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11B-MQMAS and 29Si–{11B} Double-Resonance NMR Studies on the Structure of Binary B2O3–SiO2 Glasses
Solid State Nuclear Magnetic Resonance 21, 134–144 (2002) doi:10.1006/snmr.2002.0054
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B-MQMAS and 29Si^{11B} Double-Resonance NMR Studies on the Structure of Binary B2O3^SiO2 Glasses Leo van Wu¨llen1 and Georg Schwering Max-Planck-Institut fu¨r Festko¨rperforschung, Heisenbergstrasse 1, D-70569 Stuttgart, Germany Received July 3, 2001; revised December 21, 2001; accepted February 11, 2002 Glasses in the system xB2 O3 ð1 xÞSiO2 ð0:2 x 1:0Þ were studied using 11 B multiple quantum magic angle spinning NMR spectroscopy (MQMAS), 29 Si–f11 Bg rotational echo adiabatic passage double-resonance and 29 Si–f11 Bg CP heteronuclear correlation spectroscopy. The results can be quantitatively interpreted in terms of a phase separation of the borosilicate glasses into a virtually SiO2 -free B2 O3 phase and a mixed borosilicate phase. While the MQMAS spectra allowed the site speciation and resolution of at least two different 11 B resonances, attributable to BO3=2 units consumed in boroxol rings, BO3=2 units connecting the boroxol rings and BO3=2 units involved in B–O–Si linkages, the analysis of the double-resonance data further elucidated the structure of the mixed borosilicate phase. The results indicate that only a fraction of 0:48 mol B2 O3 can be accommodated per mole SiO2 , building a mixed borosilicate network. # 2002 Elsevier Science (USA) Key Words: MQMAS; REAPDOR; HETCOR; borosilicate glasses; phase separation.
INTRODUCTION The structure of borosilicate glasses continues to be of great interest ever since the ground breaking 11 B NMR studies by Bray and coworkers [1, 2]. The numerous studies using NMR techniques [3–7], IR [7, 8] and Raman [9, 10] approaches and most recently, theoretical modeling of glass structure [11–13], have led to a wealth of information about these glasses. Starting with the 11 B wideline NMR data, models were developed [2], which were then challenged by new experimental results with the onset of more sophisticated techniques, such as 29 Si MAS NMR, 17 O MAS and multiple quantum magic angle spinning NMR spectroscopy (MQMAS) NMR [3, 6] and subsequently refined. The most prominent question with respect to glasses in this system is the degree of mixing between borate and silicate regions in the glasses, which was first assumed to be only very limited in agreement with the fact that the mixing of BO3=2 and SiO4=2 polyhedra is thermodynamically disfavored. Specifically for the glasses studied in the present work, binary borosilicate glasses, earlier models predict virtually complete phase separation. More recent studies by Stebbins et al. [6], using 17 O MQMAS spectroscopy, and Martens et al. [3] using 11 B MAS data, however, found considerable mixing of B2 O3 and SiO2 . 1
To whom correspondence should be addressed. Fax: +49-711-6891502. E-mail: wullen@jansen. mpi-stuttgart.mpg.de. 134 0926-2040/02 $35.00 # 2002 Elsevier Science (USA) All rights reserved.
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In these glasses, boron exclusively adopts a trigonally planar environment, resulting in quadrupolar broadened MAS line shapes. Owing to the rather narrow chemical shift range for the 11 B nucleus this leads to strongly overlapping lines, if more than one local environment for boron is present in the glass, severely impeding deconvolution and site speciation. In recent years, however, considerable progress has been achieved in the resolution of quadrupolar broadened MAS NMR spectra. Predominantly, MQMAS spectroscopy, introduced in 1995 by Frydman and coworkers [14–16], attracted much attention due to the versatility and ease of use of this approach. In MQMAS spectroscopy, the evolution of a multiple quantum coherence is correlated with the evolution of a single quantum coherence under the conditions of fast MAS, resulting in a separation of the isotropic chemical shifts and the quadrupolar broadening in a 2D experiment. While MQMAS spectroscopy facilitates the site speciation for nuclei exhibiting quadrupolar broadened line shapes, double-resonance techniques have emerged as a powerful tool for the characterization of intermediate range ordering phenomena in amorphous solids [17–20]. These techniques utilize the dipole–dipole coupling strength between two nuclear spins I and S to obtain information about I–S internuclear distances. From the various possible pulse sequences, the rotational echo adiabatic passage double-resonance (REAPDOR) approach [21] has been shown to be advantageous in cases in which the dephasing nucleus is of quadrupolar nature. In this work, we present a reinvestigation of the structure of binary borosilicate glasses using 11 B MQMAS NMR spectroscopy and 29 Si–f11 Bg double-resonance (REAPDOR and CP heteronuclear correlation (HETCOR)) NMR spectroscopy. The combined results from the different techniques allow for a quantitative description of the degree of mixing and of the short and intermediate range order present in the identified phases.
EXPERIMENTAL All glasses were synthesized using dried B2 O3 and SiO2 , both obtained from Aldrich. 0:1 mol% MnCO3 was added to the glass batches to shorten the long 29 Si T1 values. Glass melting was performed in platinum crucibles at temperatures of 1450–15508C. The samples were kept at this temperature for ca. 1=2 h and subsequently quenched to room temperature in air. All glass samples were optically clear. The glasses proved to be X-ray amorphous, some of the high silica glasses developed traces of crystalline SiO2 . NMR experiments were performed on a Bruker DSX 400 spectrometer operating at 9:4 T with resonance frequencies of 79.46 and 128:33 MHz for 29 Si and 11 B, respectively. A 2:5 mm rotor system was used for the MQMAS experiments, allowing rotor frequencies of 30 000 Hz and 11 B RF amplitudes of 220 kHz. The 29 Si–f11 Bg REAPDOR studies were performed using a 4 mm triple resonance MAS probe at spinning speeds of 5 kHz employing RF fields of 29 kHz for 29 Si and 70 kHz for 11 B. Chemical shifts are referenced relative to TMS and BF3 Et2 O for 29 Si and 11 B, respectively.
