J. Phys. Chem. B 2008, 112, 6747–6752
Local Structure of Magnesium in Silicate Glasses: A
25Mg
6747
3QMAS NMR Study
Keiji Shimoda,*,†,‡ Takahiro Nemoto,§ and Koji Saito‡ Institute for Study of the Earth’s Interior, Okayama UniVersity, Misasa, Tottori 682-0193, Japan, AdVanced Technology Research Laboratories, Nippon Steel Corp., 20-1 Shintomi, Futtsu, Chiba 293-8511, Japan, and JEOL Ltd., 3-1-2 Musashino, Akishima, Tokyo 196-8558, Japan ReceiVed: December 3, 2007; ReVised Manuscript ReceiVed: March 12, 2008
We have reported the 25Mg triple-quantum magic-angle spinning (3QMAS) NMR spectra of silicate glasses. The two-dimensional spectra suggest that the magnesium ions in MgSiO3, CaMgSi2O6, Ca2MgSi2O7, Mg3Al2Si3O12, and Li2MgSi2O6 glasses are mainly in octahedral environments, although in Na2MgSi2O6, K2MgSi2O6, and K2MgSi5O12 glasses they form tetrahedral species. We discussed the coordination environments of Mg based on the field strength of competing Mg2+, Ca2+, Na+, K+, and Li+ cations, and convincingly demonstrated that there is a correlation between them. These results indicate that the two-dimensional NMR spectroscopy such as MQMAS technique is a very useful method to analyze the local environments of nonframework cations in noncrystalline solids. Introduction Magnesium is an important constituent in various areas of materials science, Earth science, and biochemistry. In silicate melts, the addition of MgO involves breakdown of the SiO4 tetrahedral network, significantly influencing melt viscosities and glass-forming properties. Nevertheless, the structural role of Mg itself is still poorly understood due to the lack of information on its local environment, which has been a longstanding issue in understanding the complete picture of noncrystalline structures. The deficiency of conclusive information stems from the difficulty in extracting Mg correlation from conventional X-ray diffraction, Raman, and infrared spectroscopy. Yin et al.1 suggested that Mg is coordinated by six oxygen atoms in MgSiO3 glass by using an X-ray diffraction technique, but a recent X-ray and neutron scattering study, combined with reverse Monte Carlo (RMC) simulation, has reported tetra- to pentahedral coordination for the glass.2 The Mg coordination number for CaMgSi2O6 glass was also well-investigated by some researchers but differently concluded as tetrahedral,3 pentahedral,4,5 or octahedral coordination,6 respectively. Molecular dynamics simulations have also given different conclusions.7–9 Nuclear magnetic resonance (NMR) spectroscopy is an effective tool for collecting structural information focused on a specific element (especially light elements) in multicomponent systems. Kroeker and Stebbins6 studied the Mg environments of CaMgSi2O6 and K2MgSi5O12 glasses by using 25Mg magic angle spinning (MAS) NMR spectroscopy. They revealed that the Mg environments depend on the chemical composition and suggested the potency of the NMR technique to elucidate Mg local structure. However, 25Mg NMR study has been circumvented because of its low resonance frequency (18.4 MHz at 7.0 T) and relatively low natural abundance (10.1%). Using a high magnetic field and 25Mg isotopic enrichment drastically * Corresponding author. Present address: Institute for Study of the Earth’s Interior, Okayama University, Misasa, Tottori 682-0193, Japan. Tel: +81858-43-3766. Fax: +81-858-43-3766. E-mail: kshimoda@ misasa.okayama-u.ac.jp. † Okayama University. ‡ Nippon Steel Corp. § JEOL Ltd.
