Solid-State NMR Spectroscopy of Inorganic Materials - American

Chapter 7

Borate Glass Structure Probed with Dynamic Angle Spinning NMR 1

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J. W. Zwanziger, R. E. Youngman , and M . Braun

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Department of Chemistry, Indiana university, Bloomington, IN 47405

Dynamic Angle Spinning NMR is used to probe the short and intermediate range order of boron oxide glass and potassium borate glasses. Through comparison with crystalline potassium borates of known structure, the boron resonances are assigned to various structures, including four-fold coordinate species, and three-fold coordinate boron in three-memberedringsand in less ordered environments. The primary conclusions reached are: (1) that about 70% of the boron in boron oxide glass is bound in three-membered boroxol rings; (2) that the clustering of theseringsgivesriseto intermediate range order in this material; and (3) that upon alkali oxide modification both ring and non-ring boron are modified, with a propensity for further ring closure leading to an excess of rings that contain four-fold coordinate boron.

Boron oxide is an essential component of many industrial glasses, in part because of its exceptionally good glass forming ability. This tendency is exemplified by pure boron oxide itself, which does not crystallize from the liquid at ambient pressure [1]. Crystals have been grown only by application of pressure (20 kbar) to the melt. The resulting crystals consist of B 0 units, sharing oxygen at the corners and forming extended chains. There is a variety of evidence indicating that boron oxide glass consists of the same B 0 groups, a significantfractionof which are linked in six-membered boroxolrings(B 0 ). 3

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Present address: Department of Materials Science and Engineering, University of Illinois, 105 South Goodwin Avenue, Urbana, IL 61801. Present address: Institute of Optics and Quantum Electronics, Friedrich Schiller University, Jena, Germany.

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©1998 American Chemical Society In Solid-State NMR Spectroscopy of Inorganic Materials; Fitzgerald, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

243 This evidence includes Raman spectroscopy [2], neutron scattering [3,4], NQR [5], and NMR [6-8]. However, no crystalline form of boron oxide has been prepared that contains theserings,and indeed modeling studies of boron oxide glass have typically suggested that there is not a large fraction of rings, in contrast to the interpretations of the many e x p e r i m e n t a l s t u d i e s [ 9 , 1 0 ] . Additives such as alkali oxides are known to cause conversion of three-fold coordinate boron in B 0 groups to B0 " units, in which the boron is four-fold coordinate. Crystal structures of many such compounds are known, and contain three- and four-fold coordinate boron in bothringand chain structures. In both modified borate glasses and pure boron oxide glass the local structure around the boron atoms is quite ordered, with bonding geometries similar to what is seen in borate crystals. This means that the B 0 groups are nearly planar, and the B0 " groups nearly tetrahedral. Because of this ordering, NMR has been an exceptionally successful probe of structure in these materials. Bray and his group, in fact, established NMR as a key tool in glass science primarily through application to borates [11,12]. They were able to show that the fraction of three-fold to four-fold coordinate boron could be determinedfromthe B NMR spectrum, and also pioneered studies of 0 and B in these materials [6]. Because these experiments were performed with CW NMR and static sample techniques, the achievable resolution was not sufficient to resolve boron in similar three-fold sites, for example boron in boroxol rings and in non-ring B 0 units. Therefore, while these studies demonstrated beyond doubt what the basic building blocks of borate glasses are, they could not reveal as much about how these units are linked in the pure glass and modified borate glasses. Because both boron nuclei ( B and have substantial electric quadrupole moments, conventional high-resolution solid-state NMR methods like magic angle spinning (MAS) do not yield resolution enhancements comparable to what can be obtained on spin-Và nuclei like C and P. Recent advances in NMR methodology, however, have provided the necessary tools to obtain the several orders of magnitude improvement in resolution necessary to probe the next level of structure in borate glasses, namely the linkages between the basic units [13-16]. The purpose of this contribution is to describe the application of several of these new experiments to both boron oxide and alkali oxide borate glasses. Our primary results are: (1) a direct assignment of resonances in glassy boron oxide toringand non-ring B 0 units, thereby quantifying these two units; (2) the inference of intermediate range order due to ring-ring binding in glassy boron oxide, through interpretation of boron and oxygen NMR data; and (3) a new model of the modification process, which includes modification at both the boroxolringand non-ring boron sites. In the next section of the paper we detail the experimental methods used in these studies. Then we present results, followed by discussion and interpretation. We close with conclusions and an outline of planned future work.