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RESULTS Figure 1 contains the 29 Si and 11 B MAS NMR spectra for the xB2 O3 ð1 xÞSiO2 glasses. As observed by other authors, the 29 Si MAS resonance line is effectively independent of the xB2 O3 mole fraction. The isotropic chemical shift diso remains more or less constant at approx. 110 ppm, a value also observed for pure silica glass. The 11 B MAS signal, however, exhibits a clear dependence on the xB2 O3 mole fraction. The overall 11 B signal is shifted upfield with increasing SiO2 content. Although in some of the 11 B MAS spectra, a contribution of more than one trigonal boron site seems obvious, an attempt of a deconvolution into different quadrupolar
FIG. 1.
11
B MAS (left) and
29
Si MAS (right) NMR spectra for the studied glass samples.
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FIG. 2. 2D 11 B MQMAS spectrum of glass 1: 0:28B2 O3 0:72SiO2 . Left: isotropic projection in F1 ; bottom: corresponding 1D 11 B MAS spectrum; right: slices taken parallel to the MAS axis ðF2 Þ at the maxima in the isotropic projection, 18:1 ppm (top) and 23:1 ppm (bottom).
powder patterns would be overoptimistic, at least at the employed field of 9:4 T. However, the observed upfield shift of the center of gravity might indicate increasing mixing of the B2 O3 and SiO2 networks, as discussed by Martens and Mu. llerWarmuth [3]. The above-mentioned lack of resolution can be overcome with the help of 11 B MQMAS spectroscopy. Figure 2 exhibits the sheared two-dimensional MQMAS spectrum of glass 1; 0:28B2 O3 0:72SiO2 . The bottom spectrum parallel to the F 2axis represents the conventional MAS spectrum, whereas the projection on the F 1axis yields the isotropic spectrum. Two different signals can easily be identified; a signal at approx. 18 ppm and a broader second one at approx. 24 ppm (cf. Table 1). Applying the equations [16] diso ¼ dF2 þ ðdF1 dF2 Þ17 27 and sffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Z2Q ¼ 8:246 103 n0 dF1 diso SOQE ¼ CQ 1 þ 3
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29
TABLE 1 Si Chemical Shift Values and 11 B NMR Parameters for the Studied Glass Samples as Determined from 29 Si MAS and 11 B MQMAS
Glass #
xB2 O3
d29 iso Si (ppm)
Site 1 d11 iso B
SOQE (MHz)
d11 iso B (ppm)
Rel. fraction
SOQE (MHz)
0.14 0.27 0.33 0.37 0.44 0.47 0.53 0.57 0.61 0.68
2.8 2.9 2.8 2.8 2.8 2.8 2.8 2.8 2.9 2.8
11.0 11.6 11.7 11.9a 11.9a 11.8 12.1 12.5 11.7a 12.5
0.86 0.73 0.67 0.63 0.56 0.53 0.47 0.43 0.39 0.32
2.8 2.6 2.7 2.7 2.8 2.6 2.6 2.6 2.9 2.8
(ppm) 1 2 3 4 5 6 7 8 9 10
0.28 0.37 0.46 0.53 0.58 0.59 0.64 0.73 0.85 1.00 a
110:0 109:5 108:9 109:9 110:2 110:2 108:2 108:0 }
16.0 15.7 16.0 16.3a 16.4a 16.0 15.9 16.5 16.3a 17.3
Site 2
Rel. fraction
Measured at 10 000 Hz MAS.
translates these values to isotropic chemical shifts of 16.0 and 11:7 ppm for the two respective environments and second-order quadrupole effects (SOQE) of approx. 2:7 MHz. dF1 and dF2 are the centers of gravity in the respective dimensions, CQ is the quadrupolar coupling constant given by CQ ¼ 3e2 qQ=h; ZQ denotes the asymmetry parameter. These chemical shift values may be compared to those found by Zwanziger et al. [22–24] for a pure B2 O3 glass and by Martens and Mu. ller-Warmuth [3] for borosilicate glass. Zwanziger et al. found two signals in a pure B2 O3 glass which they assigned to boroxol rings ððBOÞ3 O3=2 Þ; ð70%; 17 ppmÞ and BO3=2 units ð30%; 13 ppmÞ connecting the rings. Martens and Mu. ller-Warmuth [3] attributed the 11 Bsignal at 13:5 ppm found in a high silica borosilicate glass to boron atoms in a mixed borosilicate glass phase. Accordingly, we assign the peak at 16:5 ppm to boroxol rings within a borate type of network, whereas the broad signal at 12:5 ppm is ascribed to a superposition of signals arising from BO3=2 units in a borate environment and BO3=2 units in a borosilicate environment. The compositional dependence of the relative fraction of the two signals, which is plotted in Fig. 3, lends support to the above assignment. Obviously, the relative fraction of the upfield signal, covering the range of 0.86 at xB2 O3 ¼ 0:28 to 0.32 at xB2 O3 ¼ 1:0 is neither compatible with a complete phase separation nor with a perfect mixing. A complete phase separation over the entire compositional range would imply a constant relative fraction of 0.32 for the upfield signal, whereas for complete mixing only one signal is expected, exhibiting a dependence of the chemical shift on the composition. The results therefore indicate a phase separation in these glasses into a borate and a mixed borosilicate glass phase. The MQMAS results were used as input parameters for the simulation of the 1D 11 B MAS spectra. The results of these simulations are consistent with the results of the MQMAS experiments.
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FIG. 3. (a) isotropic projections of the 11 B MQMAS spectra for the studied glass samples; xB2 O3 ¼ ðtop to bottomÞ 0.28, 0.37, 0.46, 0.59, 0.64, 0.73, 1.0. (b) Relative fraction of the 11 B signals near 17 ppm (open circles) and 12 ppm (filled circles) as a function of xB2 O3 . Solid lines: theoretical curves following the model discussed in the text.
