lineshape component, tentatively assigned to a P(x) unit based on its high asymmetry ACS Paragon Plus Environment

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parameter ησ. In the SiO2-P2O5-Al2O3 system the concentration of this species was found to be correlated with the concentration of five- and six-coordinated aluminum species. Thus, it was assigned to be an anionic metaphosphate species (P(2) unit) compensating the charge of the network-modifying Al3+ ions. In the present system, the charge compensating role of these species is unclear, unless there is cationic silicon, such as in SiP2O7 or Si3(PO4)4. In their crystalline states these silicon species are six-coordinate, however,34-36 and in the present glasses, the concentrations of six-coordinate Si species was found to be very small (Figure 5). Even though other researchers have also suggested the presence of metaphosphate units in binary SiO2-P2O5 glasses,13,16 we note that, if the non-axially symmetric powder pattern found in these spectra is indeed, as suggested, assigned to such an anionic metaphosphate species, there is no crystal-chemical precedence for it in binary silicophosphates, where charge compensation occurs via pyro- or orthophosphate units.34-36 31P NMR studies have shown that the chemical shift tensors of some of these units can also present asymmetry parameters of significant magnitude.43 Based on these arguments, assignment to a distorted P(1) species appears to be more plausible, which is also supported by the less negative chemical shifts observed for this species.

Figure 9. 31P static NMR spectra of glasses in the system 70 SiO2- 30-x P2O5- x B2O3.



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Figure 10. Deconvolution of representative 31P static NMR spectra (top) and species concentrations (bottom) measured in glasses in the system 70 SiO2- 30-x P2O5- x B2O3.


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31

P spin echo decay spectroscopy. Figure 11 shows results from 31P spin echo decay

spectroscopy conducted for representative samples. Approximately Gaussian decays of the normalized spin echos (I/I0) are observed as a function of the evolution time 2t1, resulting from the effect of homonuclear

31

P-31P magnetic dipole-dipole interactions. By fitting these

decays to the expression27 I/I0 = exp - {(2t1)2/M2/2}

(2)

the average strengths of these interactions were quantified in terms of dipolar second moments M2(31P-31P). As indicated in Table 3, the M2(31P-31P) values are found to decrease initially with increasing x, reflecting the decrease in the number of P-O-P linkages with decreasing P2O5 content, go through a minimum near x = 10 % and then show a tendency towards a gradual increase again, despite a decrease of phosphorus content. This compositional dependence clearly indicates that the phosphate species is not distributed statistically but is subject to some clustering. On the other hand, the M2(31P-31P) values measured in all of the glasses are significantly smaller than the value of 9.2 × 106 rad2s-2 measured in crystalline BPO4. On this basis we can rule out the limiting scenario in which extended BPO4 domains are formed. To address the question whether the different static 31P NMR lineshape components are associated with different

31

P-31P dipole-dipole coupling

strengths, we compared the echo Fourier-transforms for short and long evolution times for a sample with x = 2.5 %. No significant differences were found indicating that the P(2), P(3) and P(4) species have comparable homonuclear interaction strengths (see Supporting Materials Section).



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Figure 11. 31P spin echo decay of glasses in the system 70 SiO2- 30-x P2O5- x B2O3 and of crystalline BPO4. The inset shows a typical Gaussian analysis of the experimental data


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Table 3: Lineshape Fitting Parameters, Chemical Shift Tensor Values δiso, δax and ησ and Fractional Areas A Obtained for the Distinct Phosphate Units Observed in the Static 31P NMR Spectra of Glasses in the System 70 SiO2- 30-x P2O5- x B2O3. Included are also M2(31P-31P) Values Obtained from Spin Echo Decay Spectroscopy. δiso [ppm]

δax [ppm]

ησ

P(x) P(4) P(3)-I P(3)-II P(3)-III

±1.0 ppm -31.0 -41.9 -36.4 -40.0 -39.9

±2 ppm 77 -134 -162 -196

±0.05 0.80 0.07 0.07 0.00

A [%] ±4 16 9 19 35 21

P(x) P(4) P(3)-I P(3)-II P(3)-III

-31.0 -40.0 -36.8 -40.0 -39.3

78 -138 -167 -213

0.80 0.08 0.07 0.01

20 15 20 33 12

-

P(x) P(4) P(3)-I

-32.7 -36.4 -37.8

-84 -145

0.80 0.10

32 29 39

-

P(x) P(4) P(3)-I P(3)-II

-32.1 -36.2 -36.3 -54.5

76 -141 -98

0.96 0.08 0.09

39 40 13 8

15

P(x) P(4) P(3)-I

-32.0 -37.2 -35.4

75 -149

0.92 0.09

31 61 8

4.2

20

P(x)

-32.2

74

0.98

26

4.3

P(4)

-36.9

-

-

74

22.5

P(4)

-35.2

-

-

100

25

P(4)

-36.0

-

-

100

27.5

P(4)

-35.0

-

-

100

x[% B2O3] 2.5

5.0

7.5

10

M2(31P-31P) [106 rad² s-2] 2t1 ≤ 400 µs (±10%)