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In Solid-State NMR Spectroscopy of Inorganic Materials; Fitzgerald, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

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Experimental Methods Sample Preparation Glasses enriched to 90% in boron-10 were made by fusing together appropriate quantities of boric acid (Strem, 99.9995%), boric acid enriched to 97% in B (Aldrich) and potassium carbonate (Strem, 99.8%) at 1000°C. Depletion of the B isotope was necessary to alleviate spin diffusion in the DAS NMR experiments [7]. The melts were held at this temperature for one hour and quenched by removing the platinum cruciblefromthe furnace and cooling in a dry nitrogen atmosphere. The clear, colorless glasses were broken into small pieces and stored under nitrogen to minimize absorption of water. After obtaining Raman data on the bulk samples, the glasses were powdered for NMR analysis. X-ray powder diffraction was used to verify the amorphous character of these samples. Final compositions were checked by weight loss measurements. Potassium borate glasses, enriched in oxygen-17, were prepared by fusing together appropriate amounts of 0-enriched boron oxide and potassium carbonate. The synthesis of 0-enriched B 0 is described elsewhere [8]. After thoroughly mixing these two materials, they were melted at 1000°Cfor 15 minutes. The melts were quenched by cooling in a dry nitrogen environment. 10


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N M R Spectroscopy B NMR spectra were acquired at both 115.6 MHZ and 64.2 MHZ, using a home-built spectrometer. Sample-spinning experiments were carried out with a commercial DAS probe (Doty Scientific), under the control of the spectrometer. Typical sample rotation frequencies were 6-7 kHz. The DAS experiment was designed to remove anisotropic broadenings and yield high resolution NMR spectra of quadrupolar nuclei like B and 0 [13-16]. For these nuclei, at the magnetic field strengths used here, anisotropic terms of both first and second order are important. Conventional Magic Angle Spinning (MAS) removes the first order terms, but not the second order. Because the anisotropies depend on both / (cos0)and P (eos0), where P is a Legendre polynomial and θ is the angle between the rotor axis and the magnetic field, there is no single angle θ about which sample spinning removes both sources of broadening. DAS exploits the fact that there are pairs of angles such that n

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P (oosQ ) = -*P (cos0 ), 2

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P (cosQ ) = -*P (cos0 ). 4

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The experiment is executed with two time evolution periods, thefirstwith the sample spinning about θ and the second about θ . Because of the sign change in the anisotropy, a spin echo is formed in which all first and second order broadening is canceled out. Using this experiment, sites with 20 kHz line widths under static conditions and 5 kHz line widths under MAS yield spectra with 200 Hz-wide lines. For the DAS experiments discussed here, the rotor orientation angles used were Q = 37.38° and θ = 79.19°, with k = 1. The time to reorient the probe and re establish stable spinning between these angles was 40 msec. For the glass samples, the shifted-echo DAS sequence with hypercomplex data acquisition was used, while for the ρ

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crystalline samples only shifted-echo DAS was employed [15]. Each data set consisted of typically 64-128 scans (256 complex points) at each of 80 t points, with 15-60 seconds (for glasses) and 600 seconds (crystals) between scans to allow for full relaxation of ail sites. Each such series of data was zero-filled to 512 χ 512 before transformation. No smoothing was used in either dimension. The time necessary to acquire each spectrum variedfromabout 24 hours for glass samples in the higher field, to as much as several weeks for the crystalline samples at the low field. The major difference is that the relaxation times in the glasses are typically an order of magnitude shorter than in corresponding crystals. Downloaded by STANFORD UNIV GREEN LIBR on August 1, 2012 | http://pubs.acs.org Publication Date: March 18, 1999 | doi: 10.1021/bk-1999-0717.ch007

