12530
J. Phys. Chem. C 2008, 112, 12530–12539
The Mixed-Network Former Effect in Phosphate Glasses: NMR and XPS Studies of the Connectivity Distribution in the Glass System (NaPO3)1-x(B2O3)x Devidas Raskar, Matthias T. Rinke, and Hellmut Eckert* Institut fu¨r Physikalische Chemie, Westfa¨lische Wilhelms-UniVersita¨t Mu¨nster, Corrensstrasse 30, D 48149 Mu¨nster, Germany ReceiVed: April 23, 2008; ReVised Manuscript ReceiVed: May 27, 2008
The structural organization of sodium borophosphate glasses with composition (NaPO3)1-x(B2O3)x (0.0 e x e 0.3) has been investigated by X-ray photoelectron spectroscopy (XPS), as well as single- and doubleresonance 11B and 31P magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. O-1s XPS data provides a quantitative distinction between B-O-B, B-O-P, and P-O-P linkages as well as nonbridging oxygen atoms. 11B and 31P MAS NMR data indicate that within the compositional region 0 e x e 0.20 the entire boron inventory is present in the form of anionic BO4- units, resulting in the repolymerization of an equivalent fraction of the phosphate units (conversion of anionic metaphosphate (P(2)) into neutral branching groups (P(3)) species. Both XPS as well as 31P{11B} and 11B{31P} rotational echo double resonance (REDOR) NMR results reveal strong interactions between the two network formers boron oxide and phosphorus oxide, resulting in the dominant formation of B-O-P linkages. In addition, the shape of the REDOR curve reveals a certain tendency of these linkages to cluster, consistent with a preference of P(3) units to form more than one P-O-B linkage, even at low boron contents. The enhanced degree of network polymerization correlates with a significant increase of the glass transition temperature as a function of boron content. Introduction The large majority of technically relevant glasses are based on more than one network former species. The combination of several network formers usually offers the possibility of finetuning physical property combinations to special technological demands, and in certain cases new physical properties arise as a result of a specific interaction of the two network former species. A well-known example for this phenomenon, called the “mixed-network former effect”, is the sodium borophosphate glass system. For example, in the ion-conducting glass system (Na2O)x[(P2O5)1-y(B2O3)y]x the successive replacement of the network former species phosphorus oxide by boron oxide at constant alkali content has a profound influence on the glass transition temperatures and ion conducting properties.1–8 To understand these, often nonlinear, compositional trends at a fundamental level, detailed structural information is sought. To this end, a number of Raman and multinuclear solid-state nuclear magnetic resonance (NMR) spectroscopic studies of borophosphate glasses have appeared in the literature.5–16 Although the ability of solid-state NMR to provide structural information in alkali borophosphate glasses is well-documented,9–16 significant progress has been achieved in recent years by the application of advanced dipolar NMR techniques combined with the benefits of high magnetic field strengths and fast magic angle spinning. This has resulted in new quantitative insights about species concentrations and interatomic connectivities,15,16 making solidstate NMR the technique of choice in addressing detailed structure/property correlations. In principle, the local environ(n) (n) ments can be described in terms of the PmB and BmP nomenclature, where n is the number of bridging oxygen atoms and m is the number of heteroatomic bridges for each network * E-mail:
[email protected].
