Estimating accuracy of 17O NMR measurements in oxide glasses: Constraints and evidence from crystalline and glassy calcium and barium silicates

Journal of Non-Crystalline Solids 358 (2012) 2999–3006

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Estimating accuracy of 17O NMR measurements in oxide glasses: Constraints and evidence from crystalline and glassy calcium and barium silicates Linda M. Thompson ⁎, Ryan J. McCarty, Jonathan F. Stebbins Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305, USA

a r t i c l e

i n f o

Article history: Received 23 June 2012 Received in revised form 25 July 2012 Available online 21 August 2012 Keywords: Silicate glasses; 17 O NMR spectroscopy; Non-bridging oxygen

a b s t r a c t The use of 17O nuclear magnetic resonance (NMR) spectroscopy to measure site populations in silicate and aluminosilicate glasses has provided insights and challenges to conventional models of glass structure. In order to better understand the level of accuracy and precision achievable, we have synthesized crystalline barium metasilicate (BaSiO3), barium orthosilicate (Ba2SiO4), tricalcium silicate (Ca3SiO5), a barium silicate glass ((BaO)0.45(SiO2)0.55), and a calcium silicate glass ((CaO)0.56(SiO2)0.44), and report 17O NMR spectra for all of these. After correcting the observed intensities for quadrupolar effects, we measure an NBO content of 66.7% ± 0.6% for the BaSiO3, compared to the known value of 66.7%. Applying the same techniques for the glasses gives an NBO content of 58.8% ± 0.8% (vs. the expected 55.5% ± 1.4% from stoichiometry) for the barium silicate and 76.9% ± 1.2% (vs. 78.6% ± 1.4%) for the calcium silicate. Within our uncertainties, we find no evidence for deviation from conventional models of glass structure for the glasses studied here. We also see no NMR signal (detection limit of about 0.5%) at the expected position for “free” oxide ions (bonded only to Ca2+), as newly constrained by our data for crystalline Ca3SiO5, which contains this species. © 2012 Elsevier B.V. All rights reserved.

1. Introduction The impact of melt structure on macroscopic properties in silicate melts and glasses has been the subject of much research, reviewed in detail in a number of studies [1–3]. Techniques used to characterize glass and melt structure have included Raman spectroscopy [4,5], EXAFS [6,7], neutron diffraction [8,9], nuclear magnetic resonance [10–12], and XPS [13–15]. In particular, the quantitative and element-specific nature of NMR has provided a variety of insights into both expected and unexpected structural changes with composition and temperature in a variety of silicate and aluminosilicate glasses [e.g., 12,16–21]. Conventional wisdom for glass structure finds its roots in concepts derived from crystal structure, as presented by Zachariasen [22]. In this most common model, silicate glass structure is viewed as a polymerized network of corner-shared silica tetrahedra (SiO4) connected through bridging oxygens, which is disrupted by the introduction of modifier oxides, creating non-bridging oxygens (NBO) bonded to only one silicon tetrahedra. At very high modifier contents, for example beyond the orthosilicate composition (e.g. Ca2SiO4), “free” oxide ions not bonded to any network cations are expected to become abundant. The introduction of alumina will repolymerize the network if the modifier cation serves to compensate the additional negative charge, such that the network is thought to be fully

⁎ Corresponding author. Tel.: +1 650 723 4475. E-mail address: [email protected] (L.M. Thompson). 0022-3093/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnoncrysol.2012.07.032

repolymerized at the point where the amount of alumina present in the melt or glass is equal to the amount of modifier oxide required for exact charge balance. Neutron scattering and X-ray data have also suggested that in binary silicates, some glasses may have modifierrich and silica-rich regions [6]. Since the majority of nuclides of interest for NMR studies of silicate glasses are quadrupolar, the introduction of higher field strength magnets and two-dimensional methods such as triple-quantum magicangle spinning (3QMAS) has greatly enhanced the resolution and information content. For example, NMR has already provided several illustrations of the limits of conventional wisdom, such as five-coordinated Al in modifier-rich compositional regions of aluminosilicate glasses [12,18]. In particular, 17O NMR has provided new insights into a range of structural questions in silicate, aluminosilicate, and other oxide glasses, including the observation of NBO on the charge-balanced join and aluminum-rich regions in aluminosilicate glasses [16,23,24], the presence of Al–O–Al in violation of the “aluminum avoidance” rule [25,26], variations in silicon–oxygen bond angles and length in silicate glasses [27,28], and modifier cation disorder [29–32]. In several of these studies, the clear presence of unexpected oxygen species has challenged the conventional wisdom and expanded our understanding of how glass and melt structure changes with composition. However, in order to assess potential deviations from models in regions such as modifier-rich compositions, where the presence of NBO is expected and the question then turns to the exact quantity, we need a better understanding of the accuracy achievable with 17O NMR before we will be able to draw strong conclusions.