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Raman Spectroscopy Raman spectra were obtained on a commercial Raman spectrometer (Spectra-Code) at the Purdue University Chemistry Department Laser Facility. The excitation source was the 632.8 nm line of a 30 mW helium-neon laser and the spectral resolution was 3 cm" . The instrument uses integral holographic filtering and a liquid-nitrogen-cooled CCD detector. Data was obtained using large pieces of glassy sample, sealed in a quartz sample cuvette. The scattering intensity was integrated over 600 s of detector exposure time. Spectra were acquired under ambient pressure and temperature conditions. 1

Results Figure 1 shows a typical two-dimensional DAS spectrum of a potassium borate glass. The powder patterns obtained at different isotropic shifts are shown on therightside of thisfigure;the total powder pattern is projected at the top of the contour plot. At left is the high resolution dimension. Figure 2 shows a series of high-resolution spectra as a function of potassium oxide content. Raman spectra (not shown) of the potassium borate glasses show the same behavior seen in previous studies [2,17-21]. The dominant feature is a sharp, strong peak at 808 cm" in pure B 0 glass, which decreases in intensity with added modifier. In addition, a somewhat broader peak grows in at about 770 cm" . By a modifier level of 33%, the 808 cm' has vanished. Additionally, broad weak features in the 300-800 cm" region were seen, which at low modifier content have been assigned to motions of the loose B 0 units [22]. 1

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Discussion NMR Evidence for the Existence of Boroxol Rings While as noted in the introduction a considerable amount of experimental evidence has been interpreted as showing the existence of boroxolringsin boron oxide glass, the precision by which the fraction of boron in such groups can be determined has been questioned. In particular, Raman cross sections (the main source of evidence for rings) are quite difficult to quantify, and diffraction measurements (both neutron and x-ray) can be interpreted with models that show good agreement with the structure factors but differ greatly in the number of rings they contain. Here we present assignments of our NMR data which argue strongly for the existence of a large fraction of boron in boroxol rings, in accord with the conventional view of this system.



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30 Ο Total Shift (ppm)

40 ,

T o t a

0 S h i f t

-40 ( ) ppm

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Figure 1. (a) Two-dimensional DAS spectrum of B in (K 0)2o(B 0 ) glass. The projection at left is the high-resolution dimension. Shifts are in ppm referenced to Et OBF . Slices through the data at three key isotropic shifts are shown to the right: (b) -0.3 ppm, four-fold coordinate boron; (c) 1.0 ppm, loose B 0 units; (d) 4.7 ppm, B 0 units in rings. (Reproduced with permission from ref. 24. Copyright 1996 American Chemical Society.) 2

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Figure 2. Isotropic shift spectra of B in (K 0) (B 0 ) . glasses,fromDAS NMR. (a) x=0.0 (B 0 ); (b) x=0.08; (c) x=0.13; (d)x=0.18; (e)x=0.23; (f) x=0.28; (g)x=0.33;and(h)x=0.38. The vertical lines are drawn at the isotropic shifts of the three sites. Solid line: 4.7 ppm, theringthree-coordinate boron shift; long dashed line: 1 ppm, boron in loose B 0 units; short dashed line: -0.3 ppm, four-coordinate boron. (Reproduced with permission from ref. 24. Copyright 1996 American Chemical Society.) 2

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248 Π

The two Β resonances in boron oxide glass have isotropic shifts of 4.7 ppm and 1.0 ppm at 8.4 Tesla; these shifts are the sum of the isotropic chemical shift and the second order quadrupole shift. The contributions can be separated by performing the experiment at a different field; we have done this, andfindthe parameters given in Table I [7]. Based on the intensities (0.7 and 0.3) we assigned the down-field resonance to boroxolringboron, and the up-field resonance to non-ring B 0 units, in agreement with earlier work which suggested that between 60% and 80% of the boron are in boroxol rings. The chemical shifts as assigned in Table I are in agreement with ab initio results of Tossell [23]. Downloaded by STANFORD UNIV GREEN LIBR on August 1, 2012 | http://pubs.acs.org Publication Date: March 18, 1999 | doi: 10.1021/bk-1999-0717.ch007

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Site Intensity

Isotropic Shift (8.4 T)

Isotropic ChemicalQuadrupole Product Shiff

4.7 ppm

18.0 ppm

a

0.7

2.7 MHZ

2.6 MHZ 13.0 ppm 0.3 1.0 ppm Shifts are relative to Et OBF . The quadrupole product is given by (e Qq/h)\/l+r\ /3, with e Qqlh the quadrupole coupling and η the asymmetry. SOURCE: Adaptedfromref. 7.