The two-dimensional 29 Si–f11 Bg CP HETCOR [25] experiment for glass 6 is shown in Fig. 4. This experiment allows a direct correlation of S- and I-spin frequencies. Since the CP process is based on the dipolar coupling strength between the involved nuclei, only those 11 B nuclei involved in B–O–Si linkages can contribute to the 11 B HETCOR signal (F1 domain). The efficiency of the CP process depends largely on the spin-lock behavior of the 11 B transverse magnetization, which is governed by the adiabaticity parameter a¼
u2RF uQ uMAS
with uQ being the quadrupolar frequency, defined by uQ ¼ 3e2 qQ=ðhð2Ið2I 1ÞÞ; uMAS being the rotor frequency and uRF being the nutation frequency of the 11 B
VAN WU¨LLEN AND SCHWERING
140 ppm 60 40 20 0 -20 -40 -60
0
50
100
150
ppm
FIG. 4. 2D 29 Si–f11 Bg CP HETCOR spectrum for glass 6: 0:59B2 O3 0:41SiO2 . The projection along F2 represents the 29 Si spectrum, the projection along F1 the 11 B spectrum.
nucleus in the applied spin-lock field [26–28]. Under the condition a 1, the magnetization is effectively recovered after each full rotor period, whereas for a ffi 1 mixing of states during the rotor period leads to fast dissipation of the transverse magnetization. In the limit of a 1 (sudden regime), a fraction of the transverse magnetization can be preserved over longer spin-lock times. Since, considering the 11 B quadrupolar coupling constant of ca. 2:7 MHz in the studied glasses, it is not possible to reach the adiabatic regime with our hardware equipment, we chose to adopt the a 1 condition using uMAS ¼ 5 kHz and uRF ð11 BÞ ¼ 8 kHz. A simulation of the signal (diso ¼ 12:9 ppm; CQ ¼ 2:6 MHz and ZQ ¼ 0:3) produces values very close to those found by Martens and Mu. ller-Warmuth [3]. This is unambiguous evidence for the existence of a mixed borosilicate phase consisting of BO3=2 and SiO4=2 polyhedra. We do not find any indication for the presence of direct linkages between the boroxol rings and SiO4=2 tetrahedra, although their existence cannot be ruled out.
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1.2 1.0
xB2O3 xSiO2 ---------------------------0.28 0.72 0.46 0.54 0.58 0.42 0.59 0.41 0.73 0.27
∆ S/S0
0.8 0.6 0.4 0.2 0.0 0.000
0.002
0.004
0.006
0.008
0.010
0.012
NTR /s
FIG. 5. 29 Si–f11 Bg-REAPDOR data for glass 1: 0:28B2 O3 0:72SiO2 (filled circles); glass 3: 0:46B2 O3 0:54SiO2 (open diamonds); glass 5: 0:58B2 O3 0:42SiO2 (open circles); glass 6: 0:59B2 O3 0:41SiO2 (open squares); glass 8 0:73B2 O3 0:27SiO2 (open triangles). Solid line: theoretical 29 Si–f11 Bg REAPDOR curve for an Si–B two-spin system; dotted line: REAPDOR curve for an SiB2 three-spin system assuming a homogeneous distribution of boron; dotted line: theoretical REAPDOR curve for a statistical distribution of boron within a 0:48B2 O3 SiO2 mixed borosilicate glass phase. For the generation of the curve we did not account for the fraction of SiB4 units, since the calculation time exceeds any reasonable time frame. Since the SiB4 five-spin system contributes to only 2% of the total signal, the ( was assumed in the calculations. expected error is rather small. An Si–B distance of 2:77 A
The 29 Si–f11 Bg REAPDOR experiments were performed to further elucidate the intermediate range ordering in the mixed borosilicate phase. Figure 5 contains a compilation of the REAPDOR curves for the studied glass samples. The 29 Si–f11 Bg REAPDOR curves are mainly dominated by the number of interacting 11 B nuclei, since the 29 Si–11 B distances of the B–O–Si linkages are not expected to be subject to a large distribution. Discussing the curves on a qualitative basis, the initial slope and steepness of the curves are a measure for the strength of the dipolar coupling between 29 Si and 11 B. We note that the overall appearance of the REAPDOR curves}with the exception of glass 1}is fairly independent of the composition. This allows as a first conclusion that the dipolar coupling and hence the mixing of B2 O3 and SiO2 increases until a limiting value of xB2 O3 is reached. A further increase in the xB2 O3 mole fraction does not change the magnitude of the 29 Si–11 B dipolar coupling, i.e. the silicate network is saturated with boron at this limiting composition. For a quantitative analysis of the REAPDOR curves, we need an estimate for the distance between boron and silicon in borosilicates. Although there are no naturally occurring borosilicate minerals which parallel the connectivity pattern present in binary borosilicate glasses, i.e. a corner sharing of BO3=2 and SiO4=2 polyhedra, there are but a few known examples of synthetic borosilicate crystals. In M3 BSi2 O10 ðM ¼ Eu; Gd; SmÞ [29, 30] and Nd3 BGe1:08 Si0:92 O10 [31], BSiO5 anions are found, 6 exhibiting the above-mentioned connectivity pattern. The B–Si distances in these ( , respectively. Quantum chemical crystals are 2.788, 2.847, 2.781 and 2:760 A
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VAN WU¨LLEN AND SCHWERING
DFT calculations on model borosilicate clusters produced B–Si distances of ( [32]. We used an averaged B–Si distance of 2:77 A ( , corresponding to 2.76–2:80 A a dipolar coupling constant of 360 Hz, as input for the simulations of the REAPDOR evolution curves. The simulations were performed using the software package SIMPSON [33]. With the internuclear distance as input, the only adjustable parameter is the number of 11 B nuclei participating in the SiBn spin system. Since the geometry of the SiBn spin system is not known, we restricted the analysis to the geometry-independent initial part of the dipolar evolution curves ðDS=S0 50:6Þ [17]. An average number of approx. one boron atom per SiO4=2 tetrahedron is obtained for glass 1; 0:28B2 O3 0:72SiO2 ; and an average number of 1.7 boron atoms per SiO4=2 tetrahedron is obtained for glasses with xB2 O3 > 0:4, cf. Fig. 5.