alleviates these shortcomings. Nevertheless, 25Mg MAS NMR spectra further suffer from large second-order quadrupolar broadening, as shown previously,6,10 making it difficult to clarify its detailed environment. In this study, we applied the 25Mg 3QMAS NMR technique at high magnetic field to overcome this difficulty and shed light on the local environments surrounding Mg2+ ion in Mg-containing silicate glasses. Experimental Section Sample Preparation. Glass samples (MgSiO3, CaMgSi2O6, Ca2MgSi2O7, Mg3Al2Si3O12, Na2MgSi2O6, K2MgSi2O6, K2MgSi5O12, and Li2MgSi2O6 glasses) and crystalline standards (diopside (CaMgSi2O6), åkermanite (Ca2MgSi2O7), and K2MgSi5O12 leucite analogue) were prepared from appropriate mixtures of 99%-enriched 25MgO (Trace Sciences International Inc.), Ca(OH)2, Na2CO3, K2CO3, Li2CO3, Al2O3, and SiO2 reagents. The MgSiO3, CaMgSi2O6, Ca2MgSi2O7, and Mg3Al2Si3O12 glasses were obtained by melting the powder mixtures at 1500-1700 °C and quenching in air or water, but Na2MgSi2O6, K2MgSi2O6, K2MgSi5O12, and Li2MgSi2O6 glasses were melted at 1300 °C. Diopside and åkermanite were prepared by melting at 1500 °C and subsequent annealing at 1000 °C for 6 h, and crystalline K2MgSi5O12 was prepared according to the method of Kroeker and Stebbins.6 Samples were checked by XRD. NMR Spectroscopy. The 25Mg MAS and 3QMAS spectra were acquired on a JNM-ECA700 spectrometer (16.4 T) at the Larmor frequency of 42.9 MHz. All the MAS spectra were acquired with a spin-echo sequence11 (a pulse length of 1.8 and 3.6 µs) to avoid acoustic ringing from the transmitter and probe and with a recycle delay of 0.5 s. Longer delays did not influence the peak shape. In 3QMAS measurements, we used the conventional z-filter sequence.12 Triple-quantum (3Q) excitation has been widely used in multiple-quantum (MQ) MAS studies because the 3Q coherence is the most readily excited and converted to single-quantum coherence. Multiple-quantum coherences (pQ (for I ) -5/2, pQ can be selected as 3Q or 5Q) are excited during the evolution time t1 and the given coherence is selectively transferred to the detectable singlequantum coherence -1Q, by the second and third pulses with
10.1021/jp711417t CCC: $40.75 2008 American Chemical Society Published on Web 05/08/2008
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Figure 1. Schematic illustration of z-filter type pulse sequence (top) and coherence transfer pathway diagram (bottom) for the 3QMAS technique for spin I ) 5/2.
a phase cycling technique, just before the acquisition time t2 (Figure 1). As a result, all anisotropies can be refocused in the t2 domain. Frydman and Harwood13 demonstrated that this multiple-quantum to single-quantum transfer method yields purely isotropic spectra for half-integer quadrupolar nuclei. Because the z-filter sequence that is commonly used in the MQMAS experiments needs a high power rf field for an effective excitation of the multiple-quantum coherence, the development of a high power rf-resistant probe that provides a short 90° pulse is crucial.14,15 In this study, the 3Q excitation and conversion pulses were optimized to 6.0 and 2.5 µs, respectively, with a rf field strength of ca. 139 kHz for the silicate samples. The soft 90° pulse length of 30 µs (a field strength of 8 kHz) was adopted for signal detection followed by a recycle delay of 0.5 s. Samples set in 4-mm MAS rotors (sample volume of ca. 100 mg) were rotated at a spinning rate of 18 kHz in the measurements. Chemical shifts were externally referenced to saturated MgSO4 solution at 0.0 ppm. A series of free induction decay (FID) signals in the 3QMAS measurements were Fourier-transformed and sheared to obtain the separate isotropic and anisotropic (MAS) dimensions. The isotropic chemical shift δCS and quadrupolar product Pq were derived from the sheared two-dimensional spectra with the following equations16
17 10 δ + δ 27 1 27 2 1⁄2 170 [4S(2S - 1)]2 Pq ) δ1 - δ2) ν0 × 10-3 ( [ ] 81 4S(S + 1) - 3
δCS )
(
)
where δ1 and δ2 are the apparent shifts in isotropic and MAS dimensions, ν0 is the Zeeman frequency, and S is the spin number. The Pq is also related to the quadrupolar coupling constant Cq by the equation Pq ) Cq(1 + η2/3)1/2, where η is the asymmetry parameter (η ) 0-1). Results Figure 2 shows a comparison of the 25Mg MAS spectra for MgSiO3 and K2MgSi2O6 glasses at between 16.4 and 21.8 T. The spectral features are similar to those obtained by Kroeker et al.,10 and the central peak in the MgSiO3 glass spectra considerably overlaps with spinning side bands. As shown in the figure, higher magnetic field gives narrower spectral widths. The full width at half-maxima (fwhm) of the peaks were ca. 185 and 170 ppm for MgSiO3 glass and ca. 137 and 90 ppm for K2MgSi2O6 glass at the two fields, respectively. The spectral narrowing with increasing magnetic field strength was less
Figure 2. Comparison of 25Mg MAS NMR spectra for (a) MgSiO3 and (b) K2MgSi2O6 glasses at 16.4 and 21.8 T. The asterisks refer to spinning side bands.
