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Oxide ion speciation in potassium silicate glasses: New limits from 17O NMR
Journal of Non-Crystalline Solids 368 (2013) 17–22
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Oxide ion speciation in potassium silicate glasses: New limits from
17
O NMR
Jonathan F. Stebbins a,⁎, Sabyasachi Sen b a b
Dept. of Geological and Environmental Sciences, Stanford University,Stanford CA 94305, United States Dept. of Chemical Engineering and Materials Science, University of California at Davis,CA 95616, United States
a r t i c l e
i n f o
Article history: Received 12 January 2013 Received in revised form 16 February 2013 Available online 22 March 2013 Keywords: Glass structure; Alkali silicate; NMR; Nuclear magnetic resonance; Oxide ion
a b s t r a c t In silicate glasses and liquids, “free” oxide ions (O2− ions not bonded to any network-forming cations such as Si) are required by stoichiometry for ratios of O/Si > 4, for example in “sub-orthosilicate” compositions (b 33.3% SiO2 in MO-SiO2 or M2O-SiO2 binaries). Measurements of oxide ion activities and other thermodynamic considerations suggest however, that at higher silica contents the concentration of such ions should rapidly become very low, particularly in systems with highly basic modifier oxides such as Na2O or K2O. Recent 17O NMR studies of alkaline earth ortho- and sub-orthosilicate glasses have indeed directly shown the presence of a few percent of “free” oxide ions, but did not detect this species at a silica content of 44 mol%. In contrast, recent O 1s XPS analyses of bridging oxygen concentrations in Na- and K-silicate glasses have suggested as much as about 6% “free” oxide even at about 67% SiO2 [1,2]. In K-silicates, theoretical calculations presented here, as well as previous work on Ca and Mg silicates, suggest that this species should be readily and directly measurable by 17O. However, we show here that “free” oxide ions are not detectable in glasses with 34 and 40 mol% K2O, and therefore are not likely to be present above the 0.1 to 1% level. Measured ratios of bridging to non-bridging oxygens are within errors of those expected from analyzed compositions and an assumption of negligible “free” oxide, and 29Si MAS spectra can be accurately fitted with constraints based on this same assumption. © 2013 Elsevier B.V. All rights reserved.
1. Introduction At least at very high temperatures, continuous solutions exist between most liquid oxides (e.g. those of alkali and alkaline earths) and molten silica. One of the most fundamental questions of oxide liquid structure thus becomes the transition in the coordination environments of the oxygen anions between those in low-silica systems, which must primarily have high coordination numbers by low-valent cations (e.g. with 6 to 8 M+ or M2+ neighbors, and designated as “free” oxide ions, FO) to those in high-silica systems, which have primarily only two, Si4+ neighbors, and are designated as “bridging oxygens” (BO). In between, as in crystalline silicate compounds, oxygen anions with one Si neighbor and several lower-valent neighbors become abundant, and are denoted as “non-bridging” oxygens (NBO). This structural transition has long been at the heart of models of the chemistry of molten silicates, although until recently these were based on little or no actual structural data. Early models, often concerned with low-silica metallurgical slags, emphasized the key equilibrium between these three types of oxygen anion, through the reaction: FO þ BO ¼ 2 NBO
ð1Þ
⁎ Corresponding author at: Dept. of Geological and Environmental Sciences, Bldg. 320, room 118, Stanford University, Stanford CA 94305, United States. Tel.: +1 650 723 1140. E-mail address: [email protected] (J.F. Stebbins). 0022-3093/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnoncrysol.2013.02.024
The apparent equilibrium constant for this reaction, which ignores activity coefficients, can be simply written in terms of mole fractions as Kox = (XNBO) 2/(XFO * XBO). As reviewed in some depth [3], models of varying complexity have derived values of Kox that vary strongly with the type of oxide ion, generally predicting systematically higher FO concentrations with increases in the ionization potential of the M cation, or its corresponding cation field strength, defined as the charge divided by the square of cation-oxygen distance [4–7], or alternatively, with decreasingly basic character of the oxide. These models served well to fit observed liquidus surfaces for oxides such as CaO, MgO, and FeO in low-silica liquids, where the activities of the FO are directly constrained by equilibrium with the corresponding crystals. By stoichiometry alone, if Si is primarily four-coordinated by oxygen, significant FO must be present when the O/Si ratio is greater than 4, e.g. at silica contents below 33.3 mol% in MO-SiO2 and M2O-SiO2 binaries. When M is an alkali earth cation, such models predict activities of FO to be small (corresponding to concentrations below a few %) when “normal” glass-forming silica contents are reached, e.g. at >50% SiO2, and plummet rapidly as silica increases further. Related approaches have also proven useful for modeling the two-liquid solvi in systems such as CaO-SiO2 [8,9]. Models involving equilibria such as reaction (1) have also long been developed for alkali oxide-silica systems. Here, the activities of the oxide ion (i.e. of FO) can be constrained relatively directly by Knudsencell vapor pressure data and by electrochemical measurements [10,11], as well as by modeling of equilibria between liquids and crystalline
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silicates [12]. In the typical glass-forming region (e.g. >50% silica), FO activities are generally very low (e.g. 10−9 to 10−8) relative to the pure oxide standard state, decrease with decreasing temperature, and are systematically lower with decreasing cation field strength (e.g. K b Na). Although the relationship of such activities to concentrations of observable structural species, as well as those between the high-temperature melts and those near to the glass transition as typically sampled by spectroscopy, are uncertain, there should be some close connection if the concept of actual “free” oxide ions is to have some chemical relevance. Independent considerations of the systematics of enthalpies of reaction among metal oxides and silica, to form intermediate silicate compounds, indicate that the FO species is likely to be most stable, and thus have higher concentration, when the ionization potential or field strength of the M cation is highest, e.g. La3+ > Mg2+ > Ca2+ > Na+ > K+ [3]. All of these considerations suggest that in the glass-forming regions of Na and K silicate systems, the actual concentration of FO should be very low, even though the concept of the activity of oxide ions remains a useful concept, just as pH does in very alkaline aqueous solutions. Nonetheless, it is important to attempt to place direct structural constraints on such species and test the assumptions involved in thermodynamic treatments, generally through studies of glasses that are assumed to represent the liquid structure near to Tg. Even in glasses of relatively high silica contents (well above 50 mol%), NMR spectroscopy and other methods have shown that monovalent anions such as Cl− [13], F− [14] and even OH− [15] often are coordinated by only alkali or alkaline-earth cations, although the divalent O 2− ion is expected to be much more reactive with Si\O\Si linkages. Spectroscopic studies have revealed that some minor structural groups of low energetic stability (and thus absent in typical solution models) can reach significant concentrations, possibly for entropic reasons, for example SiO5 [16] and AlO5 [17,18] species in ambient pressure glasses, and NBO in “charge-balanced” binary systems (e.g. CaAl2O4-SiO2) [19]. Two approaches have been taken to constrain or test the concentration of FO in glass-forming silicates. The most direct methods can in principle detect a unique spectroscopic signature of such species, but results have been limited. For example, BO and NBO peaks are often partially resolved in O1s XPS spectra of silicate glasses, and an early study interpreted a shoulder about 2 eV below the NBO as indicating the presence of as much as about 10% FO in a number of Li and Zn silicates, although this was not detected or resolved in Na, Ca, Sr, or Ba silicates [20]. In more modern XPS studies of Pb, Na, K silicate glasses this feature was not observed [1,2,21]. Recent high-field 17O MAS NMR spectra of very low silica Ca-Mg silicates glasses (b34 mol% SiO2) have partially resolved peaks whose frequencies correspond to those expected for FO with mixed Ca + Mg neighbors, based on the known chemical shifts of crystalline MgO and CaO [22]. For such “sub-orthosilicate” compositions, which can only be formed into glasses by rapid-quench, containerless methods, FO species are required simply by the high O/Si ratio, if Si coordination is predominantly 4. The latter is confirmed 29Si NMR. On the other hand, in Ba- and Ca-silicate glasses with silica contents as low as 44%, careful observation of the spectral region where FO peaks are expected, again based on crystalline model compounds (BaO, CaO, Ca3SiO5), did not find any indication of FO, with detection limits of b1% [23]. Other approaches to constraining the concentration of FO in glasses are less direct and generally have larger uncertainties. Oxygen 1s XPS has been applied in numerous studies to determine relative proportions of various bridging oxygen and non-bridging oxygen species, with results often consistent with conventional assumptions about low FO contents [24–26]. In more recent work, the fractions of BO out of total oxygen signals were determined from O 1s XPS spectra of Na-, K-, and Pb-silicate glasses, which have relatively well-resolved peaks for this species [1,2,21]. Comparison with compositions (nominal or as determined by XPS analysis) lead to calculations of FO concentrations that were surprisingly high (at least in the very basic Na2O-SiO2 and K2O-SiO2 glasses), up to about 7% at 35% K2O. In contrast, in lower-silica