DISCUSSION The MQMAS results identified the boroxol ring ðBOÞ3 O3=2 , free BO3=2 - and SiO4=2 -units as the polyhedra constituting the network. The 29 Si–f11 Bg CP HETCOR experiment demonstrates a connectivity between (a fraction of) the BO3=2 -groups and SiO4=2 polyhedra, while no connection between the boroxol ring and SiO4=2 units was found. The compositional dependence of the relative fraction of the 11 B signals and the virtual unison of the 29 Si–f11 Bg REAPDOR curves can be interpreted in terms of a phase separation into a borate glass and a mixed borosilicate phase. In a xB2 O3 ð1 xÞSiO2 glass, B2 O3 can only be incorporated into the SiO2 matrix up to a limiting fraction. Excess B2 O3 is consumed in a pure borate network. The conjecture of such a phase separation reproduces the observed compositional dependence of the relative fractions of the two 11 B signals. The fraction of boron in the various possible environments can be calculated as follows. Assuming that f gives the fraction of B2 O3 per SiO2 involved in the borosilicate network, the fraction of boron in the mixed borosilicate phase of a xB2 O3 ð1 xÞSiO2 glass is given by ð1 xÞf =x. Correspondingly, 1 ð1 xÞf =x gives the fraction of boron in the pure borate network. This fraction can be subdivided into the fraction of boron consumed in boroxol rings, ð1 ð1 xÞf =xÞ 0:68, and the fraction of boron in BO3=2 -units within the borate network, ð1 ð1 xÞf =xÞ 0:32, where 0.68 and 0.32 are the relative fractions of boron involved in boroxol rings and free BO3=2 -units, respectively (cf. Fig. 3a and Table 1) [22–24]. With these equations, we can calculate the relative fractions of the various boron species as a function of the mole fraction xB2 O3 . Reasonable agreement between experimental data and calculated fractions of the individual units is obtained for f ffi 0:48. The results are plotted as solid lines in Fig. 3b, in which the calculated fractions for the free BO3=2 -units and the BO3=2 groups in the mixed borosilicate phase are combined, since the deconvolution of the signals is not possible. The agreement with the experimental data is less satisfying for glasses with xB2 O3 50:46. For these high silica glasses, the mixing of B2 O3 and SiO2 is obviously not as complete as indicated by the model.
STRUCTURE OF BINARY B2O3–SiO2 GLASSES
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The REAPDOR results are consistent with the proposed model. The SiO2 network can accommodate B2 O3 only up to a limiting fraction of 0:48B2 O3 per mole SiO2 , excess B2 O3 is consumed in a B2 O3 type of network. The limiting mole fraction xMAX B2 O3 for a completely mixed borosilicate glass is given by xMAX B2 O3 ¼ ð1 xMAX Þ 0:48. Accordingly, the overall structure of the mixed borosilicate phase should not be subject to changes, once the limiting composition of 0:32B2 O3 0:68SiO2 is exceeded. The phase is characterized by a silicon-to-boron ratio close to unity (0.96). However, a further characterization of the network organization with respect to the distribution of BO3=2 -polyhedra proves to be impossible. The assumption of a homogeneous distribution of boron within the borosilicate network (which would be characterized by ca. 1.7 B–O–Si linkages per SiO4=2 tetrahedron, taking into account the different coordination numbers for silicon and boron) as well as a statistical distribution of boron within the mixed network both lead to reasonable fits of the experimental data. For the latter scenario, the relative fraction W for an SiO4=2 tetrahedron with n Si–O–B linkages is given (to a first approximation) by ! 4 W ¼ A4n Bn n ¼ A4n Bn
4! ; n!ð4 nÞ!
where A is the probability for an Si–O–Si linkage, given by A ¼ 1=ð1:5f þ 1Þ, and B the probability for an Si–O–B linkage, given by B ¼ 1 A. Assuming a relative fraction of 0:48B2 O3 per SiO2 in the borosilicate phase, as concluded from the MQMAS experiments, the probability for an SiO4=2 -tetrahedron with n Si–O–B linkages can be calculated as 0.113 ðn ¼ 0Þ; 0:327 ðn ¼ 1Þ; 0:356 ðn ¼ 2Þ; 0:172 ðn ¼ 3Þ and 0:032 ðn ¼ 4Þ. Using these values and taking the natural abundance of the 11 B isotope (80%) into account, a simulation produces the 29 Si–f11 Bg REAPDOR curve, plotted as a slashed curve in Fig. 5. Again a dipolar coupling of 360 Hz was assumed. As can be inferred from Fig. 5, the curves for the two different scenarios (homogeneous and statistical distribution) result in equally reasonable fits to the data. CONCLUSION The presented results show that a combination of advanced NMR-techniques, 11 B MQMAS NMR, 29 Si–f11 Bg REAPDOR NMR and 29 Si–11 B CP HETCOR NMR, allows detailed insight into the network organization of glasses in the system xB2 O3 ð1 xÞSiO2 . While the CP HETCOR results present unambiguous proof for Si–O–B linkages in the borosilicate glasses, the degree of mixing could be quantified by the results of 11 B MQMAS NMR and 29 Si–f11 Bg READPOR NMR experiments. The data can consistently be described assuming a phase separation into a borate glass phase and a mixed borosilicate phase. The mixed phase adopts a boron-tosilicon ratio close to unity, which does not change with composition. A further
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characterization of the network organization in the mixed borosilicate phase with respect to the distribution of the Si–O–B linkages proved to be impossible. We are currently extending this work with respect to ternary alkali borosilicate glasses.