effective for MgSiO3 glass compared with that for K2MgSi2O6 glass. Moreover, our previous study showed that the peak positions and their widths in 25Mg MAS spectra of MgSiO3, CaMgSi2O6, Ca2MgSi2O7, and Mg3Al2Si3O12 glasses were less positive in frequency and broader than those of Na2MgSi2O6 and K2MgSi2O6 glasses. These observations indicate the different Mg environments between these two “groups”.17 The 25Mg MAS spectra of the glasses suffer from a secondorder quadrupolar broadening because of low resonance frequency and relatively large quadrupolar interaction, which smears out the detailed information in the spectra. In such cases, it is even difficult to estimate an isotropic chemical shift from a series of spectra at multiple magnetic fields.10 The MQMAS NMR technique is an excellent method for observing highresolution NMR spectra, free from second-order broadening of half-integer quadrupolar nuclei.13 Parts a-h of Figure 3 show the 25Mg 3QMAS spectra of MgSiO3, CaMgSi2O6, Ca2MgSi2O7, Mg3Al2Si3O12, Na2MgSi2O6, K2MgSi2O6, K2MgSi5O12, and Li2MgSi2O6 glasses at 16.4 T, respectively. We collected 27 000-125 400 transients for each 8 or 16 t1 increment (the t1 dwell time was 55.5 µs). The inspection of the FID signal in t1 dimension indicates that such short t1 points are marginally acceptable to get better signal-to-noise (S/N) spectra for the glasses. The 3QMAS spectrum of MgSiO3 glass shows a main peak and some complex features in the tail along quadrupolarinduced shift (QIS) axis. These features are reasonably consistent
Local Structure of Magnesium in Silicate Glasses
J. Phys. Chem. B, Vol. 112, No. 22, 2008 6749
Figure 3. 25Mg 3QMAS spectra at 16.4 T for (a) MgSiO3, (b) CaMgSi2O6, (c) Ca2MgSi2O7, (d) Mg3Al2Si3O12, (e) Na2MgSi2O6, (f) K2MgSi2O6, (g) K2MgSi5O12, and (h) Li2MgSi2O6 glasses. The chemical shift (CS) and quadrupolar-induced shift (QIS) axes are also drawn. The contours are plotted from ca. 4.2% to ca. 95.8% with an increment of 4.2% above noise level. The spectra were apodized with a Gaussian function of 600 Hz for MAS dimension, which is quite reasonable considering the fast FID decay.
with those reported at 21.8 T,17 even though it is difficult to extract the real structural component in the noisy tail due to the lower S/N ratio at 16.4 T. CaMgSi2O6, Ca2MgSi2O7, and Mg3Al2Si3O12 glasses also have an intense peak accompanied by a noisy tail, and there will be some structural features in the tail. Especially, considering the MAS and isotropic dimensions for Ca2MgSi2O7 glass spectrum may suggest two additional Mg sites other than the main peak. Contrary to these glasses, the 3QMAS spectrum of Na2MgSi2O6 glass represents a wellresolved single peak with a small plateau at large-Pq side. K2MgSi2O6 and K2MgSi5O12 glasses also show a single peak with a tail along the QIS axis (i.e., quadrupolar distribution), in agreement with the 21.8 T spectrum.17 The spectrum of Li2MgSi2O6 glass seems to have a single site. The isotropic chemical shifts δCS and quadrupolar products Pq were estimated for the most intense peak tops (Table 1).