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NMR spectroscopy is an inherently quantitative technique in that the amount of signal that can be detected in an ideal experiment is independent of the local chemical environment of the nuclei being observed. However, particularly for quadrupolar nuclides such as 17O or 27Al, accurate analysis of observed MAS central (1/2 to − 1/2 transition) peak areas can be somewhat complicated. Because the spinning speed is finite, a small but sometimes significant proportion of the central peak area can be shifted to spinning sidebands, and the satellite transitions (e.g. ±1/2–3/2 transitions) have a small component that can be hidden beneath the central peak, similar in shape and area to their spinning sidebands (as shown in Fig. 1). The required corrections depend on the quadrupolar parameters CQ and η, which generally vary from site to site. For well-resolved peaks in crystals, a rigorous methodology to account for these effects has been developed and carefully demonstrated for 27Al in yttrium aluminum garnet (YAG) [33,34], which has been employed in many subsequent studies. Additionally, chemical shift anisotropy (CSA) can result in a transfer of some signal from the central peak to the spinning sidebands, resulting in a characteristic intensity pattern in the sidebands. Although this has not been seen for many oxide materials characterized thus far by 17O NMR, large CSAs have been observed for oxygen sites coordinated by diamagnetic transition metal cations such as Ti 4+ [35,36]. The presence of large and different CSA's for different oxygen sites could, in principle, impact measured ratios of central peak areas; the impact of this, relative to that of quadrupolar effects, would vary with magnetic field. Similar to an early, critical study of aluminum sites in crystalline yttrium aluminum garnet (YAG) [33], in this study we examine three crystalline silicates and two binary silicate glasses to determine what precision and accuracy of site populations may currently be achieved with 17O MAS NMR. We also examine the oxygen spectra for evidence of “free” oxide ions, including that of a crystalline calcium silicate that contains this species in abundance, to provide constraints on the potential amount present at these compositions. 2. Experimental 2.1. Sample synthesis Crystalline barium metasilicate (BaSiO3), barium orthosilicate (Ba2SiO4), and tricalcium silicate (Ca3SiO5), as well as a calcium

r 500

400

silicate glass (nominal composition (CaO)0.555(SiO2)0.445) were prepared from reagent-grade BaCO3, CaO and 45% 17O-enriched SiO2. All reagents were dried and ground together to produce 250–500 mg of sample. To speed spin–lattice relaxation and permit faster NMR data collection, Co3O4 (0.2 wt.%) was added to all samples except for the tricalcium silicate. The barium metasilicate was decarbonated at 800 °C overnight in an argon atmosphere before being melted at 1690 °C and quenched by dipping the bottom of the platinum crucible in water. Complete crystallization was confirmed by inspection with optical microscopy as well as 29Si and 17O MAS NMR, so no additional sub-solidus heating was required. The barium orthosilicate was also decarbonated at 800 °C overnight in argon before two segments of sub-solidus heating at 1320 °C for 3 and 10 h, respectively, between which it was reground to ensure homogeneity. Complete crystallization was confirmed by 17O NMR; the crystals had a deep mauve color which is attributed to the cobalt. The tricalcium silicate was sintered twice at 1400 °C in an argon atmosphere, once for 16 h and once for 8 h, and ground in isopropyl alcohol between the two sinter cycles. The calcium silicate glass was melted in argon at 1600 °C and quenched; no crystals were detected using optical microscopy. Because of the high quench rate required to produce a barium silicate glass at the metasilicate composition, we used a previously studied 17O-enriched barium silicate glass of slightly higher silicate content (nominal composition (BaO)0.45(SiO2)0.55) [37]. Compositions for the glasses (and BaSiO3) were checked by electron microprobe analysis, and are given here for the glasses as MSx.y where x is the mol% of MO (M = Ca or Ba) and y is the mol% of SiO2. All composition information is also given in Table 1. 2.2. NMR spectroscopy For 17O, the 1D MAS NMR spectra were collected with a Varian Unity/Inova spectrometer at a 14.1 T field (81.29 MHz) and a Varian Infinity Plus spectrometer at a 9.4 T field (54.19 MHz) using Varian/ Chemagnetics “T3” probes with 3.2 mm zirconia rotors spinning at 20 kHz. The 29Si MAS NMR spectra were collected at 9.4 T (79.46 MHz) using the same probe and rotor setup. All spectra were referenced to 17O-enriched H2O or tetramethylsilane (TMS). Spectra were acquired using a single pulse excitation with pulse widths of about 0.3 μs for oxygen (short enough to allow accurate quantitation of sites with a wide range of quadrupolar coupling constants) and

* 300

200

* 100

0

-100

-200

ppm Fig. 1. 17O MAS NMR spectrum (14.1 T) for BaSiO3, showing the first spinning sidebands (ssb) on either side of the central peaks and the equivalent ssb present underneath the central peaks. Ssb for the NBO are shaded light gray, BO gray, orthosilicate (“o”) black. ‘r’ marks a background peak from the zirconia rotor and ‘*’ its ssb.