Ca- and Ba-silicate glasses, where thermodynamic approaches suggest greater stability for the FO species, ratios of BO to NBO measured from 17O NMR spectra were found to be consistent with compositions determined by EPMA and an assumption of negligible FO, although uncertainties related to peak overlap would allow a few percent of such species [23]. In previous 17O NMR studies of alkali silicate glasses, BO to NBO ratios generally were consistent with compositions and assumptions of negligible FO, although the primary goals of these efforts were usually not to test assumptions about FO contents [27–32]. Even less direct, but complementary, are studies of the silicate anionic speciation in glasses, which has been done most quantitatively by 29Si NMR. Here, peak assignments are generally made based on the chemical shifts of known crystal structures of compositions closely related to those of the glasses [33]. If concentrations of Qn species (SiO4 groups with n BO and 4-n NBO first neighbors) can be estimated by fitting spectra of glasses, the NBO/Si ratio can be calculated and compared to the composition. If this is lower than expected, one interpretation could be the presence of FO. While peaks for at least the most abundant Qn species may be better resolved in alkali silicates than in other compositions, peak overlap introduces significant uncertainty to such measurements, and analyses are quite sensitive to the exact glass composition. For example, in an extensive early study of Li-, Na-, and K-silicates to as low as 50 mol% silica, Gaussian fits yielded Qn species concentrations that were consistent with nominal compositions in the Na and K series, but which suggested significant FO in the most Li-rich (e.g. 40% Li2O) glasses [34]. Compositions were not analyzed, however, leaving questions of possible alkali loss during synthesis unresolved: alkali contents lower than expected will lead to NBO/Si ratios that are apparently too low, possibly leading to the overestimation of FO contents. A subsequent 29Si MAS NMR study of Li-rich silicate glasses with up to 64 mol% Li2O [35] found Q-speciation that was consistent with the absence of any detectable FO. Similarly, in recent work, highquality 29Si NMR spectra of binary K, Rb, and Cs silicate glasses were fitted both without and with constraints imposed by analyzed composition and assumptions of no FO [36]. Constrained fits produced consistent results, but showed that a more complex model than previously used, e.g. with two Q2 peaks, was indicated in some compositions. Alkaline earth silicate glasses would be expected to have higher FO contents because of lower basicity and higher modifier cation field strengths, which tend to stabilize anionic groups with more concentrated negative charges [37]. However, peaks for Q n species are generally unresolved in normal, 1-dimensional 29Si MAS NMR spectra. More information can be obtained from challenging, 2-D 29 Si spectra using methods such as “magic angle flipping” (MAF). Fits to such data for CaSiO3 and MgSiO3 glasses, based on differences in the chemical shift anisotropy (CSA) for different Q n species, yielded a slight deficit in NBO and thus suggested about 1% FO in both glasses [38,39]. However, given uncertainties, fits constrained by the analyzed compositions, with no FO, probably would also have been consistent with the data. In some cases, static (non-MAS) spectra can yield key information about the network structure, because of especially small CSA's for the relatively symmetrical structural Q 4 and Q0 units [34,40,41]. However, such spectra generally do not allow the independent determination of all Q n species that are present, and so usually cannot accurately fix the extent of reaction (1). A special case most relevant here is the use of this method to detect significant fractions of lower-symmetry Q1 groups in Mg2SiO4 glass, which implied the presence of complementary FO, estimated at about 6% [42]. A recent 2-D 29Si MAF NMR spectroscopic study of this glass provided a significantly more accurate estimate of the relative fraction of Q 1 groups that results in an estimate of about 4.5% FO in this glass, when re-calculated with respect to total moles of oxygen [38]. These results were extended by 29Si MAS spectra of sub-orthosilicate Ca/Mg silicate glasses, where best fits included a Q 1 peak in addition to the predominant Q 0 component [22]. These
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findings are thus consistent with the later direct detection of FO by 17 O NMR as noted above. In one report on Na- and K trisilicate glasses with extensive substitution of La2/3O for Na2O or K2O, large apparent enhancements of the Q 4 peak intensities in 29Si MAS NMR spectra of compositions with constant nominal NBO/Si suggested up to about 2% FO, which could be stabilized by the very high field strength of the La 3 + cation [43]. However, the effects of this cation on chemical shifts are not well-known, and unexpectedly large overlaps or asymmetries of the Q 2 and Q 3 peaks, and/or of the Q 3 and Q 4 peaks, could allow speciation consistent with more conventional models. In this report we examine more closely the possibility of significant fractions of “free” oxide ions in potassium silicate glasses, where recent O 1s XPS results would predict up to about 9% of this species at the 40% K2O composition [2]. Using theoretical calculations to confirm the expected chemical shift range for such species in 17O NMR spectra, we provide new, direct constraints on the abundance of this species in two carefully chosen K-silicate glass compositions. In addition, we refine the indirect constraints on the NBO/BO ratio, also from 17O NMR, as well as confirming previous models of Q n speciation from 29 Si NMR. 2. Experimental methods and data analysis Potassium silicate glasses of nominal compositions 33.3 and 40.0 mol% K2O were prepared by mixing dried K2CO3, 42% 17O-enriched SiO2, and 0.1 wt.% cobalt oxide, decarbonating at 740 °C for about 15 h, then melting at 1200 °C for about 1 h in high purity Ar. Part of the 33% K2O glass was crystallized to K2Si2O5 by reheating at 755 °C for 24 h. Portions of the 500 to 800 mg samples were coarsely crushed and loaded into MAS rotors for NMR in a glove box under dry N2. Rotor caps were tight enough to prevent significant hydration during NMR experiments: spectra collected within an hour or two after loading were identical to those collected after several days; glass samples were removed as free-running powders in the dry box at the end of experiments. ICP-OES analyses for K and Si gave compositions of 34.5 ± 1 and 40.5 ± 1 mol% K2O; the glasses will thus be referred to below as “34%” and “40%-K2O” samples. If the slight excess in K2O led to a small amount of K2SiO3 in the crystallized K2Si2O5 sample, it was not detected in the relatively low-quality 29Si NMR spectrum collected for this material (see below). NMR spectra were collected with Varian Unity/Inova 600 (14.1 Tesla) and Infinity Plus 400 (9.4 Tesla) spectrometers using 3.2 mm Varian/ Chemagnetics “T3” probes at spinning rates of 20 to 23 kHz. 29Si spectra at 119.13 MHz were referenced to tetramethylsilane (TMS); 17O spectra at 54.19 and 81.29 MHz were referenced to 17O-enriched water. Spectra were acquired with single radiofrequency (RF) pulses corresponding approximately to 30° RF tip angle for solids. Various pulse delays were tested to ensure that relative peak areas were quantitative. With the exception of the 17O spectra for crystalline K2Si2O5 (see below), no differential relaxation was observed. Optimal spectra for the glasses were obtained with pulse delays of 10 s for 29Si and 0.1 or 1s for 17O. Relaxation for 29Si in crystalline K2Si2O5 was much slower. This limited the quality of the spectra collected for this sample. For some of the 17O spectra shown, the very small but readily observable background signal from the natural-abundance 17O in the zirconia MAS rotors was subtracted using a spectrum for the empty rotor; in others, this subtraction was not done, to illustrate the sensitivity of the experiments to low-level signals. For the 14.1 T 17O spectra of the glasses, relative peak areas for BO and NBO were derived by simply fitting a Gaussian to the NBO peak (consistent with the small CQ's known for such sites and the high magnetic field used) and subtracting to yield a residual BO peak (Fig. 1). This obviated the need for fitting of the BO peak, made much broader and asymmetric by the well-known larger CQ's [44]. Areas were corrected as previously described [23] for satellite spinning sidebands under the main central peaks, using estimates of mean quadrupolar coupling constants CQ based on extensive previous
19
studies of such materials and from the spinning sidebands for the satellite transitions. The 14.1 T 29Si spectra were fitted with either 2 or 3 Gaussians, whose relative areas were constrained to be consistent with composition, assuming that all oxygens were BO or NBO, as in other recent more detailed studies of alkali silicates [36]. 3. Theoretical calculations We did not attempt to synthesize the unstable crystalline oxide K2O as a model compound for the “free” oxide ion, and K-silicates that contain this structure apparently do not exist, unlike the situation for alkaline earth oxides (e.g. CaO, BaO, Ca3SiO5), as in our previous study of Ba and Ca silicate glasses [23]. Instead, we used the density functional theory (DFT) based codes CASTEP and CASTEP-NMR (Accelrys Inc.) to estimate the 17O NMR chemical shift for the single oxygen site in the K2O crystal structure [45–48]. The crystal structure of K2O reported previously in the literature was used without further geometry optimization [49]. The gauge-including projector augmented wave (GIPAW) algorithm and the generalized gradient approximation (GGA), simplified by Perdew–Burke–Ernzerhof (PBE) exchange correlation functional, were employed [45–48]. An energy cutoff of 600 eV was used for the plane wave basis expansions. The Brillouin zone was sampled using the Monkhorst–Pack scheme and a 4 × 4 × 4 k-point grid [47]. All core–valence interactions were modeled with ultra-soft pseudopotentials. Such PBE-DFT calculations have been successfully used in the literature to estimate 17O NMR chemical shifts and electric field gradients in SiO2 and Na-silicates with reasonable accuracy (e.g. ±5 ppm for 17O chemical shift) [50,51]. These calculations yield the absolute shielding tensor principal components σxx, σyy and σzz. The cubic site symmetry for the oxygen site in the K2O crystal structure results in
NBO BO
40% K2O glass
34% K2O glass
34% K2O crystalline
ppm Fig. 1. Central regions of 17O MAS spectra (14.1 T) for the K-silicate glasses and crystalline K2Si2O5, with bridging (BO) and non-bridging (NBO) oxygen peaks labeled. For the 34%-K2O glass, the Gaussian fitted to the NBO peak, and subtracted to yield the residual BO peak, is shown. At this field, the Gaussian form is consistent with a broadened quadrupolar lineshape and a CQ of about 2 MHz for the NBO.