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B-MQMAS and 29Si^{11B} Double-Resonance NMR Studies on the Structure of Binary B2O3^SiO2 Glasses Leo van Wu¨llen1 and Georg Schwering Max-Planck-Institut fu¨r Festko¨rperforschung, Heisenbergstrasse 1, D-70569 Stuttgart, Germany Received July 3, 2001; revised December 21, 2001; accepted February 11, 2002 Glasses in the system xB2 O3 ð1 xÞSiO2 ð0:2 x 1:0Þ were studied using 11 B multiple quantum magic angle spinning NMR spectroscopy (MQMAS), 29 Si–f11 Bg rotational echo adiabatic passage double-resonance and 29 Si–f11 Bg CP heteronuclear correlation spectroscopy. The results can be quantitatively interpreted in terms of a phase separation of the borosilicate glasses into a virtually SiO2 -free B2 O3 phase and a mixed borosilicate phase. While the MQMAS spectra allowed the site speciation and resolution of at least two different 11 B resonances, attributable to BO3=2 units consumed in boroxol rings, BO3=2 units connecting the boroxol rings and BO3=2 units involved in B–O–Si linkages, the analysis of the double-resonance data further elucidated the structure of the mixed borosilicate phase. The results indicate that only a fraction of 0:48 mol B2 O3 can be accommodated per mole SiO2 , building a mixed borosilicate network. # 2002 Elsevier Science (USA) Key Words: MQMAS; REAPDOR; HETCOR; borosilicate glasses; phase separation.
INTRODUCTION The structure of borosilicate glasses continues to be of great interest ever since the ground breaking 11 B NMR studies by Bray and coworkers [1, 2]. The numerous studies using NMR techniques [3–7], IR [7, 8] and Raman [9, 10] approaches and most recently, theoretical modeling of glass structure [11–13], have led to a wealth of information about these glasses. Starting with the 11 B wideline NMR data, models were developed [2], which were then challenged by new experimental results with the onset of more sophisticated techniques, such as 29 Si MAS NMR, 17 O MAS and multiple quantum magic angle spinning NMR spectroscopy (MQMAS) NMR [3, 6] and subsequently refined. The most prominent question with respect to glasses in this system is the degree of mixing between borate and silicate regions in the glasses, which was first assumed to be only very limited in agreement with the fact that the mixing of BO3=2 and SiO4=2 polyhedra is thermodynamically disfavored. Specifically for the glasses studied in the present work, binary borosilicate glasses, earlier models predict virtually complete phase separation. More recent studies by Stebbins et al. [6], using 17 O MQMAS spectroscopy, and Martens et al. [3] using 11 B MAS data, however, found considerable mixing of B2 O3 and SiO2 . 1
To whom correspondence should be addressed. Fax: +49-711-6891502. E-mail: wullen@jansen. mpi-stuttgart.mpg.de. 134 0926-2040/02 $35.00 # 2002 Elsevier Science (USA) All rights reserved.
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In these glasses, boron exclusively adopts a trigonally planar environment, resulting in quadrupolar broadened MAS line shapes. Owing to the rather narrow chemical shift range for the 11 B nucleus this leads to strongly overlapping lines, if more than one local environment for boron is present in the glass, severely impeding deconvolution and site speciation. In recent years, however, considerable progress has been achieved in the resolution of quadrupolar broadened MAS NMR spectra. Predominantly, MQMAS spectroscopy, introduced in 1995 by Frydman and coworkers [14–16], attracted much attention due to the versatility and ease of use of this approach. In MQMAS spectroscopy, the evolution of a multiple quantum coherence is correlated with the evolution of a single quantum coherence under the conditions of fast MAS, resulting in a separation of the isotropic chemical shifts and the quadrupolar broadening in a 2D experiment. While MQMAS spectroscopy facilitates the site speciation for nuclei exhibiting quadrupolar broadened line shapes, double-resonance techniques have emerged as a powerful tool for the characterization of intermediate range ordering phenomena in amorphous solids [17–20]. These techniques utilize the dipole–dipole coupling strength between two nuclear spins I and S to obtain information about I–S internuclear distances. From the various possible pulse sequences, the rotational echo adiabatic passage double-resonance (REAPDOR) approach [21] has been shown to be advantageous in cases in which the dephasing nucleus is of quadrupolar nature. In this work, we present a reinvestigation of the structure of binary borosilicate glasses using 11 B MQMAS NMR spectroscopy and 29 Si–f11 Bg double-resonance (REAPDOR and CP heteronuclear correlation (HETCOR)) NMR spectroscopy. The combined results from the different techniques allow for a quantitative description of the degree of mixing and of the short and intermediate range order present in the identified phases.
EXPERIMENTAL All glasses were synthesized using dried B2 O3 and SiO2 , both obtained from Aldrich. 0:1 mol% MnCO3 was added to the glass batches to shorten the long 29 Si T1 values. Glass melting was performed in platinum crucibles at temperatures of 1450–15508C. The samples were kept at this temperature for ca. 1=2 h and subsequently quenched to room temperature in air. All glass samples were optically clear. The glasses proved to be X-ray amorphous, some of the high silica glasses developed traces of crystalline SiO2 . NMR experiments were performed on a Bruker DSX 400 spectrometer operating at 9:4 T with resonance frequencies of 79.46 and 128:33 MHz for 29 Si and 11 B, respectively. A 2:5 mm rotor system was used for the MQMAS experiments, allowing rotor frequencies of 30 000 Hz and 11 B RF amplitudes of 220 kHz. The 29 Si–f11 Bg REAPDOR studies were performed using a 4 mm triple resonance MAS probe at spinning speeds of 5 kHz employing RF fields of 29 kHz for 29 Si and 70 kHz for 11 B. Chemical shifts are referenced relative to TMS and BF3 Et2 O for 29 Si and 11 B, respectively.