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In alkali oxide-modified glasses, as we will discuss in more detail below, resonances with isotropic shifts of 4.7 ppm and 1.0 ppm are again observed at 8.4 Tesla, with a third resonance at -0.3 ppm appearing as the alkali oxide concentration is increased. Here we wish to discuss the results obtainedfromthe NMR spectra of potassium borate crystals [24]. The crystal structures of the compounds studied are all known, and so the boron resonances can be assigned unambiguouslyfromtheir intensities. The assignments are shown in Table Π. Thefirstpoint to note is that boron in the metaboratering,B 0 ", has an isotropic shift of 4.9 ppm, almost identical to the 4.7 ppm shift seen in the glass. The metaborateringis a planar boroxolringcapped by non-bridging oxygen. Next, in the crystals, non-ring B 0 groups are found in the range 1.0-2.3 ppm, with the down-field shift arising from a B 0 group with two B0 " neighbors. B 0 groups in modified boroxol rings have isotropic shifts within a ppm of the metaborate shift, never farther upfield than 3.8 ppm. These results on crystalline compounds are, in our opinion, only consistent with assignment of the 4.7 ppm shift in the glass to boroxolrings,and the 1.0 ppm shift to non-ring B 0 groups. This assignment leads to 70% of the boron in boroxol rings, and 30% in non-ring units, in agreement with numerous previous estimates. 3

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Intermediate Range Order of Pure B 0 Given that the building blocks of boron oxide glass are planar B 0 units, the intermediate range order is generated by how these units 2

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Table Π. Sites and Β Isotropic Shifts from Potassium Borate Crystals Structural Unit

nonring B 0

Isotropic Shift

1.0-2.3 ppm

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B0 -

-0.3 ppm

B0 : metaborate

4.9 ppm

B0 : pentaborate

3.7 ppm

B0 : triborate

3.8 ppm

B0 : di-triborate

5.4 ppm

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5.4 ppm B0 : diborate isotropic shifts at 8.4 Tesla, relative to Et OBF . SOURCE: Adapted from ref. 24. 3

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are linked together. The short-range intermediate range order results from the boroxol rings, and since some 70% of the boron are in these units, it is clear that ordering on this length scale is extensive. The way in which theserings,and the non-ring B 0 units, are linked giveriseto longer range intermediate range order. Since these bridges are made through oxygen, 0 NMR suggests itself as a suitable probe of the types of linking oxygen. The first results of such experiments were obtained by Bray and co-workers, who showed the presence of multiple oxygen sites, again within and outside of boroxol rings [6]. Recently we obtained very high-resolution 0 NMR spectra, using the Double Rotation (DOR) NMR experiment, which showed three resolved oxygen sites [8]. This experiment removes both first and second-order quadrupole broadening, as does DAS. A difficulty in using this experiment, as compared to DAS and MAS, is the comparatively low spinning rates achievable (< 1 kHz). This givesriseto spectra with a great many spinning sidebands, making accurate intensity determination problematic. To circumvent this problem we performed time-dependent simulations of the spectra, which account for the sidebands, and used the interaction parameters and intensities to simulate MAS spectra, which we could record with excellent signal to noise and little interference from sidebands. The results of these consistency checks were that the spectra arise from three sites with relative populations 0.5±0.1, 0.3±0.1, and 0.2±0.1. The interaction parameters (chemical shift and quadrupole coupling) of the three sites are similar and all indicate oxygen in bridging configurations. To assign these sites we constructed a model based on the assumption that four resolvable oxygen sites could be present: oxygen in boroxol rings, oxygen bridging boroxol rings, oxygen bridgingringsand non-ring B0 , andfinallyoxygen bridging two non-ring B 0 units. Two facts were used to constrain the concentrations of these four units: the ratio of oxygen to boron in the glass (3/2) and the fraction of boron present in rings (0.7, based on our NMR data on these samples) [7,8]. This is sufficient to 3