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σxx = σyy = σzz = σiso where σiso is the isotropic shielding. The isotropic chemical shift δiso was obtained from σiso, using the relationship: δiso = −(σiso − σref) where σref is the isotropic shielding of a reference material. The 17O chemical shifts of the oxygen sites in α-quartz, GeO2 polymorphs, MgO and SrO crystals were used as references in this study that resulted in an average value of σref = 227 ± 10 ppm. The 17 O NMR δiso, thus calculated for K2O yields a value of 290 ± 10 ppm. It may be noted here that a relatively recent study indicated the failure of such calculations in the accurate prediction of 17O NMR chemical shift in CaO [52]. This failure was attributed to the partially covalent character of the Ca\O bonds, an issue that is not of significant concern here due to the strong ionic character of the K\O bonds compared to that of the Ca\O bonds. 4. Results As in previous 17O MAS NMR studies of alkali and alkaline earth silicate glasses [23,27–30,53], the spectra of the K-silicate glasses (Fig. 1) contain two overlapping peaks readily attributable to BO (lower frequency, broader) and NBO (higher frequency, narrower), based on data for crystals with known structures [44,54]. Mean CQ values are about 2 MHz for the NBO peaks of both and about 4 to 4.5 MHz for the BO peaks. Derived relative peak areas (Table 1) are fully consistent with the conventional view that nearly all oxygens in such compositions are BO or NBO. The NBO peak position, and probably its mean isotropic chemical shift, is higher for the lower silica glass. This effect has been seen in previous 17O MAS and 3QMAS studies of Na- and Ca-silicate glasses, where it was attributed to changes with composition in the coordination environment of the modifier cation and consequent effects on mean NBO-Si distances [27,32,55]. The 17O spectrum for the crystalline K2Si2O5 resembles those of the glasses, with somewhat narrower peaks as expected from its more ordered structure, giving confidence to peak assignments. The NBO peak for the crystalline sample lies at a somewhat lower frequency than those in the glasses, suggesting slightly higher chemical shifts in the latter, again possibly due to differences in the distributions of modifier ions and NBO. The spectrum of the crystalline sample has features indicating more resolved quadrupolar lineshapes, but we have not attempted to fit the data as at least 15 crystallographically known oxygen sites are expected [56]. Relative areas of the overall NBO and BO peaks are, however, consistent with composition within error (Table 1). Unlike the case of the glasses, significant changes in both BO and NBO peak shapes as a function of pulse delay were noted. This differential relaxation suggests the presence of more than one K2Si2O5 polymorph with different relaxation rates, as surmised in a previous 29 Si NMR study [56]. As discussed above, the results of our theoretical calculations indicate that “free” oxide ions in coordination environments similar to
crystalline K2O should have chemical shifts in the range of roughly 280 to 300 ppm. K\O bonds are expected to be highly ionic, and this should contribute to relatively low CQ values and thus to less quadrupolar broadening than BO and even NBO peaks. The latter expectation is supported by the low CQ's and narrow 17O NMR peaks observed for “free” oxide ion sites in crystalline Ca3SiO5, even for sites that have relatively high distortions from ideal octahedral geometry [23], for the five-coordinated O1 oxide ion site in wadsleyite (high-pressure β-Mg2SiO4) [57], and the ready observation of such sites in “sub-orthosilicate” Ca-Mg glasses [22]. CQ values for oxide ions in relatively ionic crystals such as TiO2 and ZrO2 are typically less than about 1.5 MHz as well, even though such sites typically have low symmetry [44]. A wide frequency range around that expected for “free” oxide ions was explored by collecting spectra at several spinning speeds at 14.1 T, and at 23 kHz at 9.4 T for both glasses (Fig. 2). At the lower field, NBO and BO peaks show increased quadrupolar broadening, but the maximum spinning speed corresponds to a 50% greater separation in ppm of the spinning sidebands and hence a wider clear baseline range at a single spinning speed. The high 17O enrichment level in the glasses, rapid relaxation, and collection of 100's of thousands of acquisitions produced high signal-to-noise ratios, up to about 1000 for the NBO peaks. Using the tiny background signal for the “free” oxide ion in the zirconia rotor (about 0.5% relative area) as a model of a relatively narrow peak, we estimate a detection limit for an additional peak in the range of 150 to 400 ppm to be about 0.1%. With one of the satellite spinning sidebands (about 5% relative area) as a model for a broader peak, we estimate the detection limit to be about 0.5 to 1%. At these limits, no additional peaks attributable to “free” oxide ions are detected in this region of the spectra. 29 Si for the two glasses and for crystalline K2Si2O5, collected at 14.1 T to improve sensitivity for the relatively small samples, are shown in Fig. 3. The latter is noisy because of the sample's slow spin-lattice relaxation time, as has been noted in previous studies of crystalline alkali silicates [27,56], probably resulting from the exclusion of common
*
rotor
*
a *
b
* *
Table 1 Compositional analyses and NMR results for glassy and crystalline K-silicates. 29
Si NMR, constrained fit
NBO
4
17
Sample
mol% K2O ±1%
Q
34% K2O glass
34.5
K2Si2O5 crystal
–
3.6% –100.0* 12.0* –
40% K2O glass
40.5
– –
Q
3
87.4% –90.3 10.9 100% –92.5 3.5 63.9% –89.6 10.6
Q
2
9.1% –80.0* 8.7 –
c
O NMR ±1.5%
Nominal ±1%
41%
41.7%
39%
41.7%
51%
50.8%
*
*
* *
*
d
ppm 36.1%* –81.2 10.0
Notes: Fitting results for 29Si NMR spectra of glasses list peak areas, centers, and FWHM, the latter two in ppm. Constraints were applied (marked by *) so that component peak areas are consistent with composition, assuming no FO. The 29Si spectrum for the crystal was not fitted.