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RESULTS Figure 1 contains the 29 Si and 11 B MAS NMR spectra for the xB2 O3 ð1 xÞSiO2 glasses. As observed by other authors, the 29 Si MAS resonance line is effectively independent of the xB2 O3 mole fraction. The isotropic chemical shift diso remains more or less constant at approx. 110 ppm, a value also observed for pure silica glass. The 11 B MAS signal, however, exhibits a clear dependence on the xB2 O3 mole fraction. The overall 11 B signal is shifted upfield with increasing SiO2 content. Although in some of the 11 B MAS spectra, a contribution of more than one trigonal boron site seems obvious, an attempt of a deconvolution into different quadrupolar
FIG. 1.
11
B MAS (left) and
29
Si MAS (right) NMR spectra for the studied glass samples.
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FIG. 2. 2D 11 B MQMAS spectrum of glass 1: 0:28B2 O3 0:72SiO2 . Left: isotropic projection in F1 ; bottom: corresponding 1D 11 B MAS spectrum; right: slices taken parallel to the MAS axis ðF2 Þ at the maxima in the isotropic projection, 18:1 ppm (top) and 23:1 ppm (bottom).
powder patterns would be overoptimistic, at least at the employed field of 9:4 T. However, the observed upfield shift of the center of gravity might indicate increasing mixing of the B2 O3 and SiO2 networks, as discussed by Martens and Mu. llerWarmuth [3]. The above-mentioned lack of resolution can be overcome with the help of 11 B MQMAS spectroscopy. Figure 2 exhibits the sheared two-dimensional MQMAS spectrum of glass 1; 0:28B2 O3 0:72SiO2 . The bottom spectrum parallel to the F 2axis represents the conventional MAS spectrum, whereas the projection on the F 1axis yields the isotropic spectrum. Two different signals can easily be identified; a signal at approx. 18 ppm and a broader second one at approx. 24 ppm (cf. Table 1). Applying the equations [16] diso ¼ dF2 þ ðdF1 dF2 Þ17 27 and sffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Z2Q ¼ 8:246 103 n0 dF1 diso SOQE ¼ CQ 1 þ 3
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29
TABLE 1 Si Chemical Shift Values and 11 B NMR Parameters for the Studied Glass Samples as Determined from 29 Si MAS and 11 B MQMAS
Glass #
xB2 O3
d29 iso Si (ppm)
Site 1 d11 iso B
SOQE (MHz)
d11 iso B (ppm)
Rel. fraction
SOQE (MHz)
0.14 0.27 0.33 0.37 0.44 0.47 0.53 0.57 0.61 0.68
2.8 2.9 2.8 2.8 2.8 2.8 2.8 2.8 2.9 2.8
11.0 11.6 11.7 11.9a 11.9a 11.8 12.1 12.5 11.7a 12.5
0.86 0.73 0.67 0.63 0.56 0.53 0.47 0.43 0.39 0.32
2.8 2.6 2.7 2.7 2.8 2.6 2.6 2.6 2.9 2.8
(ppm) 1 2 3 4 5 6 7 8 9 10
0.28 0.37 0.46 0.53 0.58 0.59 0.64 0.73 0.85 1.00 a
110:0 109:5 108:9 109:9 110:2 110:2 108:2 108:0 }
16.0 15.7 16.0 16.3a 16.4a 16.0 15.9 16.5 16.3a 17.3
Site 2
Rel. fraction
Measured at 10 000 Hz MAS.
translates these values to isotropic chemical shifts of 16.0 and 11:7 ppm for the two respective environments and second-order quadrupole effects (SOQE) of approx. 2:7 MHz. dF1 and dF2 are the centers of gravity in the respective dimensions, CQ is the quadrupolar coupling constant given by CQ ¼ 3e2 qQ=h; ZQ denotes the asymmetry parameter. These chemical shift values may be compared to those found by Zwanziger et al. [22–24] for a pure B2 O3 glass and by Martens and Mu. ller-Warmuth [3] for borosilicate glass. Zwanziger et al. found two signals in a pure B2 O3 glass which they assigned to boroxol rings ððBOÞ3 O3=2 Þ; ð70%; 17 ppmÞ and BO3=2 units ð30%; 13 ppmÞ connecting the rings. Martens and Mu. ller-Warmuth [3] attributed the 11 Bsignal at 13:5 ppm found in a high silica borosilicate glass to boron atoms in a mixed borosilicate glass phase. Accordingly, we assign the peak at 16:5 ppm to boroxol rings within a borate type of network, whereas the broad signal at 12:5 ppm is ascribed to a superposition of signals arising from BO3=2 units in a borate environment and BO3=2 units in a borosilicate environment. The compositional dependence of the relative fraction of the two signals, which is plotted in Fig. 3, lends support to the above assignment. Obviously, the relative fraction of the upfield signal, covering the range of 0.86 at xB2 O3 ¼ 0:28 to 0.32 at xB2 O3 ¼ 1:0 is neither compatible with a complete phase separation nor with a perfect mixing. A complete phase separation over the entire compositional range would imply a constant relative fraction of 0.32 for the upfield signal, whereas for complete mixing only one signal is expected, exhibiting a dependence of the chemical shift on the composition. The results therefore indicate a phase separation in these glasses into a borate and a mixed borosilicate glass phase. The MQMAS results were used as input parameters for the simulation of the 1D 11 B MAS spectra. The results of these simulations are consistent with the results of the MQMAS experiments.
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FIG. 3. (a) isotropic projections of the 11 B MQMAS spectra for the studied glass samples; xB2 O3 ¼ ðtop to bottomÞ 0.28, 0.37, 0.46, 0.59, 0.64, 0.73, 1.0. (b) Relative fraction of the 11 B signals near 17 ppm (open circles) and 12 ppm (filled circles) as a function of xB2 O3 . Solid lines: theoretical curves following the model discussed in the text.