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250 constrain the oxygen site populations leaving only a single degree of freedom. For example, consider the ring oxygen. Call the number ofringboron atoms R; then there are also R ring oxygen atoms, since theringcomposition is B 0 . The fraction of ring oxygen is then R/N, where Ν is the total number of oxygen atoms. But N/M = 3/2, where M is the total number of boron atoms. Therefore R/N = 2R/3M = 0.7x2/3 = 0.47, since for the fraction of boron inringsRIM = 0.7. We thus assign the experimentally observed resonance with intensity 0.5 as the ring oxygen site (note that there are no adjustable parameters in this assignment). For the other three oxygen sites two independent equations may be derived, following reasoning similar to the above. The solutions of these equations may be found in terms of one of the three unknowns, and they show that there is only one region of solution space that corresponds to the observed intensities of 0.5, 0.3, and 0.2: when the population of ring to non-ring bridging oxygen is small. A small concentration of ring to non-ring connections suggests that the glass consists of regions rich in boroxolrings,and regions poor in rings. From the size of rings and the assumption that eachringon the surface of aring-richregion makes a single connection to non-ring B 0 units, we estimated the size of thering-richregions to be about 20-40 Â. Interestingly, recent light-scattering measurements at the glass transition in B 0 [25,26] are consistent with structural inhomogeneities on this length scale. Indeed, intermediate-range order is conjectured to be universal in glass formation. Our data suggests its structural origin in this particular material.


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Modification by Conversion from Three-fold to Four-fold Coordination As stated in the introduction, there is considerable evidence showing that as modifiers are added to borate glass, the initial structural change is the conversion of three-fold coordinate boron to four-fold coordination. Because of the presence of both boroxolringsand nonring B 0 units in the pure glass and melt [22], this modification could happen at either site or both. Considerable work using Raman spectroscopy indicates that the pure boroxol rings are modified in some way, as the sharp 808 cm" peak indicative of the boroxolringsdiminishes in intensity and is replaced by a broader feature at about 770 cm" . This pattern is observed also in the present study, as mentioned above in Section 3. The DAS NMR data presented in Figure 2 also provides a way to follow the structural changes upon modification. In this Figure it is clear that as more modifier is added, the less intense peak at 1.0 ppm decreases in intensity, while a new feature grows in at -0.3 ppm. The strong feature at 4.7 ppm decreases slightly. These three resonances are all well-fit by Gaussian lineshapes, and from suchfitsthe relative intensities of the three resonances can be extracted. To make such a determination wefixedthe isotropic shift of each resonance at its observed value in the 0% modification sample (for the 4.7 ppm and 1.0 ppm features) or the 30% modification (the -0.3 ppm feature). Then, the intensities and widths were allowed to vary freely in fitting the spectra for different modifier content. The results for the relative intensities are shown in Figure 3. The widths do not change significantly with added modifier. We verified that the DAS NMR experiment gives reliable relative intensities for this system by recording spectra of crystalline potassium borates, and comparing the 3

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251 boron site intensities against the populations derived from the crystal structures. This is a significant check because the three-fold and four-fold boron have quite different quadrupole interaction parameters, and hence different NMR nutation behavior. We found that by working at high pulse powers both the three-fold and four-fold site intensities are correctly reproduced. This strategy was used to acquire all the glass data presented here. At 0% modifier (pure B 0 glass) the features in the NMR spectra have been assigned previously as boroxolringboron (4.7 ppm) and non-ring B 0 units (1.0 ppm) [7]. [Note that these are isotropic shifts, not chemical shifts, and as such include both isotropic chemical shift and second-order quadrupole effects. The latter are fielddependent, so these shifts are specific to 8.4 Tesla, the field at which the measurements were made.] This assignment is based on intensities, but agrees with recent ab initio calculations [23]. The feature at -0.3 ppm is due to four-coordinate boron. We studied this feature in detail with B NMR (chosen, due to its integer spin and lower gyromagnetie ratio, to accentuate quadrupole effects as much as possible) and found it to be remarkably insensitive not only to the modifier content in the glass, but even to the chemical nature of the modifier (K 0, Rb 0, Na20 etc.) and the physical structure (glass or crystal). The four-fold coordinate sites thus have a very robust structure and NMR shift signature. The three-fold sites are more difficult to assign in the modified glasses, and we turn to this problem in the next section. 2