Fig. 2. Higher frequency regions of the 17O MAS spectra for the K-silicate glasses, with vertical scales ×10 relative to Fig. 1. (a) 40%-K2O glass, 9.4 T, 23 kHz spinning rate, rotor background subtracted; (b) 34%-K2O glass, 9.4 T, 23 kHz spinning rate, rotor background subtracted; (c) 40%-K2O glass, 14.1 T, 23 kHz spinning rate; (d) 40%-K2O glass, 14.1 T, 15 kHz spinning rate to move sideband position and clear baseline from 300 to 400 ppm. In (c) and (d), the rotor background was not subtracted, to illustrate the sensitivity of the spectra. Spinning sidebands are marked by asterisks.
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magnetic impurity ions, including the added Co 2+, from the crystal structure. The peak is centered at –92 ppm, is relatively broad (about 3 ppm FWHM) and probably contains at least three partially-resolvable components as previously observed [33,56]. A complex spectrum is indeed expected, as six Q3 sites are distinguished in the X-ray crystal structure [56]. The largest components of the spectra of the two glasses are also at about –90 ppm (Table 1), confirming their assignments to the predominant Q3 species as in numerous previous studies [33,34,36,58]. Because the 40%-K2O glass is expected to contain b1% of the Q 4 species [36], we fitted its 29Si spectrum with only two components, corresponding to Q 3 and Q 2 species (Table 1). Given the low resolution between these, unconstrained fits have relatively high uncertainties. A fit that is constrained so that the NBO/Si content, calculated from the peak areas, matches the analyzed composition, is shown in Fig. 3. Fits with a small Q4 peak, and/or with two Q2 peaks, would yield slightly smaller residuals [36]. We fitted the spectrum of the 34%-K2O with three Gaussians, corresponding to Q 4, Q 3 and Q 2 species (Table 1),
crystalline
34% K2O glass
40% K2O glass
-50
-70
-90
ppm
-110
-120
Fig. 3. 29Si MAS NMR spectra at 14.1 T for K-silicate glasses and crystalline K2Si2O5. Experimental data are marked by dots, constrained fits by solid lines.
21
again constrained to match the composition with the assumption of negligible “free” oxide ions. From this fit, the estimated value of the apparent equilibrium constant is about 0.004 for the speciation reaction 3
4
2Q ¼ Q þ Q
2
ð3Þ
which is consistent with recent, in-depth studies of network speciation in alkali silicate glasses [36]. 5. Discussion Several recent studies of O 1s XPS spectra of alkali silicate glasses have demonstrated the improvements in resolution and in quantitation made possible by better compensation of surface charging and of correction for instrumental effects such as changes in observed peak areas with time of exposure to the X-ray beam [1,21]. At least for ordered silicate crystals, improved spectral fits with asymmetric, twocomponent line shapes were also demonstrated [21]. In the Na2O-SiO2 system, BO peaks were relatively well-resolved from the main NBO peak, which was assumed to also contain signal from FO if present. Systematic comparisons of BO contents (from fits with symmetrical Gaussian/Lorentzian lineshapes), and of derived FO contents, were made between new and published XPS and both new and published 29 Si MAS NMR data. At relatively high silica contents, agreement of most data sets is good within the uncertainties, and these are consistent with either the conventional view of a large (effectively infinite) value of Kox for reaction (1) or values as low as about 14 (roughly 1% FO at 67 mol% SiO2). However, significantly larger discrepancies were noted at lower silica contents. For example, the NMR data for Na-silicates of Maekawa et al. [34] at about 50% silica predict a much lower FO content (zero within error) than the XPS data. It was suggested that these might be explained by differences in sample preparation. This would require, for example, that the samples of the recent XPS/NMR study with nominal compositions of 50% SiO2 [1] had actual Na2O contents a few percent lower than those of the same nominal composition described in the earlier NMR study [34]. In a second recent study, fits to O 1s XPS spectra for K-silicate glasses again yielded proportions of BO and an estimate of Kox for reaction (1) of 2.0 [2]. A systematic comparison of XPS data and published 29Si MAS NMR results, including recent data on glasses with independently analyzed bulk compositions [36], again showed good agreement at high silica contents (about 75 to 90%). At lower silica contents, where data can give much stronger constraints on Kox, an even larger discrepancy than noted in the Na-silicate glasses is apparent, as the NMR data are again most consistent with large to infinite values as well as being in good agreement between several studies. Suggested possible explanations for these differences focused on sample preparation. However, alkali loss during melting, if confined to the NMR samples (and as obviated by the chemical analyses in the more recent data set [36]) would be expected to yield BO contents values that were too high, the opposite of the observed difference with the XPS data. Differences in Q n speciation resulting from different cooling rates and thus different fictive temperatures have been measured in alkali silicates [41] and are consistent with in-situ high temperature Raman spectroscopic studies [59]. These are not expected to be easily measurable unless special rapid-quench methods are used. In any case, if speciation reactions such as (3) are displaced as a function of T or Tf, the overall BO/NBO content is not affected. Of course, fictive temperature effects could be present for the FO reaction (1), and would be very interesting to investigate in systems in which this species can be directly measured. For the 40 and 34% K2O glasses studied here, a Kox value of 2.0 predicts “free” oxide ion concentrations of about 6 and 9% respectively, and NBO concentrations of 32 and 29%. The former are much larger than the estimated direct detection limits in the 17O NMR spectra of 0.1 to 1%. The NBO estimates are well outside of the range of uncertainty of the
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analyzed 17O NMR spectra, which are consistent only with large values of Kox that predict NBO contents of about 50 and 41% respectively. Given the relatively low resolution of peaks for different Qn species that is typical of 29Si spectra of alkali silicates glasses, fits to such data may provide somewhat less certain constraints on the oxygen speciation than do 17O NMR spectra when NBO peaks are well-resolved. We note that this resolution is often not present, as in Li, Na, and Mg binary silicate glasses, where 17O NBO and BO peaks are well-resolved only in 2-D, DAS or 3QMAS NMR spectra [28–30,32,53]. The model based on O 1s XPS data [2] would predict NBO/Si of about 0.73 for the 34%-K2O glass and about 0.86 for the 40%-K2O glass, compared to 1.05 and 1.33 for a large value of Kox (e.g. >200). For the latter glass, this would require that the 29Si spectrum, if approximated by two components, would have to be comprised of about 86% Q3 and about 14% Q 4, instead of predominantly Q2 and Q3 species, and would thus have an asymmetry opposite to that which is observed. For the 34%-K2O glass, a Q4 peak of about 26% area would be required, compared to the fitted result of 4%. While various constrained fits other than those chosen here are possible, reinterpretation of the spectra to be consistent with the fits to the XPS data would probably require peak widths and/or positions that are not reasonable in light of our knowledge of chemical shift systematics in glasses and crystals. We thus conclude that direct results from 17O NMR, and indirect results from both 17O NMR and 29Si NMR on potassium silicate glasses, are consistent with negligible FO contents, and substantially disagree with recent results from O 1s XPS spectra. The latter are also indirect in the sense of being the result of measurements of BO contents. The reason for this apparent discrepancy is not known, but could be related to uncertainties in glass compositions, in the line shapes chosen to fit spectra, or to assumptions about the relationship of the sample volume observed by XPS, which is most sensitive to the top few nm of the material, to the bulk structure: most of the signal originates from the upper 0–6 nm [26]. The more conventional view that the “free” oxide species is of low concentration in most glass-forming alkali and alkaline earth compositions at relatively high silica contents (e.g. >50 mol% SiO2) is strongly supported by the 17O NMR data presented here. However, the importance of this species in thermodynamic models, at least for low-silica liquids, suggests that further measurements of its concentration by direct methods, especially if this can be done in the melts themselves, would be very interesting. Finally, we note that binary silicate glasses with “intermediate” oxides of much more electronegative cations, such as PbO, SnO, and Sb2O3, which often have low cation coordination numbers and in which the chemical distinctions among BO, NBO, and FO are blurred, are probably much better candidates for significant concentrations of oxide ions not bonded to Si. Both O 1s XPS [21] and 29Si NMR data provide indirect evidence for this type of species (e.g. Pb\O\Pb linkages) at relatively low silica contents [60,61]. Acknowledgments This research was supported by NSF grant EAR 1019596 to JS. SS was supported by NSF grant DMR GOALI 1104869. We thank Jingshi Wu and Guangchao Li for help with ICP analyses. References [1] 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. [2] R. Sawyer, H.W. Nesbitt, R.A. Secco, J. Non-Cryst. Solids 358 (2012) 290. [3] P.C. Hess, in: R.B. Hargraves (Ed.), Physics of Magmatic Processes, Princeton University Press, Princeton, NJ, 1980, p. 3.