Discussion To elucidate the Mg coordination state in the silicate glasses, the isotropic chemical shifts obtained here should be compared with the previous studies. However, the relationship between chemical shift and Mg-O distance, or Mg coordination state, has not been established clearly because of the difficulty of 25Mg data acquisition as mentioned above. MacKenzie and Meinhold18 examined the 25Mg MAS NMR spectra of crystalline materials and confirmed that the reliable δCS of 6-fold Mg-containing silicates ranges from 5 to 14 ppm. In the case of 4-fold Mg minerals, the δCS’s of spinel (MgAl2O4) and åkermanite (Ca2MgSi2O7) are 52 and 49 ppm, respectively.6,17 The chemical shifts of 5- and 8-fold Mg minerals are not well-known.19,20 Thus, the main peak in the MgSiO3 glass spectrum is attributable to 6-fold Mg species. It is also possible to assign the underlying
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TABLE 1: Isotropic Chemical Shift δCS and Quadrupolar Product Pq for the Most Intense Peaks in the 25Mg 3QMAS Spectra at 16.4 T
a
glass samples
δCS (ppm)a
Pq (MHz)a
MgSiO3 CaMgSi2O6 Ca2MgSi2O7 Mg3Al2Si3O12 Na2MgSi2O6 K2MgSi2O6 K2MgSi5O12 Li2MgSi2O6
14 16 14 14 41 38 37 14
2.8 2.7 3.9 3.2 3.5 2.9 3.1 2.9
Errors are estimated to be within 2 ppm and 0.4 MHz.
sites in the tail, if any, to 6-fold Mg with large Pq values based on the corresponding spectrum at 21.8 T.17 CaMgSi2O6, Ca2MgSi2O7, and Mg3Al2Si3O12 glasses also have a main peak with a chemical shift of ca. 15 ppm, indicating their octahedral coordination environments. Again, they will have other 6-fold sites with large Pq’s in the tail. In the case of Na2MgSi2O6, K2MgSi2O6, and K2MgSi5O12 glasses, we obtained a welldefined peak with a chemical shift of ca. 40 ppm. It should be noted that there might be an additional feature (ca. 30 ppm) having large Pq (ca. 6 MHz) in Na2MgSi2O6 glass. The chemical shift range for these sites is less positive in frequency than those for spinel and åkermanite. However, our 3QMAS spectrum of crystalline K2IVMgSi5O12 (not shown) shows a single peak with an isotropic chemical shift of 37 ppm, being consistent with its dry-synthesized disordered crystal structure.21 Thus, we conclude that the Mg2+ ions mainly locate in tetrahedral environments in the Na- and K-containing glasses, although the large-Pq site in Na2MgSi2O6 may be related to a 5-fold Mg. The chemical shift of Li2MgSi2O6 glass is, however, out of the range for Naand K-containing glasses and is similar to that for MgSiO3 glass (Table 1). This strongly indicates that the Li2MgSi2O6 glass has octahedral Mg species as a main structural component. Even at relatively high magnetic field and with a long accumulation time, we could not resolve any possible large-Pq sites in the noisy tail for 6-fold Mg-containing glasses, whereas we could collect better S/N spectra for 4-fold Mg glasses, where they have a noiseless smooth tail, with a moderate accumulation time ( 5 MHz, if any. A combination of the higher magnetic field and a specially designed MQMAS probe enduring high power pulse will provide the more detailed information on Mg local structure in the future. The Mg local structures in silicate glasses and melts have been under debate until recently. Waseda and Toguri23 presented that Mg2+ ion was tetrahedrally coordinated by oxygen atoms (djMg-O ) 2.14 Å) by analyzing an X-ray radial distribution function (RDF) of 44MgO-56SiO2 glass. On the other hand, Yin et al.1 considered that Mg2+ was octahedrally coordinated in MgSiO3 glass on the basis of the RDF analysis according to the quasicrystalline model. Even recently, Wilding et al.2 reported that the average Mg coordination number was 4.5 (djMg-O ) 2.2 Å) for the MgSiO3 glass by using X-ray and neutron scattering combined with a RMC simulation. Such a contrast may partly arise from the difficulty of the determination of the coordination number from the RDF analysis, especially for nonframework cations, which often locate in distorted sites.24