L.M. Thompson et al. / Journal of Non-Crystalline Solids 358 (2012) 2999–3006

shown as the intensity uncertainty (± one standard deviation). Intensity corrections were also calculated for the glass spectra, as detailed in the Results section.

Table 1 Glass and crystalline sample compositions in mol% from electron microprobe measurements, and calculated NBO contents. Sample

mol% CaOa

mol% BaOa

mol% SiO2a

Calculated Onb / Ototal

BaS45.55 CS56.44 BaSiO3

– 56.4 –

43.4 – 49.4

56.6 43.6 50.6

0.555b 0.786b 0.667

3001

3. Results 3.1. Barium metasilicate (BaSiO3) and orthosilicate (Ba2SiO4)

a

Error range on electron microprobe analysis estimated on all values at ±0.5 mol%. b Error range estimated from electron microprobe work to be ±0.014 except for BaSiO3 where the NBO is estimated from site occupancy and has no associated error estimate.

The structure of crystalline barium metasilicate contains three different oxygen sites. Two non-bridging oxygen are each bound to three Ba atoms, while one bridging oxygen is also coordinated to two Ba [40]. The 17O MAS spectra (Fig. 2) clearly show two main peaks; a lower frequency peak with a clear quadrupolar shape, which is assigned to the bridging oxygen, and a higher frequency peak with multiple components, which is assigned to the two non-bridging oxygen, consistent with what was previously observed in a study of the alkaline earth metasilicates [41]. The small peak at about 185 ppm is assigned to the NBO in barium orthosilicate (Ba2SiO4), whose presence is confirmed by a 29Si spectra showing a sharp main peak at − 80 ppm, corresponding to the Q 2 species present in the metasilicate [42], with a small second peak at − 70.4 ppm corresponding to the Q 0 species present in the orthosilicate [43] The 17 O MAS NMR data at both fields (9.4 and 14.1 T) were simulated to determine the NMR parameters for each site in the metasilicate, given in Table 2. The results of these simulations are presented in Fig. 2. The presence of the small peak at 185 ppm in the oxygen spectra indicated the presence of a barium orthosilicate phase in the metasilicate sample, confirmed by the 29Si spectrum. To account for any contribution from orthosilicate peaks to the NBO peaks in our metasilicate sample, a separate barium orthosilicate sample was synthesized. Its 29Si spectrum confirmed that this was about 95% of the desired phase; its 17O spectrum shows the presence of a small peak at approximately 163 ppm in addition to the larger peak at 185 ppm, analogous to observations and calculations for forsterite

0.6 μs for silicon, corresponding to about 30° radio frequency tip angles in the solid. Delays of 5–10 s (300 s for 29Si) for crystals and 0.1 s for glasses were used between pulses to optimize the signal-to-noise ratio; no differential relaxation was observed between signals for different sites in the glasses or crystals at this delay. All plotted spectra are normalized to the highest peak maxima. The 17O spectra for BaSiO3 were integrated using the Varian VNMR software. NMR parameters were obtained by simulating the spectra at multiple magnetic fields and manually changing the parameters to find the best agreement between the experimental and simulated spectra at all fields using the STARS simulation package in VNMR [38]. Matching the observed data, the spectra were simulated with a difference from the magic angle of 0.03°. Intensity corrections were calculated following the procedure outlined by Massiot et al. [33], which accounts for effects of magnetic field, spinning speed and quadrupolar parameters on central peak areas, and were also checked by simulating the sideband manifold for each. The glass spectra were quantified using both VNMR, for comparison with BaSiO3, and the software package DMfit [39], as detailed in a previous study [24]. In all cases, the intensity analysis was repeated 10 times (with slightly different baseline endpoints in VNMR and by resetting and re-optimizing in DMfit) to provide an estimate of fit reproducibility,

NBO

NBO

14.1 T

9.4 T BO

BO

o

o

sim, total

sim, NBO

sim, NBO sim, BO

250

Ba2SiO4

250

200

150

100

50

0

ppm 200

150

100

50

0

ppm Fig. 2. Experimental 17O MAS NMR spectra for crystalline BaSiO3 collected at fields shown (top), with the simulated spectra below (Table 2). The latter include all transitions. The visible barium orthosilicate impurity peak is marked by ‘o’; spectrum for pure-phase Ba2SiO4 is shown at the bottom.