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Oxide ion speciation in potassium silicate glasses: New limits from
17
O NMR
Jonathan F. Stebbins a,⁎, Sabyasachi Sen b a b
Dept. of Geological and Environmental Sciences, Stanford University,Stanford CA 94305, United States Dept. of Chemical Engineering and Materials Science, University of California at Davis,CA 95616, United States
a r t i c l e
i n f o
Article history: Received 12 January 2013 Received in revised form 16 February 2013 Available online 22 March 2013 Keywords: Glass structure; Alkali silicate; NMR; Nuclear magnetic resonance; Oxide ion
a b s t r a c t In silicate glasses and liquids, “free” oxide ions (O2− ions not bonded to any network-forming cations such as Si) are required by stoichiometry for ratios of O/Si > 4, for example in “sub-orthosilicate” compositions (b 33.3% SiO2 in MO-SiO2 or M2O-SiO2 binaries). Measurements of oxide ion activities and other thermodynamic considerations suggest however, that at higher silica contents the concentration of such ions should rapidly become very low, particularly in systems with highly basic modifier oxides such as Na2O or K2O. Recent 17O NMR studies of alkaline earth ortho- and sub-orthosilicate glasses have indeed directly shown the presence of a few percent of “free” oxide ions, but did not detect this species at a silica content of 44 mol%. In contrast, recent O 1s XPS analyses of bridging oxygen concentrations in Na- and K-silicate glasses have suggested as much as about 6% “free” oxide even at about 67% SiO2 [1,2]. In K-silicates, theoretical calculations presented here, as well as previous work on Ca and Mg silicates, suggest that this species should be readily and directly measurable by 17O. However, we show here that “free” oxide ions are not detectable in glasses with 34 and 40 mol% K2O, and therefore are not likely to be present above the 0.1 to 1% level. Measured ratios of bridging to non-bridging oxygens are within errors of those expected from analyzed compositions and an assumption of negligible “free” oxide, and 29Si MAS spectra can be accurately fitted with constraints based on this same assumption. © 2013 Elsevier B.V. All rights reserved.
1. Introduction At least at very high temperatures, continuous solutions exist between most liquid oxides (e.g. those of alkali and alkaline earths) and molten silica. One of the most fundamental questions of oxide liquid structure thus becomes the transition in the coordination environments of the oxygen anions between those in low-silica systems, which must primarily have high coordination numbers by low-valent cations (e.g. with 6 to 8 M+ or M2+ neighbors, and designated as “free” oxide ions, FO) to those in high-silica systems, which have primarily only two, Si4+ neighbors, and are designated as “bridging oxygens” (BO). In between, as in crystalline silicate compounds, oxygen anions with one Si neighbor and several lower-valent neighbors become abundant, and are denoted as “non-bridging” oxygens (NBO). This structural transition has long been at the heart of models of the chemistry of molten silicates, although until recently these were based on little or no actual structural data. Early models, often concerned with low-silica metallurgical slags, emphasized the key equilibrium between these three types of oxygen anion, through the reaction: FO þ BO ¼ 2 NBO
ð1Þ
⁎ Corresponding author at: Dept. of Geological and Environmental Sciences, Bldg. 320, room 118, Stanford University, Stanford CA 94305, United States. Tel.: +1 650 723 1140. E-mail address: [email protected] (J.F. Stebbins). 0022-3093/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnoncrysol.2013.02.024
The apparent equilibrium constant for this reaction, which ignores activity coefficients, can be simply written in terms of mole fractions as Kox = (XNBO) 2/(XFO * XBO). As reviewed in some depth [3], models of varying complexity have derived values of Kox that vary strongly with the type of oxide ion, generally predicting systematically higher FO concentrations with increases in the ionization potential of the M cation, or its corresponding cation field strength, defined as the charge divided by the square of cation-oxygen distance [4–7], or alternatively, with decreasingly basic character of the oxide. These models served well to fit observed liquidus surfaces for oxides such as CaO, MgO, and FeO in low-silica liquids, where the activities of the FO are directly constrained by equilibrium with the corresponding crystals. By stoichiometry alone, if Si is primarily four-coordinated by oxygen, significant FO must be present when the O/Si ratio is greater than 4, e.g. at silica contents below 33.3 mol% in MO-SiO2 and M2O-SiO2 binaries. When M is an alkali earth cation, such models predict activities of FO to be small (corresponding to concentrations below a few %) when “normal” glass-forming silica contents are reached, e.g. at >50% SiO2, and plummet rapidly as silica increases further. Related approaches have also proven useful for modeling the two-liquid solvi in systems such as CaO-SiO2 [8,9]. Models involving equilibria such as reaction (1) have also long been developed for alkali oxide-silica systems. Here, the activities of the oxide ion (i.e. of FO) can be constrained relatively directly by Knudsencell vapor pressure data and by electrochemical measurements [10,11], as well as by modeling of equilibria between liquids and crystalline
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silicates [12]. In the typical glass-forming region (e.g. >50% silica), FO activities are generally very low (e.g. 10−9 to 10−8) relative to the pure oxide standard state, decrease with decreasing temperature, and are systematically lower with decreasing cation field strength (e.g. K b Na). Although the relationship of such activities to concentrations of observable structural species, as well as those between the high-temperature melts and those near to the glass transition as typically sampled by spectroscopy, are uncertain, there should be some close connection if the concept of actual “free” oxide ions is to have some chemical relevance. Independent considerations of the systematics of enthalpies of reaction among metal oxides and silica, to form intermediate silicate compounds, indicate that the FO species is likely to be most stable, and thus have higher concentration, when the ionization potential or field strength of the M cation is highest, e.g. La3+ > Mg2+ > Ca2+ > Na+ > K+ [3]. All of these considerations suggest that in the glass-forming regions of Na and K silicate systems, the actual concentration of FO should be very low, even though the concept of the activity of oxide ions remains a useful concept, just as pH does in very alkaline aqueous solutions. Nonetheless, it is important to attempt to place direct structural constraints on such species and test the assumptions involved in thermodynamic treatments, generally through studies of glasses that are assumed to represent the liquid structure near to Tg. Even in glasses of relatively high silica contents (well above 50 mol%), NMR spectroscopy and other methods have shown that monovalent anions such as Cl− [13], F− [14] and even OH− [15] often are coordinated by only alkali or alkaline-earth cations, although the divalent O 2− ion is expected to be much more reactive with Si\O\Si linkages. Spectroscopic studies have revealed that some minor structural groups of low energetic stability (and thus absent in typical solution models) can reach significant concentrations, possibly for entropic reasons, for example SiO5 [16] and AlO5 [17,18] species in ambient pressure glasses, and NBO in “charge-balanced” binary systems (e.g. CaAl2O4-SiO2) [19]. Two approaches have been taken to constrain or test the concentration of FO in glass-forming silicates. The most direct methods can in principle detect a unique spectroscopic signature of such species, but results have been limited. For example, BO and NBO peaks are often partially resolved in O1s XPS spectra of silicate glasses, and an early study interpreted a shoulder about 2 eV below the NBO as indicating the presence of as much as about 10% FO in a number of Li and Zn silicates, although this was not detected or resolved in Na, Ca, Sr, or Ba silicates [20]. In more modern XPS studies of Pb, Na, K silicate glasses this feature was not observed [1,2,21]. Recent high-field 17O MAS NMR spectra of very low silica Ca-Mg silicates glasses (b34 mol% SiO2) have partially resolved peaks whose frequencies correspond to those expected for FO with mixed Ca + Mg neighbors, based on the known chemical shifts of crystalline MgO and CaO [22]. For such “sub-orthosilicate” compositions, which can only be formed into glasses by rapid-quench, containerless methods, FO species are required simply by the high O/Si ratio, if Si coordination is predominantly 4. The latter is confirmed 29Si NMR. On the other hand, in Ba- and Ca-silicate glasses with silica contents as low as 44%, careful observation of the spectral region where FO peaks are expected, again based on crystalline model compounds (BaO, CaO, Ca3SiO5), did not find any indication of FO, with detection limits of b1% [23]. Other approaches to constraining the concentration of FO in glasses are less direct and generally have larger uncertainties. Oxygen 1s XPS has been applied in numerous studies to determine relative proportions of various bridging oxygen and non-bridging oxygen species, with results often consistent with conventional assumptions about low FO contents [24–26]. In more recent work, the fractions of BO out of total oxygen signals were determined from O 1s XPS spectra of Na-, K-, and Pb-silicate glasses, which have relatively well-resolved peaks for this species [1,2,21]. Comparison with compositions (nominal or as determined by XPS analysis) lead to calculations of FO concentrations that were surprisingly high (at least in the very basic Na2O-SiO2 and K2O-SiO2 glasses), up to about 7% at 35% K2O. In contrast, in lower-silica