The two-dimensional 29 Si–f11 Bg CP HETCOR [25] experiment for glass 6 is shown in Fig. 4. This experiment allows a direct correlation of S- and I-spin frequencies. Since the CP process is based on the dipolar coupling strength between the involved nuclei, only those 11 B nuclei involved in B–O–Si linkages can contribute to the 11 B HETCOR signal (F1 domain). The efficiency of the CP process depends largely on the spin-lock behavior of the 11 B transverse magnetization, which is governed by the adiabaticity parameter a¼
u2RF uQ uMAS
with uQ being the quadrupolar frequency, defined by uQ ¼ 3e2 qQ=ðhð2Ið2I 1ÞÞ; uMAS being the rotor frequency and uRF being the nutation frequency of the 11 B
VAN WU¨LLEN AND SCHWERING
140 ppm 60 40 20 0 -20 -40 -60
0
50
100
150
ppm
FIG. 4. 2D 29 Si–f11 Bg CP HETCOR spectrum for glass 6: 0:59B2 O3 0:41SiO2 . The projection along F2 represents the 29 Si spectrum, the projection along F1 the 11 B spectrum.
nucleus in the applied spin-lock field [26–28]. Under the condition a 1, the magnetization is effectively recovered after each full rotor period, whereas for a ffi 1 mixing of states during the rotor period leads to fast dissipation of the transverse magnetization. In the limit of a 1 (sudden regime), a fraction of the transverse magnetization can be preserved over longer spin-lock times. Since, considering the 11 B quadrupolar coupling constant of ca. 2:7 MHz in the studied glasses, it is not possible to reach the adiabatic regime with our hardware equipment, we chose to adopt the a 1 condition using uMAS ¼ 5 kHz and uRF ð11 BÞ ¼ 8 kHz. A simulation of the signal (diso ¼ 12:9 ppm; CQ ¼ 2:6 MHz and ZQ ¼ 0:3) produces values very close to those found by Martens and Mu. ller-Warmuth [3]. This is unambiguous evidence for the existence of a mixed borosilicate phase consisting of BO3=2 and SiO4=2 polyhedra. We do not find any indication for the presence of direct linkages between the boroxol rings and SiO4=2 tetrahedra, although their existence cannot be ruled out.
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1.2 1.0
xB2O3 xSiO2 ---------------------------0.28 0.72 0.46 0.54 0.58 0.42 0.59 0.41 0.73 0.27
∆ S/S0
0.8 0.6 0.4 0.2 0.0 0.000
0.002
0.004
0.006
0.008
0.010
0.012
NTR /s
FIG. 5. 29 Si–f11 Bg-REAPDOR data for glass 1: 0:28B2 O3 0:72SiO2 (filled circles); glass 3: 0:46B2 O3 0:54SiO2 (open diamonds); glass 5: 0:58B2 O3 0:42SiO2 (open circles); glass 6: 0:59B2 O3 0:41SiO2 (open squares); glass 8 0:73B2 O3 0:27SiO2 (open triangles). Solid line: theoretical 29 Si–f11 Bg REAPDOR curve for an Si–B two-spin system; dotted line: REAPDOR curve for an SiB2 three-spin system assuming a homogeneous distribution of boron; dotted line: theoretical REAPDOR curve for a statistical distribution of boron within a 0:48B2 O3 SiO2 mixed borosilicate glass phase. For the generation of the curve we did not account for the fraction of SiB4 units, since the calculation time exceeds any reasonable time frame. Since the SiB4 five-spin system contributes to only 2% of the total signal, the ( was assumed in the calculations. expected error is rather small. An Si–B distance of 2:77 A
The 29 Si–f11 Bg REAPDOR experiments were performed to further elucidate the intermediate range ordering in the mixed borosilicate phase. Figure 5 contains a compilation of the REAPDOR curves for the studied glass samples. The 29 Si–f11 Bg REAPDOR curves are mainly dominated by the number of interacting 11 B nuclei, since the 29 Si–11 B distances of the B–O–Si linkages are not expected to be subject to a large distribution. Discussing the curves on a qualitative basis, the initial slope and steepness of the curves are a measure for the strength of the dipolar coupling between 29 Si and 11 B. We note that the overall appearance of the REAPDOR curves}with the exception of glass 1}is fairly independent of the composition. This allows as a first conclusion that the dipolar coupling and hence the mixing of B2 O3 and SiO2 increases until a limiting value of xB2 O3 is reached. A further increase in the xB2 O3 mole fraction does not change the magnitude of the 29 Si–11 B dipolar coupling, i.e. the silicate network is saturated with boron at this limiting composition. For a quantitative analysis of the REAPDOR curves, we need an estimate for the distance between boron and silicon in borosilicates. Although there are no naturally occurring borosilicate minerals which parallel the connectivity pattern present in binary borosilicate glasses, i.e. a corner sharing of BO3=2 and SiO4=2 polyhedra, there are but a few known examples of synthetic borosilicate crystals. In M3 BSi2 O10 ðM ¼ Eu; Gd; SmÞ [29, 30] and Nd3 BGe1:08 Si0:92 O10 [31], BSiO5 anions are found, 6 exhibiting the above-mentioned connectivity pattern. The B–Si distances in these ( , respectively. Quantum chemical crystals are 2.788, 2.847, 2.781 and 2:760 A
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VAN WU¨LLEN AND SCHWERING
DFT calculations on model borosilicate clusters produced B–Si distances of ( [32]. We used an averaged B–Si distance of 2:77 A ( , corresponding to 2.76–2:80 A a dipolar coupling constant of 360 Hz, as input for the simulations of the REAPDOR evolution curves. The simulations were performed using the software package SIMPSON [33]. With the internuclear distance as input, the only adjustable parameter is the number of 11 B nuclei participating in the SiBn spin system. Since the geometry of the SiBn spin system is not known, we restricted the analysis to the geometry-independent initial part of the dipolar evolution curves ðDS=S0 50:6Þ [17]. An average number of approx. one boron atom per SiO4=2 tetrahedron is obtained for glass 1; 0:28B2 O3 0:72SiO2 ; and an average number of 1.7 boron atoms per SiO4=2 tetrahedron is obtained for glasses with xB2 O3 > 0:4, cf. Fig. 5.