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A Unified Model for Borate Modification Chemistry As noted above, Raman spectra strongly indicate that the symmetric boroxol ring is modified, presumably being converted to triborate (two three-fold and one four-fold coordinate boron in aring)and more complex ring structures. Earlier work from our laboratory, using DAS NMR, indicated that substantial modification happens as well at the non-ring B 0 sites, which we observed in rubidium borate glasses to decrease markedly in intensity as modifier was added [27]. Moreover, the NMR resonance initially assigned to the boroxolringboron in B 0 decreased only slightly in intensity with added modifier. These observations together suggested an interpretation in contradiction with Raman studies, namely that the ring sites are not modified to any great extent, while the non-ring B 0 groups are preferentially converted to B 0 \ Here we use our more complete data set, which includes both NMR and Raman spectra, to describe a unified model of modification [24]. First, our NMR results (Figure 3) show that the resonance assigned to loose B 0 units decreases strongly with R; a fit to the data yields 3

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L = 0.35 - 0.75i?,

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where X is thefractionof boron in loose B 0 units. Likewise, the fraction of boron in all four-fold coordinate sites can be taken as 3

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whereZ and Rg axe the fractions of boron in loose andring-boundfour-fold sites, respectively. This estimation assumes that each modifier ion produces a single B0 * unit, and hence that no non-bridging oxygen are formed. At the modifier concentrations considered here this is a reasonably accurate assumption. Finally, the fraction of three fold boron inringsof all kinds, Rg , follows from normalization, as 4

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Rg =1 - I

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- L - Rg = 0.65 - 0.25Λ

(4)

Lines corresponding to the above model are plotted on Figure 3, along with the data, and show reasonable agreement. We assume in this plot that all three-fold coordinate boron inringshave equivalent isotropic shifts, and thus overlap in the resonance at 4.7 ppm. To check this assumption we have examined a variety of crystalline potassium borates, of known structure, and found that the three-fold ring sites are always within 1 ppm of the 4.7 ppm boroxolringshift. The above equations give a general model in agreement with our NMR data on the partitioning of the three-coordinate boron between rings and loose units. Our NMR data do not resolve individual modified ring structures, so there are a variety of ways to realize this model. We proceed by assuming that the modifiedringsinclude tetraborate and diborate groups, in accord with earlier analyses of alkali borate glasses. Then the glasses are taken to consist of 5 basic units: boroxolrings,tetraborate and diborate units, and loose B 0 and B0 ". To estimate the fractions of these building blocks we need 5 independent constraints. One is provided by normalization, and Eqs. 2 and 3 provide two more. Our Raman data give an estimate of thefractionof boroxol rings. In this data we see the 808 cm* band decreasing roughly linearly, and vanishing by roughly the composition R = 0.5 (this is in reasonable agreement with earlier B NMR studies [28,29]). From the NMR data on B 0 we find B , thefractionof boron in boroxol rings, to be 0.65, and so the fraction of boroxolringsthemselves is 0.38. The boroxol ringfractionin the modified glass is thus taken as 0.38 - 0.76R, that is, a linear function that vanishes at R = 0.5. Finally, the diborate concentration is taken from Jellison and Bray's results [28], who found that the ratio of boron in diborate groups to boron in rings is roughly zero for R < 0.1, and then increases approximately linearly to about 0.68 by R = 0.5. From Eqs. 2, 3, normalization, and the concentrations of boroxol rings and diboraterings,an estimate of the various units in the glasses can be made as a function of composition. Figure 4 shows the results, expressed in terms of thefractionof boron present in each type of site. The relative concentrations of boroxol, tetraborate, and diborateringsare quite similar to earlier determinations [28,29], Here, however, account is taken explicitly of the loose units, so the concentrations of the ring species are reduced in absolute terms. The agreement between theringfractionsexhibited in Figure 4 and those obtained in earlier works [28,29] is encouraging because here the only ingredient takenfromthose works is the diborate concentration. The boroxolringfractionis set by our Raman results, and the loose 3- and 4-foldfractionsby the DAS NMR results. Figure 4 thus represents a model that is consistent with our present high-resolution NMR results on 3