Ca- and Ba-silicate glasses, where thermodynamic approaches suggest greater stability for the FO species, ratios of BO to NBO measured from 17O NMR spectra were found to be consistent with compositions determined by EPMA and an assumption of negligible FO, although uncertainties related to peak overlap would allow a few percent of such species [23]. In previous 17O NMR studies of alkali silicate glasses, BO to NBO ratios generally were consistent with compositions and assumptions of negligible FO, although the primary goals of these efforts were usually not to test assumptions about FO contents [27–32]. Even less direct, but complementary, are studies of the silicate anionic speciation in glasses, which has been done most quantitatively by 29Si NMR. Here, peak assignments are generally made based on the chemical shifts of known crystal structures of compositions closely related to those of the glasses [33]. If concentrations of Qn species (SiO4 groups with n BO and 4-n NBO first neighbors) can be estimated by fitting spectra of glasses, the NBO/Si ratio can be calculated and compared to the composition. If this is lower than expected, one interpretation could be the presence of FO. While peaks for at least the most abundant Qn species may be better resolved in alkali silicates than in other compositions, peak overlap introduces significant uncertainty to such measurements, and analyses are quite sensitive to the exact glass composition. For example, in an extensive early study of Li-, Na-, and K-silicates to as low as 50 mol% silica, Gaussian fits yielded Qn species concentrations that were consistent with nominal compositions in the Na and K series, but which suggested significant FO in the most Li-rich (e.g. 40% Li2O) glasses [34]. Compositions were not analyzed, however, leaving questions of possible alkali loss during synthesis unresolved: alkali contents lower than expected will lead to NBO/Si ratios that are apparently too low, possibly leading to the overestimation of FO contents. A subsequent 29Si MAS NMR study of Li-rich silicate glasses with up to 64 mol% Li2O [35] found Q-speciation that was consistent with the absence of any detectable FO. Similarly, in recent work, highquality 29Si NMR spectra of binary K, Rb, and Cs silicate glasses were fitted both without and with constraints imposed by analyzed composition and assumptions of no FO [36]. Constrained fits produced consistent results, but showed that a more complex model than previously used, e.g. with two Q2 peaks, was indicated in some compositions. Alkaline earth silicate glasses would be expected to have higher FO contents because of lower basicity and higher modifier cation field strengths, which tend to stabilize anionic groups with more concentrated negative charges [37]. However, peaks for Q n species are generally unresolved in normal, 1-dimensional 29Si MAS NMR spectra. More information can be obtained from challenging, 2-D 29 Si spectra using methods such as “magic angle flipping” (MAF). Fits to such data for CaSiO3 and MgSiO3 glasses, based on differences in the chemical shift anisotropy (CSA) for different Q n species, yielded a slight deficit in NBO and thus suggested about 1% FO in both glasses [38,39]. However, given uncertainties, fits constrained by the analyzed compositions, with no FO, probably would also have been consistent with the data. In some cases, static (non-MAS) spectra can yield key information about the network structure, because of especially small CSA's for the relatively symmetrical structural Q 4 and Q0 units [34,40,41]. However, such spectra generally do not allow the independent determination of all Q n species that are present, and so usually cannot accurately fix the extent of reaction (1). A special case most relevant here is the use of this method to detect significant fractions of lower-symmetry Q1 groups in Mg2SiO4 glass, which implied the presence of complementary FO, estimated at about 6% [42]. A recent 2-D 29Si MAF NMR spectroscopic study of this glass provided a significantly more accurate estimate of the relative fraction of Q 1 groups that results in an estimate of about 4.5% FO in this glass, when re-calculated with respect to total moles of oxygen [38]. These results were extended by 29Si MAS spectra of sub-orthosilicate Ca/Mg silicate glasses, where best fits included a Q 1 peak in addition to the predominant Q 0 component [22]. These
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findings are thus consistent with the later direct detection of FO by 17 O NMR as noted above. In one report on Na- and K trisilicate glasses with extensive substitution of La2/3O for Na2O or K2O, large apparent enhancements of the Q 4 peak intensities in 29Si MAS NMR spectra of compositions with constant nominal NBO/Si suggested up to about 2% FO, which could be stabilized by the very high field strength of the La 3 + cation [43]. However, the effects of this cation on chemical shifts are not well-known, and unexpectedly large overlaps or asymmetries of the Q 2 and Q 3 peaks, and/or of the Q 3 and Q 4 peaks, could allow speciation consistent with more conventional models. In this report we examine more closely the possibility of significant fractions of “free” oxide ions in potassium silicate glasses, where recent O 1s XPS results would predict up to about 9% of this species at the 40% K2O composition [2]. Using theoretical calculations to confirm the expected chemical shift range for such species in 17O NMR spectra, we provide new, direct constraints on the abundance of this species in two carefully chosen K-silicate glass compositions. In addition, we refine the indirect constraints on the NBO/BO ratio, also from 17O NMR, as well as confirming previous models of Q n speciation from 29 Si NMR. 2. Experimental methods and data analysis Potassium silicate glasses of nominal compositions 33.3 and 40.0 mol% K2O were prepared by mixing dried K2CO3, 42% 17O-enriched SiO2, and 0.1 wt.% cobalt oxide, decarbonating at 740 °C for about 15 h, then melting at 1200 °C for about 1 h in high purity Ar. Part of the 33% K2O glass was crystallized to K2Si2O5 by reheating at 755 °C for 24 h. Portions of the 500 to 800 mg samples were coarsely crushed and loaded into MAS rotors for NMR in a glove box under dry N2. Rotor caps were tight enough to prevent significant hydration during NMR experiments: spectra collected within an hour or two after loading were identical to those collected after several days; glass samples were removed as free-running powders in the dry box at the end of experiments. ICP-OES analyses for K and Si gave compositions of 34.5 ± 1 and 40.5 ± 1 mol% K2O; the glasses will thus be referred to below as “34%” and “40%-K2O” samples. If the slight excess in K2O led to a small amount of K2SiO3 in the crystallized K2Si2O5 sample, it was not detected in the relatively low-quality 29Si NMR spectrum collected for this material (see below). NMR spectra were collected with Varian Unity/Inova 600 (14.1 Tesla) and Infinity Plus 400 (9.4 Tesla) spectrometers using 3.2 mm Varian/ Chemagnetics “T3” probes at spinning rates of 20 to 23 kHz. 29Si spectra at 119.13 MHz were referenced to tetramethylsilane (TMS); 17O spectra at 54.19 and 81.29 MHz were referenced to 17O-enriched water. Spectra were acquired with single radiofrequency (RF) pulses corresponding approximately to 30° RF tip angle for solids. Various pulse delays were tested to ensure that relative peak areas were quantitative. With the exception of the 17O spectra for crystalline K2Si2O5 (see below), no differential relaxation was observed. Optimal spectra for the glasses were obtained with pulse delays of 10 s for 29Si and 0.1 or 1s for 17O. Relaxation for 29Si in crystalline K2Si2O5 was much slower. This limited the quality of the spectra collected for this sample. For some of the 17O spectra shown, the very small but readily observable background signal from the natural-abundance 17O in the zirconia MAS rotors was subtracted using a spectrum for the empty rotor; in others, this subtraction was not done, to illustrate the sensitivity of the experiments to low-level signals. For the 14.1 T 17O spectra of the glasses, relative peak areas for BO and NBO were derived by simply fitting a Gaussian to the NBO peak (consistent with the small CQ's known for such sites and the high magnetic field used) and subtracting to yield a residual BO peak (Fig. 1). This obviated the need for fitting of the BO peak, made much broader and asymmetric by the well-known larger CQ's [44]. Areas were corrected as previously described [23] for satellite spinning sidebands under the main central peaks, using estimates of mean quadrupolar coupling constants CQ based on extensive previous
19
studies of such materials and from the spinning sidebands for the satellite transitions. The 14.1 T 29Si spectra were fitted with either 2 or 3 Gaussians, whose relative areas were constrained to be consistent with composition, assuming that all oxygens were BO or NBO, as in other recent more detailed studies of alkali silicates [36]. 3. Theoretical calculations We did not attempt to synthesize the unstable crystalline oxide K2O as a model compound for the “free” oxide ion, and K-silicates that contain this structure apparently do not exist, unlike the situation for alkaline earth oxides (e.g. CaO, BaO, Ca3SiO5), as in our previous study of Ba and Ca silicate glasses [23]. Instead, we used the density functional theory (DFT) based codes CASTEP and CASTEP-NMR (Accelrys Inc.) to estimate the 17O NMR chemical shift for the single oxygen site in the K2O crystal structure [45–48]. The crystal structure of K2O reported previously in the literature was used without further geometry optimization [49]. The gauge-including projector augmented wave (GIPAW) algorithm and the generalized gradient approximation (GGA), simplified by Perdew–Burke–Ernzerhof (PBE) exchange correlation functional, were employed [45–48]. An energy cutoff of 600 eV was used for the plane wave basis expansions. The Brillouin zone was sampled using the Monkhorst–Pack scheme and a 4 × 4 × 4 k-point grid [47]. All core–valence interactions were modeled with ultra-soft pseudopotentials. Such PBE-DFT calculations have been successfully used in the literature to estimate 17O NMR chemical shifts and electric field gradients in SiO2 and Na-silicates with reasonable accuracy (e.g. ±5 ppm for 17O chemical shift) [50,51]. These calculations yield the absolute shielding tensor principal components σxx, σyy and σzz. The cubic site symmetry for the oxygen site in the K2O crystal structure results in
NBO BO
40% K2O glass
34% K2O glass
34% K2O crystalline
ppm Fig. 1. Central regions of 17O MAS spectra (14.1 T) for the K-silicate glasses and crystalline K2Si2O5, with bridging (BO) and non-bridging (NBO) oxygen peaks labeled. For the 34%-K2O glass, the Gaussian fitted to the NBO peak, and subtracted to yield the residual BO peak, is shown. At this field, the Gaussian form is consistent with a broadened quadrupolar lineshape and a CQ of about 2 MHz for the NBO.