5.6

3.8

4.8


-


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DISCUSSION Site Speciations and Connectivities. The results of the present study give clear evidence of preferred boron-phosphorus interactions within the modifier free glass system 70 SiO2- 30-x P2O5- x B2O3 and serve to explain the apparent homogeneous nucleation of BPO4 when certain BP silicate glasses are cerammed. This is clear from the nearly exclusive formation of B(4) units when small amounts of B2O3 are added to SiO2-P2O5 glass, and likewise from the exclusive formation of P(4) units when small amounts of P2O5 are added to SiO2-B2O3 glass. Their chemical shifts and REDOR data indicate that the dominant B(4)-I units have close to the maximum number of four B-O-P linkages. We can compare the experimental B(4) concentrations with those expected for a limiting scenario, in which all B2O3 added to the glass reacts with phosphate, following the reaction scheme B(3) + P(3) -> B(4) + P(4) in analogy to the formation of BPO4 (see Figure 12). In this scenario all of the boron atoms are linked to phosphorus as long as the concentration of phosphorus exceeds that of boron (P/B > 1) and all of the P atoms are linked to boron atoms as long as the concentration of boron exceeds that of phosphorus (B/P > 1).The results illustrate that in glasses containing up to x = 5 mole % B2O3, the number of B(4) units is indeed close to the maximum possible number. Likewise, the sole observation of the P(4) species in the highboron end glasses also argues for this chemical ordering model. However, in the intermediate region (10 ≤ x ≤ 20) deviations from this scenario are clearly evident. An analogous comparison can be made for the number of B-O-P connectivities, which can be assessed independently from the

11

B{31P} REDOR experiments and the

31

P NMR

speciations. We base our estimation of the total number of B-O-P linkages, from REDOR on the following considerations: Each B-O-P linkage contributes ca. 4.2 ± 0.4 × 106 rad2s-2 to the second moment value. From the M2(11B{31P}) values and the 11B chemical shifts we can then conclude that the B(4)-I units contain on average 3.5±0.4 B-O-P linkages, while the B(4)-II units have about 2 ± 0.2 B-O-P linkages on average. We further consider the average number of B-O-P linkages involving the B(3) units, by dividing the corresponding M2 values measured for these species by the number 4.2 ± 0.4 × 106 rad2s-2 for a single linkage. Thus, we obtain: = 2x × {f(BO4–I) × 3.5 + f(BO4-II) × 2.0 + f(BO3) × M2/4.2× 106}

(3)

Independent of this evaluation, we can estimate the number of P-O-B linkages, from the fractional area of the P(4) units, for which we assume complete boron connectivity as in BPO4, i.e. ACS Paragon Plus Environment

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= 2 × (30-x) × f(P(4)) × 4.

(4) 11

Here the symbols f denote the fractional signal areas in the corresponding B and 31P resonances. Figure 13 illustrates that the and values are indeed close to each other, especially when considering the experimental errors of their determination. Also, Figure 13 shows significant deviations from the limiting scenario maximizing the number of B-O-P linkages. Consistent with the site speciations shown in Figure 12, we note that the largest deviations occur in the middle region near 10 % ≤ x ≤ 20 %, indicating that this compositional range is characterized by the maximum extent of chemical disorder.

Structural Integration and Spatial Distributions. While the results document a strong tendency of B/P interactions in this glass system, similar to the analogous situation in the related SiO2-P2O5-Al2O3 glass system,42 Figures 12 and 13 show that this “chemical ordering tendency” falls short of a macroscopic phase separation scenario. The numbers of B(4) and P(4) units as well as the number of B-O-P connectivities fall consistently below the maximum possible values. The clearest argument against a phase separation model actually comes from the spectra of samples near the x = 15 composition (B/P = 1), which show a substantial amount of B(3), P(3) and P(x) units and values of nB(P) and np(B) that are much lower than those predicted by the phase separation scenario. These results suggest that there are still some B-O-Si linkages in glasses with x ≤ 15, and P-O-Si linkages in glasses with x ≥ 15. This situation is completely different from that in SiO2-P2O5-Al2O3 glasses, where the composition x = 15% cannot be prepared in the glassy state.42 Another strikingly different result from that obtained with the former glass system is the noticeable tendency of the M2(31P -31P) values to increase with decreasing P content, suggesting that the sizes of these “BPO4like” nanodomains may increase. In contrast, in the analogous SiO2-P2O5-Al2O3 glasses the M2(31P-31P) values show the expected decrease with decreasing phosphorus content, signifying a more effective dispersal of the phosphorus species. The difference between both systems may be related to the different tendencies towards homoatomic linkages involving the group III oxide species present in excess of the 1:1 stoichiometry. While B(3)-O-B(3) linkages are easily established between these neutral network former species, resulting in the formation of boroxol rings, a corresponding mechanism is not possible in the aluminosilicophosphate system. In that system the aluminum oxide in excess of the Al/P ratio of 1:1 must be accommodated in the form of structures involving multiple aluminum coordination states (Al(4), Al(5) and Al(6)), with aluminum acting both as a network former and a network modifier species.