Similarly for CaMgSi2O6 glass, Mg K-edge X-ray absorption spectroscopy (XAS) suggested the average coordination number of 5 (djMg-O ) 1.99-2.12 Å).4,5,25 From its O K-edge extended electron energy loss fine structure (EXELFS) curve, Tabira3 indirectly suggested the tetrahedral coordination of Mg2+ ion (djMg-O ) 1.94 Å). However, the previous 25Mg MAS NMR study showed that the chemical shift of CaMgSi2O6 glass corresponded to that of the crystalline counterpart, diopside.6 The present 25Mg 3QMAS technique gives a more definite conclusion than the MAS technique. Although we cannot definitely confirm the absence of 4- and 5-fold Mg because of broad 2D spectra at 16.4 T, the 3QMAS spectra reveal the predominance of 6-fold Mg species in MgSiO3, CaMgSi2O6, Ca2MgSi2O7, Mg3Al2Si3O12, and Li2MgSi2O6 glasses. This result suggests that the Mg structures in MgSiO3 and CaMgSi2O6 glasses are relatively close to the crystalline counterparts, enstatite and diopside, where magnesium is in octahedral sites,26–28 but those in Ca2MgSi2O7 and Mg3Al2Si3O12 glasses are different from the crystalline counterparts, åkermanite and pyrope, where magnesium is in a tetrahedral and a dodecahedral (i.e., 8-fold) site, respectively.29,30 The rough estimates of the average Mg-O distances provide ca. 2.05 Å for MgSiO3, CaMgSi2O6, Ca2MgSi2O7, Mg3Al2Si3O12, and Li2MgSi2O6 glasses and ca. 1.96 Å for Na2MgSi2O6, K2MgSi2O6, and K2MgSi5O12 glasses, respectively, on the basis of the chemical shift and average Mg-O distance for diopside and åkermanite. The estimated distance for 6-fold Mg glasses appears to be smaller than the experimental values mentioned above, but close to the sum of ionic radii (VIMg-O ) 2.08 Å)31 and the value from MD simulation.9 An X-ray emission study indicated that K2MgSi2O6 glass has the average Mg coordination number of 4.32 The previous 25Mg MAS NMR similarly suggested that K2MgSi5O12 glass has a tetrahedral environment as in crystalline K2MgSi5O12,6 which is clearly confirmed in this study by using 25Mg 3QMAS NMR spectroscopy. Also, George and Stebbins33 have reported the isotropic chemical shift of 27-40 ppm for (Na2O)0.28(MgO)0.18(SiO2)0.54 melt by a high-temperature 25Mg NMR technique, although they concluded the Mg coordination number of 5-6. It is considered that the coordination environment of Mg2+ ion depends on the cation field strength of competing modifier ions. The cation field strength, Z/d2 (Z is the modifier charge and d is M-O distance) increases in the order of K (0.12), Na (0.18), Li (0.26), Ca (0.36), and Mg (0.46-0.53).31 This indicates an increase in bond strength in the order of K-O, Na-O, Li-O, Ca-O, and Mg-O. Thus, in the Na2MgSi2O6, K2MgSi2O6, and K2MgSi5O12 glasses, Mg2+ ions strongly and effectively bind with four oxygen atoms over competing Na+ and K+ ions.6 The structural difference between K-Mg and Ca-Mg silicate glasses has also been discovered by an 17O 3QMAS NMR study.34 In K-Mg silicate glass, Mg2+ was found to bind preferably with nonbridging oxygens (NBO), and K+ was bound with bridging oxygens (BO), although both cations should be associated with NBO only in principle. In contrast, Ca-Mg glass had no such bonding