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π

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Figure 3. Intensities, obtained from spectral fits, of the three main features in the DAS NMR spectra of B in (K 0) (B 0 ) . glasses as a function of R = x/l-x, the ratio of modifier to boron oxide concentration. Squares: 4.7 ppm peak. Triangles: 1.0 ppm peak. Circles: -0.3 ppm peak. Solid lines arefromthe model of Eqs. 24. (Reproduced with permission from ref. 24. Copyright 1996 American Chemical Society.) n

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/ JC

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Figure 4. Fraction of boron in each type of site for the model outlined in the text. • : boroxol rings (Β ); V: loose B 0 groups (L ); o: 3-coordinate boron in tetraborate groups (7*); 0: 4-coordinate boron in tetraborate (7*); X: three-and four-coordinate boron in diborate (D , D ); +: loose B0 " groups (L ). 3

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the loose units, Raman spectra of the boroxolrings,and earlier B wide-line NMR results on the modifiedringfractions. Conclusions We have presented results of several studies on borate glass structure, made primarily with high-resolution and multidimensional NMR methods. In pure B 0 glass we have used Double Rotation NMR to probe the oxygen environments, and found that three independent sites could be resolved. We constructed a model to interpret these sites, which suggests that the boroxolringshave a tendency to aggregate in the glass. This would explain the intermediate range order observed in scattering studies in this material. In potassium borate glasses we were able to follow the modification using twodimensional Dynamic Angle Spinning NMR. These experiments resolve three sites, assigned to ring, loose B0 , and four-fold coordinate boron. The partitioning of the boron among these sites can be modeled in a simple way that is in reasonable agreement with the data. Then, by combining the intensities of these sites with Raman spectra and earlier estimations of the diborate concentration, a full model is developed which gives the concentrations of five structural units as a function of modifier: boroxol rings, tetraborate and diborate groups, and loose B 0 and B0 " groups. The model is consistent with the present DAS NMR data, Raman spectra, and the earlier studies of the modified rings. We believe it to be a significant result in thisfield,because for the first time the loose units are explicitly included in a structural model. While the majority of boron atoms are present in rings, at low modifier concentrations the majority of structural units are loose B0 . Thus taking them into account is important. Finally, the model provides a unified treatment of a variety of different experiments, each capable of probing different elements of the glass structure.


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Acknowledgments This research was supported by the National Science Foundation under Grants DMR-9115787 and DMR-9508625. Literature Cited [1] Uhlmann, D.; Hays, J.; Turnbull, D. Phys. Chem. Glasses 1967, 8, 1. [2] Bril, T. Philips Res. Rep. Suppl. 1976, 2, 1.

[3] Hannon, Α.; Wright, Α.; Blackman, J.; Sinclair, R. J. Non-Cryst. Solids 1995, 182, 78. [4] Hannon, Α.; Grimley, D.; Hulme, R.; Wright, Α.; Sinclair, R. J. Non-Cryst. Solids 1994, 177, 299. [5] Gravina, S.; Bray, P. J. J. Magn. Reson. 1990, 89, 515. [6] Jellison, Jr., G. E.; Panek, L. E.; Bray, P. J.; Rouse, Jr., G. B. J. Chem. Phys. 1977, 66, 802. [7] Youngman, R. E.; Zwanziger, J. W. J. Non-Cryst. Solids 1994, 168, 293. [8] Youngman, R. E.; Haubrich, S.; Zwanziger, J. W.; Janicke, M.; Chmelka, B. Science 1995, 269, 1416.


255 [9] Soules, T. J. Chem. Phys. 1980, 73, 4032. [10] Soppe, W.; van der Marel, C.; van Gunsteren, W.; den Hartog, H. J. Non-Cryst. Solids 1988, 103, 201.

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Solid-State NMR Spectroscopy of Inorganic Materials - American

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