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Table 2 Fitted 17O NMR parameters for crystalline BaSiO3, Ba2SiO4, and Ca3SiO5. Site

δiso (ppm)a

CQ (MHz)b

ηc

BaSiO3 Obr Onb Onb

89.5 161.5 170.5

3.8 1.9 2.4

0.34 0.49 0.34

Ba2SiO4 O(1,2)d O(1,2)d O(3)d

191.2 165.5 187.0

1.9 1.9 1.7

0.1 ± 0.1 0.1 ± 0.1 0.3 ± 0.1

Ca3SiO5 Mean NBO Mean “free” oxide

126.0 270.0

2.1e 1.3e

– –

a

Error range estimated at ±1 ppm. Error range estimated at ±0.2 MHz. c Error range estimated at ±0.05 unless otherwise noted. d All sites are NBO. Multipliticies (relative site populations) are 1, 1 and 2 for O(1), O(2), O(3) respectively, allowing assignment of O(3) only. e PQ value (see text). b

[44]. Peak areas are in the expected 2:1:1 ratio for the three known oxygen sites in the structure [45], with the first two contributions overlapped under the 185 ppm peak. Fitted parameters are given in Table 2: those for the O(3) site can be assigned based on its doubled intensity; we do not attempt here to distinguish between O(1) and O(2). This spectrum was used to subtract out the orthosilicate contribution from the metasilicate spectrum before the intensity analyses were done. From the structure, it is known that the NBO content of BaSiO3 is 66.7%. Intensities were first measured by integration, giving an uncorrected NBO content of 67.6 ± 0.5% (Table 3). Using the NMR parameters determined from the spectra and adjusting the intensities for quadrupolar interactions [33] the corrected NBO content is 66.7 ± 0.6%. This is in agreement with the value obtained by simulating the sidebands to determine the contribution to the central peak from each transition. For comparison, an alternate, less rigorous procedure of measuring the intensity of the satellite spinning sidebands on either side of each central transition peak, averaging them, and subtracting the result from the observed central peak, results in a calculated NBO content of 66.0 ±0.5%. A graphical representation of these spinning sidebands beneath the main peak is shown in Fig. 1. 3.2. Barium silicate glass The spectrum for the barium silicate glass features two peaks, shown in Fig. 3. At lower frequency is an asymmetric peak centered near 60 ppm attributed to the bridging oxygen (Si–O–Si). The more symmetric higher frequency peak centered near 155 ppm is assigned to NBO based on data from crystalline compounds (including this study) and other barium silicate glasses [37,41]. Although the peaks are broader than in the calcium silicate glass (Fig. 3), the increased peak separation, due to a large effect of Ba on isotropic chemical shifts of NBO, allows a relatively straightforward measurement of the intensities.

Table 3 Predicted and experimental NBO contents in crystalline BaSiO3 and barium and calcium silicate glasses from composition and intensity measurements. Method

BaSiO3

BS45.55

CS56.44

Predicted from composition Uncorrected experimental Corrected (per [33]) Sideband subtraction only

0.667 0.676 (5) 0.667 (6) 0.660 (5)

0.555 0.597 0.588 0.578

0.786 0.776 0.769 0.754

(14) (3) (8) (4)

(14) (4) (12) (6)

The compositional information from the electron microprobe can be used in conjunction with the “standard” model to predict an NBO content of 55.5 ± 1.4%. The uncorrected measured peak areas give an NBO content of 59.7 ± 0.3%, with the uncertainty given based only on the reproducibility of the fitting procedure used. Several assumptions must be made to correct the intensities using the method described for the barium metasilicate, including mean values for CQ and η, which are likely present as a range rather than a single value in the glass. For these calculations we used the CQ and η values as determined for the crystalline barium metasilicate (averaging these values for the NBO) with substantially larger uncertainties to reflect the range likely present in the glass. (Multiple quantum MAS spectra could provide more accurate estimates of the mean values of CQ and η in the glass and reduce this contribution to the uncertainty slightly, but with considerably lower sensitivity to minor species and much longer spectra acquisition times.) This increases the uncertainty on the correction factor; the corrected NBO content is 58.8 ± 0.8%. For comparison, the NBO content calculated by subtracting out the sidebands under the central peaks is 57.8 ± 0.4%, again based on the areas of the observed sidebands as described above. Estimated uncertainties given here are measures of the precision and reproducibility of the data and analysis; overall accuracies are discussed below. 3.3. Tricalcium silicate (Ca3SiO5) The structure for tricalcium silicate (the most abundant phase in Portland cement) is complex and contains multiple oxygen sites: in the most commonly cited description of the stoichiometric, room-temperature triclinic polymorph there are 45 distinct oxygens in the unit cell, 20% of which are “free” oxide ions coordinated only by Ca; the remainder are NBO [46]. An 17O spectrum is shown in Fig. 4. The peak centered at about 125 ppm is attributed to the non-bridging oxygen based on previous data from calcium silicate crystals and glasses, which found a shift to higher frequency for NBO with increasing Ca content [47]. The peaks from 260 to 280 ppm are attributed to the “free” oxide ion in the structure, based on the known shift for crystalline CaO at 294 ppm [48]. The latter is present as a minor impurity in this sample, as well, providing an accurate internal frequency standard. The quadrupolar coupling constant (CQ) for cubic CaO is known to be zero [48]. We have not attempted to fit the spectra for Ca3SiO5. However, analysis of the centers of gravity (centroids) of the “free” oxide ion peak group at 14.1 and 9.4 T indicates that the mean PQ (= CQ[1 + η 2 / 3] 1/2, where η is the quadrupolar asymmetry parameter and can range from 0 to 1) is 1.3 ± 0.1 MHz, and the mean isotropic chemical shift (δiso) is 270 ± 1 ppm. Presumably because of the relatively ionic character of the Ca\O bonding, this PQ value remains quite small in spite of significant distortion from ideal octahedral geometry for many of these sites in this phase [46]. These observations are also consistent with the changes in chemical shift (38 ppm) and PQ (1.3 MHz) observed for the “free” oxide ion observed in wadsleyite, a high pressure form of Mg2SiO4 [49] when compared with MgO (47 ppm, 0 MHz) [48]. A similar analysis of the NBO peak in Ca3SiO5, undoubtedly comprised of overlapping contributions from multiple sites, gives a mean PQ of 2.1 ± 0.1 MHz and δiso of 126 ± 1 ppm, comparable to data on other alkaline earth silicates. When peak areas are corrected as described above for BaSiO3, the “free” oxide peak was found to comprise 20 ± 1% of the total signal for the Ca3SiO5 phase, as expected from the known structure. 3.4. Calcium silicate glass The 17O spectrum for the calcium silicate glass features two partially-resolved peaks, seen in detail in Fig. 5 and compared to that for the barium silicate in Fig. 3. The non-bridging oxygen component is centered around 110 ppm, while the asymmetric peak centered