preference on oxygen and showed a range of (Ca,Mg)-NBO environments. Such a difference has been also attributed to the difference in cation field strength of the modifier ions.34 Although Li+ is an alkali metal ion as well as Na+ and K+, the present study suggests the predominance of octahedral Mg coordination in Li2MgSi2O6 glass. This indicates that the Mg-O bond competes comparably with the relatively strong Li-O bond, resulting in MgO6 and various (Li,Mg)-NBO environments. Figure 4 presents a relationship between the average cation field strength of mixed modifier ions and isotropic
Local Structure of Magnesium in Silicate Glasses
Figure 4. 25Mg isotropic chemical shifts as a function of average cation field strength of modifier ions, Z/d2. Solid and open squares are from the present study (glass) and the previous high temperature studies (melt),33,35 respectively. The square size is comparable to the estimated error in ppm. Key: 1, MgSiO3; 2, CaMgSi2O6; 3, Ca2MgSi2O7; 4, Mg3Al2Si3O12; 5, Na2MgSi2O6; 6, K2MgSi2O6; 7, K2MgSi5O12; 8, Li2MgSi2O6; 9, (Na2O)0.28(MgO)0.18(SiO2)0.54; 10, (MgO)0.25(CaO)0.25(SiO2)0.50; 11, (MgO)0.14(CaO)0.41(Al2O3)0.06(SiO2)0.39; 12, (MgO)0.21(CaO)0.25(Al2O3)0.04(SiO2)0.50; 13, (Na2O)0.14(K2O)0.14(MgO)0.18(SiO2)0.54; 14, (CaO)0.29(MgO)0.14(SiO2)0.57. Data points 9-13 and 14 are from refs 33 and 35, respectively.
chemical shift. The glasses with higher Z/d2 have lower frequencies in 25Mg chemical shift. The data points are clearly distinguishable between 4-fold and 6-fold Mg-containing glasses in this study, and there appears to be a discontinuity between the average field strength of 0.30 and 0.33. This may indicate that the Mg coordination number decreases from 6 to 4 (or 5) when the Li/Mg ratio is increased to be average Z/d2 e 0.3 in Li2O-MgO-SiO2 join but remains 6 when Ca/Mg is changed in CaO-MgO-SiO2 join. Figure 4 also includes the isotropic chemical shifts provided by high-temperature 25Mg NMR studies. The high-temperature liquid NMR spectra directly give isotopic chemical shifts. They are different from the present results. It may be possible to deduce that a negative frequency shift of (Na2O)0.28(MgO)0.18(SiO2)0.54 melt with increasing temperature33,35 is caused by the increase of Mg-O distances by thermal expansion. The estimated Mg-O distance at 1400 °C is 2.00 Å, intermediate between IVMg-O and VMg-O distances at high temperature.31 In contrast, the Mg coordination may decrease to ca. 5 in CaMgSi2O6 melt33 by removing the one oxygen with weaker bonding from the first coordination sphere of distorted MgO6, due to thermal expansion. This will result in a chemical shift with slightly higher frequency compared to that of CaMgSi2O6 glass presented here. Then, the estimated distance of 2.02 Å is close to the VMg-O distance at high temperature. Our results further predict that the high-Z/d2 divalent cations such as Fe2+ (0.44-0.51) and Ni2+ (0.48-0.55) may have the local environments similar to Mg2+, i.e., the coordination increases with increasing Z/d2 of coexisting modifier cations. Because of the great difficulties of transition metals 57Fe and 61Ni NMR signal detection, Fe and Ni K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy has widely been applied to investigate the local environments of such ions in silicate glasses. Waychunas et al.36 have reported the tetrahedral Fe2+ in (Na,K)2Fe2+Si3O8 glasses, although Rossano et al.37 reported 4-fold ferrous ion in CaFe2+Si2O6 glass against our prediction. As mentioned above, their RDF analysis may underestimate the coordination number of the distorted FeO6.