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σxx = σyy = σzz = σiso where σiso is the isotropic shielding. The isotropic chemical shift δiso was obtained from σiso, using the relationship: δiso = −(σiso − σref) where σref is the isotropic shielding of a reference material. The 17O chemical shifts of the oxygen sites in α-quartz, GeO2 polymorphs, MgO and SrO crystals were used as references in this study that resulted in an average value of σref = 227 ± 10 ppm. The 17 O NMR δiso, thus calculated for K2O yields a value of 290 ± 10 ppm. It may be noted here that a relatively recent study indicated the failure of such calculations in the accurate prediction of 17O NMR chemical shift in CaO [52]. This failure was attributed to the partially covalent character of the Ca\O bonds, an issue that is not of significant concern here due to the strong ionic character of the K\O bonds compared to that of the Ca\O bonds. 4. Results As in previous 17O MAS NMR studies of alkali and alkaline earth silicate glasses [23,27–30,53], the spectra of the K-silicate glasses (Fig. 1) contain two overlapping peaks readily attributable to BO (lower frequency, broader) and NBO (higher frequency, narrower), based on data for crystals with known structures [44,54]. Mean CQ values are about 2 MHz for the NBO peaks of both and about 4 to 4.5 MHz for the BO peaks. Derived relative peak areas (Table 1) are fully consistent with the conventional view that nearly all oxygens in such compositions are BO or NBO. The NBO peak position, and probably its mean isotropic chemical shift, is higher for the lower silica glass. This effect has been seen in previous 17O MAS and 3QMAS studies of Na- and Ca-silicate glasses, where it was attributed to changes with composition in the coordination environment of the modifier cation and consequent effects on mean NBO-Si distances [27,32,55]. The 17O spectrum for the crystalline K2Si2O5 resembles those of the glasses, with somewhat narrower peaks as expected from its more ordered structure, giving confidence to peak assignments. The NBO peak for the crystalline sample lies at a somewhat lower frequency than those in the glasses, suggesting slightly higher chemical shifts in the latter, again possibly due to differences in the distributions of modifier ions and NBO. The spectrum of the crystalline sample has features indicating more resolved quadrupolar lineshapes, but we have not attempted to fit the data as at least 15 crystallographically known oxygen sites are expected [56]. Relative areas of the overall NBO and BO peaks are, however, consistent with composition within error (Table 1). Unlike the case of the glasses, significant changes in both BO and NBO peak shapes as a function of pulse delay were noted. This differential relaxation suggests the presence of more than one K2Si2O5 polymorph with different relaxation rates, as surmised in a previous 29 Si NMR study [56]. As discussed above, the results of our theoretical calculations indicate that “free” oxide ions in coordination environments similar to
crystalline K2O should have chemical shifts in the range of roughly 280 to 300 ppm. K\O bonds are expected to be highly ionic, and this should contribute to relatively low CQ values and thus to less quadrupolar broadening than BO and even NBO peaks. The latter expectation is supported by the low CQ's and narrow 17O NMR peaks observed for “free” oxide ion sites in crystalline Ca3SiO5, even for sites that have relatively high distortions from ideal octahedral geometry [23], for the five-coordinated O1 oxide ion site in wadsleyite (high-pressure β-Mg2SiO4) [57], and the ready observation of such sites in “sub-orthosilicate” Ca-Mg glasses [22]. CQ values for oxide ions in relatively ionic crystals such as TiO2 and ZrO2 are typically less than about 1.5 MHz as well, even though such sites typically have low symmetry [44]. A wide frequency range around that expected for “free” oxide ions was explored by collecting spectra at several spinning speeds at 14.1 T, and at 23 kHz at 9.4 T for both glasses (Fig. 2). At the lower field, NBO and BO peaks show increased quadrupolar broadening, but the maximum spinning speed corresponds to a 50% greater separation in ppm of the spinning sidebands and hence a wider clear baseline range at a single spinning speed. The high 17O enrichment level in the glasses, rapid relaxation, and collection of 100's of thousands of acquisitions produced high signal-to-noise ratios, up to about 1000 for the NBO peaks. Using the tiny background signal for the “free” oxide ion in the zirconia rotor (about 0.5% relative area) as a model of a relatively narrow peak, we estimate a detection limit for an additional peak in the range of 150 to 400 ppm to be about 0.1%. With one of the satellite spinning sidebands (about 5% relative area) as a model for a broader peak, we estimate the detection limit to be about 0.5 to 1%. At these limits, no additional peaks attributable to “free” oxide ions are detected in this region of the spectra. 29 Si for the two glasses and for crystalline K2Si2O5, collected at 14.1 T to improve sensitivity for the relatively small samples, are shown in Fig. 3. The latter is noisy because of the sample's slow spin-lattice relaxation time, as has been noted in previous studies of crystalline alkali silicates [27,56], probably resulting from the exclusion of common
*
rotor
*
a *
b
* *
Table 1 Compositional analyses and NMR results for glassy and crystalline K-silicates. 29
Si NMR, constrained fit
NBO
4
17
Sample
mol% K2O ±1%
Q
34% K2O glass
34.5
K2Si2O5 crystal
–
3.6% –100.0* 12.0* –
40% K2O glass
40.5
– –
Q
3
87.4% –90.3 10.9 100% –92.5 3.5 63.9% –89.6 10.6
Q
2
9.1% –80.0* 8.7 –
c
O NMR ±1.5%
Nominal ±1%
41%
41.7%
39%
41.7%
51%
50.8%
*
*
* *
*
d
ppm 36.1%* –81.2 10.0
Notes: Fitting results for 29Si NMR spectra of glasses list peak areas, centers, and FWHM, the latter two in ppm. Constraints were applied (marked by *) so that component peak areas are consistent with composition, assuming no FO. The 29Si spectrum for the crystal was not fitted.
Fig. 2. Higher frequency regions of the 17O MAS spectra for the K-silicate glasses, with vertical scales ×10 relative to Fig. 1. (a) 40%-K2O glass, 9.4 T, 23 kHz spinning rate, rotor background subtracted; (b) 34%-K2O glass, 9.4 T, 23 kHz spinning rate, rotor background subtracted; (c) 40%-K2O glass, 14.1 T, 23 kHz spinning rate; (d) 40%-K2O glass, 14.1 T, 15 kHz spinning rate to move sideband position and clear baseline from 300 to 400 ppm. In (c) and (d), the rotor background was not subtracted, to illustrate the sensitivity of the spectra. Spinning sidebands are marked by asterisks.