L.M. Thompson et al. / Journal of Non-Crystalline Solids 358 (2012) 2999–3006

3003 NBO

NBO BO

BO

250

200

150

100

50

0 250

200

150

ppm (BaO)0.45(SiO2)0.55 Fig. 3.

17

100

50

0

ppm (CaO)0.56(SiO2)0.44

O MAS NMR spectra (14.1 T) for BS45.55 and CS56.44 glasses. Data for crystalline BaSiO3 (14.1 T) is also shown below the glass spectrum for comparison.

around 60 ppm is attributed to the bridging oxygen, based on previous studies of binary calcium silicate glasses [21,47]. As has been previously noted both in this study and others [24,37], the oxygen peaks in the calcium silicate glass are narrower than those observed in the barium silicate glass, possibly because the much smaller Ca2+ ion more effectively orders NBO in its first coordination shell, resulting in more order around the oxygen sites as well. No peaks are observed in the region where free oxide ions are expected (based on the tricalcium silicate and calcium oxide data), with our limit of detection estimated to be about 0.5% for a peak similar in width to those observed for the non-bridging oxygen. Similar to the barium silicate glass, the uncorrected peak areas were measured to give an NBO content of 77.6 ± 0.4%. This initial

estimate compares with an NBO content of 78.6 ± 1.4% calculated from the microprobe data using the “standard” model. The corrected NBO peak area is 76.9 ± 1.2%; the apparent increase in uncertainty here is a function of the increased NBO content, as the percent error is similar to that observed in the barium silicate glass (approximately 1.5% of the measured value for each). The NBO content can also be calculated by subtracting the satellite sidebands from under the central peaks, with the value estimated as 75.4 ± 0.6%. 4. Discussion 4.1. Accuracy of site populations from

17

O MAS NMR

Barium metasilicate was chosen to assess how well we could quantify the NBO content because there are only three crystallographically

NBO

Ca3SiO5

CaO

CS56.44 glass

“free” oxide

r r

vs x10

*

400 350

300

250

200

150

100

50

0

300

200

100

0

ppm

ppm Fig. 4. 17O MAS NMR spectrum (14.1 T) for crystalline Ca3SiO5. “CaO” marks an impurity phase; * indicates a spinning sideband.

Fig. 5. 17O MAS NMR spectrum (9.4 T, where sideband separation is greater) for the CS56.44 glass, with the region from 140 ppm to 400 ppm also shown at an enlarged vertical scale. ‘r’ marks background peaks from the zirconia rotor.