DISCUSSION The MQMAS results identified the boroxol ring ðBOÞ3 O3=2 , free BO3=2 - and SiO4=2 -units as the polyhedra constituting the network. The 29 Si–f11 Bg CP HETCOR experiment demonstrates a connectivity between (a fraction of) the BO3=2 -groups and SiO4=2 polyhedra, while no connection between the boroxol ring and SiO4=2 units was found. The compositional dependence of the relative fraction of the 11 B signals and the virtual unison of the 29 Si–f11 Bg REAPDOR curves can be interpreted in terms of a phase separation into a borate glass and a mixed borosilicate phase. In a xB2 O3 ð1 xÞSiO2 glass, B2 O3 can only be incorporated into the SiO2 matrix up to a limiting fraction. Excess B2 O3 is consumed in a pure borate network. The conjecture of such a phase separation reproduces the observed compositional dependence of the relative fractions of the two 11 B signals. The fraction of boron in the various possible environments can be calculated as follows. Assuming that f gives the fraction of B2 O3 per SiO2 involved in the borosilicate network, the fraction of boron in the mixed borosilicate phase of a xB2 O3 ð1 xÞSiO2 glass is given by ð1 xÞf =x. Correspondingly, 1 ð1 xÞf =x gives the fraction of boron in the pure borate network. This fraction can be subdivided into the fraction of boron consumed in boroxol rings, ð1 ð1 xÞf =xÞ 0:68, and the fraction of boron in BO3=2 -units within the borate network, ð1 ð1 xÞf =xÞ 0:32, where 0.68 and 0.32 are the relative fractions of boron involved in boroxol rings and free BO3=2 -units, respectively (cf. Fig. 3a and Table 1) [22–24]. With these equations, we can calculate the relative fractions of the various boron species as a function of the mole fraction xB2 O3 . Reasonable agreement between experimental data and calculated fractions of the individual units is obtained for f ffi 0:48. The results are plotted as solid lines in Fig. 3b, in which the calculated fractions for the free BO3=2 -units and the BO3=2 groups in the mixed borosilicate phase are combined, since the deconvolution of the signals is not possible. The agreement with the experimental data is less satisfying for glasses with xB2 O3 50:46. For these high silica glasses, the mixing of B2 O3 and SiO2 is obviously not as complete as indicated by the model.
STRUCTURE OF BINARY B2O3–SiO2 GLASSES
143
The REAPDOR results are consistent with the proposed model. The SiO2 network can accommodate B2 O3 only up to a limiting fraction of 0:48B2 O3 per mole SiO2 , excess B2 O3 is consumed in a B2 O3 type of network. The limiting mole fraction xMAX B2 O3 for a completely mixed borosilicate glass is given by xMAX B2 O3 ¼ ð1 xMAX Þ 0:48. Accordingly, the overall structure of the mixed borosilicate phase should not be subject to changes, once the limiting composition of 0:32B2 O3 0:68SiO2 is exceeded. The phase is characterized by a silicon-to-boron ratio close to unity (0.96). However, a further characterization of the network organization with respect to the distribution of BO3=2 -polyhedra proves to be impossible. The assumption of a homogeneous distribution of boron within the borosilicate network (which would be characterized by ca. 1.7 B–O–Si linkages per SiO4=2 tetrahedron, taking into account the different coordination numbers for silicon and boron) as well as a statistical distribution of boron within the mixed network both lead to reasonable fits of the experimental data. For the latter scenario, the relative fraction W for an SiO4=2 tetrahedron with n Si–O–B linkages is given (to a first approximation) by ! 4 W ¼ A4n Bn n ¼ A4n Bn
4! ; n!ð4 nÞ!
where A is the probability for an Si–O–Si linkage, given by A ¼ 1=ð1:5f þ 1Þ, and B the probability for an Si–O–B linkage, given by B ¼ 1 A. Assuming a relative fraction of 0:48B2 O3 per SiO2 in the borosilicate phase, as concluded from the MQMAS experiments, the probability for an SiO4=2 -tetrahedron with n Si–O–B linkages can be calculated as 0.113 ðn ¼ 0Þ; 0:327 ðn ¼ 1Þ; 0:356 ðn ¼ 2Þ; 0:172 ðn ¼ 3Þ and 0:032 ðn ¼ 4Þ. Using these values and taking the natural abundance of the 11 B isotope (80%) into account, a simulation produces the 29 Si–f11 Bg REAPDOR curve, plotted as a slashed curve in Fig. 5. Again a dipolar coupling of 360 Hz was assumed. As can be inferred from Fig. 5, the curves for the two different scenarios (homogeneous and statistical distribution) result in equally reasonable fits to the data. CONCLUSION The presented results show that a combination of advanced NMR-techniques, 11 B MQMAS NMR, 29 Si–f11 Bg REAPDOR NMR and 29 Si–11 B CP HETCOR NMR, allows detailed insight into the network organization of glasses in the system xB2 O3 ð1 xÞSiO2 . While the CP HETCOR results present unambiguous proof for Si–O–B linkages in the borosilicate glasses, the degree of mixing could be quantified by the results of 11 B MQMAS NMR and 29 Si–f11 Bg READPOR NMR experiments. The data can consistently be described assuming a phase separation into a borate glass phase and a mixed borosilicate phase. The mixed phase adopts a boron-tosilicon ratio close to unity, which does not change with composition. A further
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characterization of the network organization in the mixed borosilicate phase with respect to the distribution of the Si–O–B linkages proved to be impossible. We are currently extending this work with respect to ternary alkali borosilicate glasses.
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