J.F. Stebbins, S. Sen / Journal of Non-Crystalline Solids 368 (2013) 17–22
magnetic impurity ions, including the added Co 2+, from the crystal structure. The peak is centered at –92 ppm, is relatively broad (about 3 ppm FWHM) and probably contains at least three partially-resolvable components as previously observed [33,56]. A complex spectrum is indeed expected, as six Q3 sites are distinguished in the X-ray crystal structure [56]. The largest components of the spectra of the two glasses are also at about –90 ppm (Table 1), confirming their assignments to the predominant Q3 species as in numerous previous studies [33,34,36,58]. Because the 40%-K2O glass is expected to contain b1% of the Q 4 species [36], we fitted its 29Si spectrum with only two components, corresponding to Q 3 and Q 2 species (Table 1). Given the low resolution between these, unconstrained fits have relatively high uncertainties. A fit that is constrained so that the NBO/Si content, calculated from the peak areas, matches the analyzed composition, is shown in Fig. 3. Fits with a small Q4 peak, and/or with two Q2 peaks, would yield slightly smaller residuals [36]. We fitted the spectrum of the 34%-K2O with three Gaussians, corresponding to Q 4, Q 3 and Q 2 species (Table 1),
crystalline
34% K2O glass
40% K2O glass
-50
-70
-90
ppm
-110
-120
Fig. 3. 29Si MAS NMR spectra at 14.1 T for K-silicate glasses and crystalline K2Si2O5. Experimental data are marked by dots, constrained fits by solid lines.
21
again constrained to match the composition with the assumption of negligible “free” oxide ions. From this fit, the estimated value of the apparent equilibrium constant is about 0.004 for the speciation reaction 3
4
2Q ¼ Q þ Q
2
ð3Þ
which is consistent with recent, in-depth studies of network speciation in alkali silicate glasses [36]. 5. Discussion Several recent studies of O 1s XPS spectra of alkali silicate glasses have demonstrated the improvements in resolution and in quantitation made possible by better compensation of surface charging and of correction for instrumental effects such as changes in observed peak areas with time of exposure to the X-ray beam [1,21]. At least for ordered silicate crystals, improved spectral fits with asymmetric, twocomponent line shapes were also demonstrated [21]. In the Na2O-SiO2 system, BO peaks were relatively well-resolved from the main NBO peak, which was assumed to also contain signal from FO if present. Systematic comparisons of BO contents (from fits with symmetrical Gaussian/Lorentzian lineshapes), and of derived FO contents, were made between new and published XPS and both new and published 29 Si MAS NMR data. At relatively high silica contents, agreement of most data sets is good within the uncertainties, and these are consistent with either the conventional view of a large (effectively infinite) value of Kox for reaction (1) or values as low as about 14 (roughly 1% FO at 67 mol% SiO2). However, significantly larger discrepancies were noted at lower silica contents. For example, the NMR data for Na-silicates of Maekawa et al. [34] at about 50% silica predict a much lower FO content (zero within error) than the XPS data. It was suggested that these might be explained by differences in sample preparation. This would require, for example, that the samples of the recent XPS/NMR study with nominal compositions of 50% SiO2 [1] had actual Na2O contents a few percent lower than those of the same nominal composition described in the earlier NMR study [34]. In a second recent study, fits to O 1s XPS spectra for K-silicate glasses again yielded proportions of BO and an estimate of Kox for reaction (1) of 2.0 [2]. A systematic comparison of XPS data and published 29Si MAS NMR results, including recent data on glasses with independently analyzed bulk compositions [36], again showed good agreement at high silica contents (about 75 to 90%). At lower silica contents, where data can give much stronger constraints on Kox, an even larger discrepancy than noted in the Na-silicate glasses is apparent, as the NMR data are again most consistent with large to infinite values as well as being in good agreement between several studies. Suggested possible explanations for these differences focused on sample preparation. However, alkali loss during melting, if confined to the NMR samples (and as obviated by the chemical analyses in the more recent data set [36]) would be expected to yield BO contents values that were too high, the opposite of the observed difference with the XPS data. Differences in Q n speciation resulting from different cooling rates and thus different fictive temperatures have been measured in alkali silicates [41] and are consistent with in-situ high temperature Raman spectroscopic studies [59]. These are not expected to be easily measurable unless special rapid-quench methods are used. In any case, if speciation reactions such as (3) are displaced as a function of T or Tf, the overall BO/NBO content is not affected. Of course, fictive temperature effects could be present for the FO reaction (1), and would be very interesting to investigate in systems in which this species can be directly measured. For the 40 and 34% K2O glasses studied here, a Kox value of 2.0 predicts “free” oxide ion concentrations of about 6 and 9% respectively, and NBO concentrations of 32 and 29%. The former are much larger than the estimated direct detection limits in the 17O NMR spectra of 0.1 to 1%. The NBO estimates are well outside of the range of uncertainty of the
22
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analyzed 17O NMR spectra, which are consistent only with large values of Kox that predict NBO contents of about 50 and 41% respectively. Given the relatively low resolution of peaks for different Qn species that is typical of 29Si spectra of alkali silicates glasses, fits to such data may provide somewhat less certain constraints on the oxygen speciation than do 17O NMR spectra when NBO peaks are well-resolved. We note that this resolution is often not present, as in Li, Na, and Mg binary silicate glasses, where 17O NBO and BO peaks are well-resolved only in 2-D, DAS or 3QMAS NMR spectra [28–30,32,53]. The model based on O 1s XPS data [2] would predict NBO/Si of about 0.73 for the 34%-K2O glass and about 0.86 for the 40%-K2O glass, compared to 1.05 and 1.33 for a large value of Kox (e.g. >200). For the latter glass, this would require that the 29Si spectrum, if approximated by two components, would have to be comprised of about 86% Q3 and about 14% Q 4, instead of predominantly Q2 and Q3 species, and would thus have an asymmetry opposite to that which is observed. For the 34%-K2O glass, a Q4 peak of about 26% area would be required, compared to the fitted result of 4%. While various constrained fits other than those chosen here are possible, reinterpretation of the spectra to be consistent with the fits to the XPS data would probably require peak widths and/or positions that are not reasonable in light of our knowledge of chemical shift systematics in glasses and crystals. We thus conclude that direct results from 17O NMR, and indirect results from both 17O NMR and 29Si NMR on potassium silicate glasses, are consistent with negligible FO contents, and substantially disagree with recent results from O 1s XPS spectra. The latter are also indirect in the sense of being the result of measurements of BO contents. The reason for this apparent discrepancy is not known, but could be related to uncertainties in glass compositions, in the line shapes chosen to fit spectra, or to assumptions about the relationship of the sample volume observed by XPS, which is most sensitive to the top few nm of the material, to the bulk structure: most of the signal originates from the upper 0–6 nm [26]. The more conventional view that the “free” oxide species is of low concentration in most glass-forming alkali and alkaline earth compositions at relatively high silica contents (e.g. >50 mol% SiO2) is strongly supported by the 17O NMR data presented here. However, the importance of this species in thermodynamic models, at least for low-silica liquids, suggests that further measurements of its concentration by direct methods, especially if this can be done in the melts themselves, would be very interesting. Finally, we note that binary silicate glasses with “intermediate” oxides of much more electronegative cations, such as PbO, SnO, and Sb2O3, which often have low cation coordination numbers and in which the chemical distinctions among BO, NBO, and FO are blurred, are probably much better candidates for significant concentrations of oxide ions not bonded to Si. Both O 1s XPS [21] and 29Si NMR data provide indirect evidence for this type of species (e.g. Pb\O\Pb linkages) at relatively low silica contents [60,61]. Acknowledgments This research was supported by NSF grant EAR 1019596 to JS. SS was supported by NSF grant DMR GOALI 1104869. We thank Jingshi Wu and Guangchao Li for help with ICP analyses. References [1] 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. [2] R. Sawyer, H.W. Nesbitt, R.A. Secco, J. Non-Cryst. Solids 358 (2012) 290. [3] P.C. Hess, in: R.B. Hargraves (Ed.), Physics of Magmatic Processes, Princeton University Press, Princeton, NJ, 1980, p. 3.
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