3004

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distinct oxygen sites with good resolution between bridging and non-bridging oxygen, due to the well-known effect that longer cation-NBO distances, as generated for larger cations such as Ba 2+, move chemical shifts to higher frequencies [50]. Estimating the uncertainty introduced by the initial intensity measurements is done by either integrating the spectra from a variety of endpoints (in VNMR) or refitting and re-optimizing the spectra fit (in DMfit) to constrain the variation in intensities due to small variations in the fit parameters. Additionally, we have measured the uncertainty caused by slight variations in the CQ and η values. Correcting the intensities using the procedure of Massiot et al. [33] gives good agreement with the expected NBO content, showing that in a crystalline compound, 17O NMR can accurately assess oxygen speciation. The sideband pattern showed no indication of large and different chemical shift anisotropies (CSAs) for the different sites; additionally, all calculations were done with the assumption that CSAs were negligible and provided good agreement with the expected data, suggesting that neglect of possible CSA contributions will not have large effects on analysis of spectra such as those of alkaline earth glasses. With that verification, we can begin to more completely assess NMR accuracy in glasses, which can be more complex and challenging. There are multiple sources of potential error in the quantification of 17O NMR results in glasses. Although the resolution between BO and NBO peaks in the barium silicate glass is better than that of the calcium silicate glass, there is still some peak overlap. Peak fitting with DMfit, done consistently here using one Gaussian peak for the NBO and a line shape for the BO based on the CzSimple model [24,51], provided the best fit for the observed spectra. The potential absolute uncertainty here is relative to the size of the peak, and thus will be smaller for smaller peaks. The 17O spectra for the calcium silicate and barium silicate glasses both display broader peaks than observed in the crystalline samples, presumably because of variations in bond angle, bond length, and coordination environment. For example, both the bridging oxygen and non-bridging oxygen peaks in the barium silicate glass are broader than those observed in the calcium silicate glass, which could indicate increasing disorder in the BO angles or that the bridging oxygen also ‘sees’ the modifier ion. This disorder will also be reflected in a range of NMR parameters (such as η and CQ), which are used in the intensity correction procedure of Massiot et al. [33]. We have calculated the impact that variations in the η and CQ values would have on the correction factor and have included this in our calculations, both for the barium metasilicate and for the barium and calcium silicate glasses; because of the increased uncertainty in the parameters for the glasses, the error was correspondingly larger there. However, this contribution to the uncertainty is more influenced by the change in parameters than the size of the peaks and thus will likely remain relatively constant for a given type of material, although it may be influenced by a change in coordination environment and therefore composition. The uncertainty in our final estimate for the NBO content given here reflects both the relative error of the initial peak fitting and the constant error introduced by the range of correction factors. In previous studies, we have estimated the contribution of the satellite transitions to the central peak intensity by subtracting out the sideband intensities, hidden beneath the central peaks, before calculating the NBO content [23,24]. We have done the same calculation here for comparison purposes and find it to provide a reasonable estimate of the intensity within the achievable uncertainty; this is likely because the CQ values for bridging versus non-bridging oxygen in the glasses studied thus far do not vary over a huge range. This means that the applied correction factors to the central transition intensity are similar and may be neglected, leaving only the sideband contribution to be subtracted out, which we have done. Additionally, using sideband subtraction eliminates some of the error introduced by potential variation in the CQ and η values, since the sideband features the same distribution, thereby potentially decreasing the absolute error

(although introducing another contribution of relative uncertainty from the sideband fitting). Once we have finished computing the NBO content and providing an estimate for the uncertainty, it is necessary to compare it to the expected NBO content for the sample, as predicted by conventional models in which all oxygen ions are either NBO or BO. It is important to note that the uncertainty in glass composition results in an important addition to the uncertainty in the predicted amount of NBO. Within our measured uncertainties, we find that the results are in relatively good agreement with the standard model, although the precisions noted above (Table 3) may somewhat underestimate all contributions to the uncertainty in the overall accuracy due to overlap of broad peaks and, particularly for the barium silicate glass, difficulty in determining a baseline due to broad sidebands. This is bolstered by the fact that although we measured less NBO than predicted by the standard model in the calcium silicate glass, we measured more NBO than predicted in the barium silicate glass, suggesting that discrepancies are more likely to be randomly associated with the vagaries of peak fitting and compositional analysis rather than a systematic effect of either the spectroscopy or glass structure. If there were, for example, substantial amounts of unseen “free” oxide ions in these glasses (see below), we would have expected to see NBO contents of both glasses to be significantly less than those predicted, instead of both falling within the range of uncertainty. 4.2. “Free” oxide ions The presence of “free” oxide ions in silicate glasses and melts has been a visible question in recent literature [14,15,52,53] as well as a suggestion in several previous 29Si NMR studies [54–56]. Oxide ion activities determined by thermodynamic studies at much lower SiO2 contents in MO-SiO2 liquids (M = Ca, Fe, Ni, etc.) point to extremely low free oxide ion concentrations at higher silica contents (e.g. those studied here) but are challenging to interpret at the glass transition versus the liquidus temperature as well as on extrapolation in composition [57]. Such results have long suggested the conventional view that for the silica contents in the normal glass-forming range (well above the orthosilicate composition, with ≫33 mol% SiO2), the reaction below goes essentially to completion, as written from the point of view of the oxygen [23,24] and assuming (for convenience) octahedral coordination for the divalent modifier cation: h

O−2Si1=4 BO

i0

h i0 h i– þ O−6 M1=6 → 2 O−Si1=4 þ “free” oxide

NBO

2þ

M

modifier ion

:

ð1Þ

Therefore, deviations from the expected NBO content have been used to suggest that this reaction does not go all the way to completion and that “free” oxide ions are present in a glass. For example, measurements of Q-speciation using 29Si NMR suggested a higher number of bridging oxygen than expected in both calcium silicate [56] and sodium–lanthanum and potassium–lanthanum glasses [54], with the authors suggesting in both cases that the presence of 1–2% of “free” oxide ions could explain the discrepancy. It is important to note, however, that more recent 29Si NMR studies of alkali silicate glasses (15–50% alkali oxide) have found that better fits can be obtained, consistent with the absence of “free” oxide species, if structurally plausible additional Si sites are included, e.g. two types of Q 2 species [58]. Recent XPS work on sodium silicate [14] and potassium silicate [15] glasses were interpreted as suggesting a substantial “free” oxide ion presence (5–10% of the total oxygen) at SiO2 contents of 50 to 65% because of apparent deficits in measured NBO contents. More definitively, recent work on forsterite glass (Mg2SiO4) and sub-orthosilicate compositions (SiO2 b 33.3 mol%) in calcium–magnesium silicates (where “free” oxide ions are required by stoichiometry) found clear evidence in Raman and 29Si spectroscopy for Q1 species, which requires the presence of

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“free” oxide from reaction (1) [52,59,60]. Finally, direct observation of the “free” oxide ion using 17O NMR has been reported for such a low-silica composition [53], and very recent two-field 17O MAS NMR has shown that CQ values for such ions are small, and thus the resulting peak is narrow enough to be readily detectable [61]. The failure of reaction (1) to go to completion is not the only possible mechanism for changing the NBO content from standard models, although it is the most likely in a binary silicate. In aluminosilicate glasses, for example, triclusters have been suggested to explain anomalous viscosity data [62,63] although their direct observation has been limited to 17O NMR data for a calcium aluminate glass [64] and suggested by recent diffraction studies of an Al-rich barium aluminate [65]. The presence of triclusters could increase the NBO content of a silicate glass by the following reaction: h i0 h iþ h i− 2 O−2Si1=4 → O−3Si1=4 þ O−Si1=4 :

ð2Þ

However, this type of tricluster in an Al-free binary glass is extremely unlikely due to the overbonding of the oxygen; the incorporation of aluminum into the melt reduces the overbonding and could stabilize Al-containing triclusters, as suggested for high aluminum glasses with few NBO [66]. Likewise, a change in the silicon coordination could also change the NBO content by the following reaction: h

O−Si1=4

i−

h i0 h i−0:2 þ 4 O−2Si1=4 →5 O−Si1=4 Si1=5 :

ð3Þ

An increase in high-coordinated silicon would result in a decrease in NBO present without requiring the formation of free oxide ions. Although tiny amounts of five-coordinated silicon have been observed in binary silicate glasses [67], the role of this species in affecting NBO contents is probably much more important at high pressure [68,69]. To provide the best possible opportunity for the observation of free oxide in the 17O NMR, we looked at a calcia-rich glass near to the limit of what can be formed using standard quench rates. The crystalline tricalcium silicate spectra provide a reasonable estimate for the range of chemical shifts expected for “free” oxide ions in a glass, where it would also be likely to interact with the Ca 2 + ions in a distorted octahedral structure. The results for this crystal (as well as for wadsleyite) also show that CQ values for such sites are likely to be small, even when not strictly cubic, because Ca\O interactions are much more ionic than those in Si\O bonds. This reduces the likelihood of substantial peak broadening by quadrupolar interactions and accompanying lowered sensitivity. Examination of that region in the calcium silicate glass shows no detectable signal. Assuming a width similar to BO and NBO peaks, we estimate a detection limit of 0.5%. If the 17O NMR peak for the “free” oxide ion was hypothesized to be unexpectedly broad (and thus undetected), we can independently use measured NBO and BO contents to assess the possibility for this species, as was done in recent XPS studies [14,15]. Using reaction (1) and assuming that this is the only source for deviation from what is expected, we can estimate the upper limit for free oxide content in the glass based on the observed ratio of NBO to BO. The formation of 1 mol of free oxide consumes 2 mol of NBO and produces an additional mole of bridging oxygen. For the Ca silicate glass, within the uncertainties for this sample, the slight deficit in the observed NBO content allows a maximum percentage of free oxide of about 4%, but this assumes the maximum amount of ‘missing’ NBO. For the Ba silicate, our estimated NBO content is slightly higher than nominal, allowing essentially no “free” oxide ion. We conclude that within the uncertainties in fitting the spectra, there is no evidence to support the presence of free oxide in either glass. We note that in a detailed (and complex) fitting of 2-dimensional 29Si spectra for a CaSiO3 glass, a slight deficit in calculated NBO content allowed for a small percentage of “free” oxide ion [56], but that this could well be zero given the uncertainties.