J. Phys. Chem. B, Vol. 112, No. 22, 2008 6751 In the case of Ni2+, the EXAFS studies have provided the Ni coordination number of 4 in K2NiSi3O8 glass, 4-5 in Na2NiSi3O8 glass, and ca. 5 in CaNiSi2O6 glass.38,39 The 25Mg 3QMAS spectra suggest the possibility of multiple Mg environments, instead of a quadrupolar distribution of the main peak site, especially in 6-fold Mg-containing glasses, although it is less clear due to relatively low S/N ratio at 16.4 T. The main peaks in the spectra have small Pq values of ca. 3 MHz (Table 1) and indicate that these sites are relatively regular, stable environments. The Pq value of the main peak in CaMgSi2O6 glass spectrum (2.7 MHz) is relatively close to that of diopside (2.2 MHz),17 but the spectral feature of the tail would suggest the existence of more distorted octahedral environments of Mg2+ in the glassy state (eca. 6 MHz). An asymmetric Mg-O peak in a RDF curve2 also confirms such polyhedral distortions in MgSiO3 glass, supporting the assertion that MgSiO3 glass has four Mg-O bonds at 2.08 Å and two additional bonds at 2.5 Å.1 Thus, the present study gives some information on the site distortion for the nonframework cations in noncrystalline materials. Conclusions We applied 25Mg 3QMAS NMR spectroscopy to elucidate the Mg local structure in silicate glasses and showed that magnesium ions in MgSiO3, CaMgSi2O6, Ca2MgSi2O7, Mg3Al2Si3O12, and Li2MgSi2O6 glasses are mainly in MgO6 octahedral environments, which contrasts with the previous diffraction and XAS studies. On the other hand, Na2MgSi2O6, K2MgSi2O6, and K2MgSi5O12 glasses have a tetrahedral Mg coordination. We also found that the Mg coordination number changed discontinuously according to the cation field strengths of the competing modifier cations in the glass. Moreover, the two-dimensional spectra imply the structural complexity of the Mg local environment, i.e., the multiplicity of the Mg sites, in 6-fold Mg glasses, although the definitive conclusion needs further improvement in excitation efficiency, for example, by the application of higher magnetic field strength above 22 T or the other sophisticated two-dimensional NMR techniques. Acknowledgment. This work is supported by Special Coordination Funds for Promoting Science and Technology in Japan. We express our gratitude to T. Nishiura for making the 25Mg-enriched samples, and M. Hatakeyama for his assistance on NMR measurements. We would like to acknowledge anonymous reviewers for their constructive comments and suggestions. References and Notes (1) Yin, C. D.; Okuno, M.; Morikawa, H.; Marumo, F. J. Non-Cryst. Solids 1983, 55, 131. (2) Wilding, M. C.; Benmore, C. J.; Tangeman, J. A.; Sampath, S. Chem. Geol. 2004, 213, 281. (3) Tabira, Y. Mater. Sci. Eng. B 1996, 41, 63. (4) Ildefonse, P.; Calas, G.; Flank, A. M.; Lagarde, P. Nucl. Instrum. Methods Phys. Res., Sect. B 1995, 97, 172. (5) Li, D.; Peng, M.; Murata, T. Can. Mineral. 1999, 37, 199. (6) Kroeker, S.; Stebbins, J. F. Am. Mineral. 2000, 85, 1459. (7) Kubicki, J. D.; Lasaga, A. C. Phys. Chem. Minerals 1991, 17, 661. (8) Matsui, M. Geophys. Res. Lett. 1996, 23, 395. (9) Shimoda, K.; Okuno, M. J. Phys.: Condens. Matter 2006, 18, 6531. (10) Kroeker, S.; Neuhoff, P. S.; Stebbins, J. F. J. Non-Cryst. Solids 2001, 293-295, 440. (11) Kunwar, A. C.; Turner, G. L.; Oldfield, E. J. Magn. Reson. 1986, 69, 124. (12) Amoureux, J. -P.; Fernandez, C.; Steuernagel, S. J. Magn. Reson. A 1996, 123, 116. (13) Frydman, L.; Harwood, J. S. J. Am. Chem. Soc. 1995, 117, 5367.
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