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Figure 12: Concentration of BPO4 units deduced from the fractional areas of B(4) and P(4) in the system 70 SiO2- 30-x P2O5- x B2O3. The experimental data are compared with predicted values for a macroscopic BPO4 segregation model (maximal chemical ordering scenario).


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Figure 13: number of B-O-P linkages deduced via eq. (3) from M2(11B{31P}) and deduced via eq. (4) from the concentration of P(4) units in in the system 70 SiO2- 30-x P2O5- x B2O3. The data are compared with predicted values for a macroscopic BPO4 segregation model (maximum chemical ordering scenario).



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Figure 14: Dependence of Tg and the average number of bridging oxygen species per network former unit, N(X-O-X), on glass composition x in the system 70 SiO2- 30-x P2O5- x B2O3. Correlation between Tg and Network Connectivity. Based on the structural information given above we can now discuss the compositional dependence of the glass transition temperature, Tg. Previous studies of ion-conducting mixed-network former glasses have pointed towards a relationship of this parameter with the overall degree of network connectivity, as expressed by the average number N(X-O-X) of bridging oxygen atoms present.44-46 The relationship appears to hold particularly well, if the concentration of the network modifier species is kept constant and only the ratio of the different network former species is being varied. Thus, the present system, which is exclusively based on network former species may appear as a suitable test case of this relationship. While experimental data for N(X-O-X) cannot be specified from our study, as the identity of the P(x) species is uncertain, we can compare the Tg trend with the prediction of the chemical ordering scenario discussed in Figures 12 and 13. As shown in Figure 14, this scenario predicts a maximum of Tg at x = 15 %, where the number of four-coordinate species is expected to be at a maximum value of 2.0. Figure 14 illustrates that the Tg data follow the predicted trend in general, with the important difference, however, that the maximum is found at x = 12.5 %, rather than x = 15 %. From our NMR data we can provide two possible explanations that may contribute to this important deviation: first of all, Figures 12 and 13 indicate that the deviations from the


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chemical ordering model are particularly severe in the glasses with x = 15 % and 20 %, suggesting that the experimental network connectivity in these glasses is significantly lower than that predicted from the chemical ordering model. Secondly, (and possibly more importantly) one may argue that the presence of the anionic P(x) species leads to an additional strengthening of the glass structure owing to Coulombic interactions between the cationic Si4+ and the anionic meta- or pyrophosphate units. The same phenomenon is observed for the compositional dependence of binary (100-x) SiO2 – x P2O5 glasses. In this glass system Tg initially shows the predicted decrease with increasing P2O5 content up to about 15 mole% P2O5, as Si(4) units are successively replaced by P(3) units in the network. However, for xvalues above 15 % Tg is not found to decrease any further, while both 31P NMR17 and Raman scattering data indicate the appearance of a similar P(x) species as found in the present glass system.47 This deviation from the simple connectivity prediction may again arise from additional stabilization of the network via Coulombic interactions between cationic Si4+ species and the anionic P(x) units. In the present ternary glass system the stabilization of the glass structure by these Columbic interactions will be more important in the P-rich glasses (where the total contribution of this P(x) species to the network is larger) than in the B-rich glasses, thereby contributing an asymmetry to the Tg(x) dependence that is not predicted from the chemical ordering scenario. Thus, the shift of the Tg – maximum to x = 10 mole % can be well-explained on the basis of our NMR data.

CONCLUSIONS In summary, the detailed speciation and structural connectivity of the pure network former glass 70 SiO2- 30-x P2O5- x B2O3 glasses has been studied in detail by complementary spectroscopic approaches, in particular dipolar solid state NMR experiments. While the system is characterized by a clear chemical ordering tendency towards the formation of B-O-P linkages, this chemical preference is somewhat less pronounced compared to the situation in the compositionally analogous aluminophosphosilicate glass system. In the limit of high boron oxide contents the data suggest that the sizes of the BPO4 nanodomains tend to increase. These nanodomains are likely responsible for the homogeneous nucleation of BPO4 observed by MacDowell in his study of boron phosphate glass-ceramics. Based on the quantitative structural information provided by solid state NMR, the experimentally found compositional dependence of the glass transition temperature Tg(x) can be explained in terms of the average degree of network connectivity (average number of bridging oxygen atoms per ACS Paragon Plus Environment


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network former unit), However, the network is additionally stabilized by Coulombic interactions between cationic Si4+ species and anionic P(x) units (x = 1 or 2), which are particularly important in the glasses with high P2O5 contents. The results of the present study illustrate the power and potential of advanced solid state NMR techniques for quantifying medium-range order in glasses and for relating physical glass properties to structure in mixed network former glasses.

ASSOCIATED CONTENT Supporting Information. Field dependent

11

B MAS-NMR spectra and

31

P MAS-NMR

spectra of representative glasses.

AUTHOR INFORMATION *corresponding author, email [email protected]. Notes: The authors declare no competing financial interest.

ACKNOWLEDGEMENTS. Funding by FAPESP, grant number 2013/07793-6 (CERTEV – Center for Research, Technology and Education in Vitreous Materials) is most gratefully acknowledged.

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ToC Graphic


The Structure of Borophosphosilicate Pure Network Former Glasses

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