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Higher cation field strength modifier cations are better able to stabilize more localized negative charge and may promote the formation of “free” oxide ions, as determined by thermodynamic analyses of low silica melts [57]. Thus, the suggestion of minor (ca. 2%) oxide ion content in lanthanum–sodium silicate glasses, based on NBO estimates derived by fitting of 29Si NMR spectra [54] may be more expected than those derived from XPS fits in alkali silicates [14,15], although it is unclear in both cases whether full uncertainties in composition, or the possible effects of asymmetrical peak shapes, were included in the error estimates. In any case, further study of this possible variation from standard models, ideally by methods that allow direct detection [53], will clearly be interesting. 5. Conclusion We have synthesized three crystalline silicates and two silicate glasses in order to refine what level of accuracy and precision is possible with 17O NMR in these systems. Using the correction procedure developed by Massiot et al. [33], we find excellent agreement between the predicted NBO content (66.7%) and the measured NBO content (66.7% ± 0.6%) for barium metasilicate (BaSiO3). Applying this to a barium silicate and calcium silicate glass gives reasonable agreement both for the full correction procedure and for the technique of subtracting out the spinning sidebands only, compared with the NBO ratio predicted by sample composition; some error remains unaccounted for, likely due to peak overlap in the fitting procedure. Within our uncertainties, we find no evidence for deviation from conventional wisdom in these glasses, such as the presence of “free” oxide ions in detectable quantities in the regions where they would be observed, per data from the tricalcium silicate (Ca3SiO5). Acknowledgments This research was supported by the National Science Foundation, EAR 1019596. We thank Aaron Palke for providing some of the EPMA data, Namjun Kim for assistance with the spectra for Ca3SiO5, and helpful comments from an anonymous reviewer. References [1] B.O. Mysen, P. Richet, Silicate Glasses and Melts, Properties and Structure, Elsevier, Amsterdam, 2005. [2] B.O. Mysen, M.J. Toplis, Am. Mineral. 92 (2007) 933–946. [3] J.F. Stebbins, P.F. McMillan, D.B. Dingwell, in, Mineralogical Society of America, Washington, DC, 1995. [4] B.O. Mysen, D. Virgo, I. Kushiro, Am. Mineral. 66 (1981) 678–701. [5] D.R. Neuville, B.r.O. Mysen, Geochim. Cosmochim. Acta 60 (1996) 1727–1737. [6] G.N. Greaves, J. Non-Cryst. Solids 71 (1985) 203–217. [7] G.N. Greaves, S. Sen, Adv. Phys. 56 (2007) 1–166. [8] L. Cormier, G. Calas, B. Beuneu, J. Non-Cryst. Solids 357 (2011) 926–931. [9] F. Kargl, A. Meyer, Chem. Geol. 213 (2004) 165–172. [10] J.F. Stebbins, Nature 330 (1987) 465–467. [11] S.K. Lee, J.F. Stebbins, Am. Mineral. 84 (1999) 937–945. [12] D.R. Neuville, L. Cormier, V. Montouillout, D. Massiot, J. Non-Cryst. Solids 353 (2007) 180–184. [13] K.N. Dalby, H.W. Nesbitt, V.P. Zakaznova-Herzog, P.L. King, Geochim. Cosmochim. Acta 71 (2007) 4297–4313. [14] H.W. Nesbitt, G.M. Bancroft, G.S. Henderson, R. Ho, K.N. Dalby, Y. Huang, Z. Yan, J. Non-Cryst. Solids 357 (2011) 170–180. [15] R. Sawyer, H.W. Nesbitt, R.A. Secco, J. Non-Cryst. Solids 358 (2012) 290–302. [16] J.F. Stebbins, Z. Xu, Nature 390 (1997) 60–62. [17] S. Sen, R.E. Youngman, J. Phys. Chem. B 108 (2004) 7557–7564. [18] D.R. Neuville, L. Cormier, D. Massiot, Chem. Geol. 229 (2006) 173–185. [19] S.K. Lee, J.F. Stebbins, J. Phys. Chem. B 104 (2000) 4091–4100. [20] J.F. Stebbins, Rev. Mineral. Geochem. 32 (1995) 191–246. [21] J.R. Allwardt, S.K. Lee, J.F. Stebbins, Am. Mineral. 88 (2003) 949–954. [22] W.H. Zachariasen, J. Am. Chem. Soc. 54 (1932) 3841–3851. [23] J.F. Stebbins, E.V. Dubinsky, K. Kanehashi, K.E. Kelsey, Geochim. Cosmochim. Acta 72 (2008) 910–925. [24] L.M. Thompson, J.F. Stebbins, Am. Mineral. 96 (2011) 841–853. [25] S.K. Lee, J.F. Stebbins, J. Non-Cryst. Solids 270 (2000) 260–264. [26] E.V. Dubinsky, J.F. Stebbins, Am. Mineral. 91 (2006) 753–761. [27] F. Angeli, O. Villain, S. Schuller, S. Ispas, T. Charpentier, Geochim. Cosmochim. Acta 75 (2011) 2453–2469.

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