former type. Although we were recently able to estimate such species concentrations for sodium borophosphate glasses with compositions (Na2O)0.4[(P2O5)1-x(B2O3)x]0.6 based on a detailed analysis of 31P and 11B magic-angle spinning (MAS) NMR spectra,17 the structural description of this glass system is still incomplete. Here we wish to demonstrate that important additional structural constraints can be obtained by advanced dipolar NMR methods and X-ray photoelectron spectroscopic (XPS) results. In multicomponent phosphate glasses, XPS not only serves to distinguish between bridging and nonbridging oxygen atoms18,19 but can, in favorable cases, also discriminate between different types of bridging oxygen atoms.18,20,21 In the present study we have used the combination of XPS, Raman scattering, and advanced solid-state NMR techniques to develop a comprehensive and quantitative structural model for the glass system (NaPO3)1-x(B2O3)x. Experimental Section Sample Preparation. Sodium polyphosphate (Merck 99.9%) and diboron trioxide (Merck, 99.95%) were used as starting materials. To obtain water-free reagents, B2O3 was kept at 393 K for more than 48 h in an oven. Sodium polyphosphate was heated to 800 °C and was quenched on a copper plate to obtain glassy NaPO3. The weighed chemicals (NaPO3 and B2O3) were ground and mixed thoroughly in their desired proportions. The materials were melted in an open silica glass crucible at 1073 to 1173 K for about 1 h. The homogeneous melts were quenched on a copper plate to obtain the glass. Because of the hygroscopic character of the borophosphate glasses, all samples were kept in plastic containers with desiccant. Glass transition temperatures (Tg) were determined as “onset points” from differential scanning calorimetry (DSC), using a NETZSCH DSC-204 instrument, operated with a linear heating ramp of 10 K/min. Raman scatting experiments were conducted on a LabRAM HR
10.1021/jp8035549 CCC: $40.75 2008 American Chemical Society Published on Web 07/23/2008
The Mixed-network Former Effect in Phosphate Glass 800 high-resolution spectrometer, using the 532.18 nm line of a Nd/YAG laser operating at 8.6 mW as the excitation source. The spectra were collected at room temperature for 420 s with a spectral resolution of 1 cm-1. XPS. X-ray photoelectron spectra were measured using an AXIS-ULTRA spectrometer (KRATOS, Manchester, UK) in ultrahigh vacuum ( 0.125 the concentration of B-O-P linkages lies below the largest possible number. For (4) these glasses, better agreement with a scenario of m ) 3 (B3P units) is obtained. Consistent with this conclusion are subtle changes in both the isotropic 11B chemical shift and the M2(11B{31P}) values. In principle, this conclusion can also be tested against the 31P{11B} REDOR data, as the number of boron-oxygen-phosphorus linkages inferred from both double resonance experiments must be identical. For example, in the x ) 0.2 sample, a network comprised of 120 network former species consists of 80 phosphorus and 40 boron atoms. If the latter makes, on average, three heterolinkages, then this will produce a total of 120 B-O-P linkages, to be distributed over 80 P atoms. Thus, on average 1.5 P-O-B linkages are expected per phosphorus atom. Considering that each P-O-B link contributes about (15-20) × 106 rad2/s2 to M2(31P{11B}) in silver borophosphate glasses,16 the experimental M2 value of 25.4 × 106 rad2/s2 is in good agreement with this model, whereas it is too low in comparison to the predicted value of (30-40) × 106 rad2/s2 (P-O-B linkages per phosphorus atom) if the number of B-O-P linkages is maximized. For the x ) 0.1 sample, 80 B-O-P linkages are distributed over 90 P atoms if m ) 4. In this case, the experimental M2(31P{11B}) value of 12.6 × 106 rad2/s2 is a little lower than expected from this scenario, suggesting that m might be somewhat lower than 4, in contrast to the XPS result. More insight into this apparent contradiction can be obtained by a closer examination of the REDOR data of Figure 10, as described below. Connectivity Clustering. Figure 10 reveals that the parabolic fit made for the x ) 0.1 sample within the usual region ∆S/S0 e 0.2 is particularly poor in this case. Deviations of this kind can be observed if wide distributions of dipole-dipole couplings
12536 J. Phys. Chem. C, Vol. 112, No. 32, 2008 TABLE 4:
11B
Raskar et al.
and Dipolar NMR Parameters for Glasses in the System (NaPO3)1-x(B2O3)x
x
δ (11B) [ppm] ((0.5)
CQ(11B) ((0.05 MHz)
0.000 0.025 0.050 0.075 0.100 0.125 0.150 0.175 0.200 0.300
a
-2.95 -2.91 -2.93 -2.91 -2.87 -2.85 -2.76 -2.73 -2.40
435 a
a
a
465
M2(31P {11B}) [106 rad2 s-2] ((10%)
M2(11B{31P}) [106 rad2 s-2] ((10%)
a
a
a
a
a
a
a
a
a
f1
a
0.43
a
a
12.6
18.4
a
a
a
a
a
a
a
a
a
a
a
a
25.4
15.2 13.5
550 600
0.49 a
a
Not determined.
Figure 8. 31P MAS NMR spectra of (NaPO3)1-x(B2O3)x glasses. Spinning sidebands are indicated by asterisks.
Figure 9. Deconvolution of the 31P MAS NMR spectrum of the glass with x ) 0.1.
Figure 7. 11B{31P} REDOR data measured on representative (NaPO3)1-x(B2O3)x glasses.
are present, and Figure 10 suggests this to be the case for the x ) 0.10 sample. As a matter of fact, the REDOR curve of this sample levels off near ∆S/S0 ) 0.3, rather than near 0.6 as expected for a 31P-11B two-spin system with the 11B quadru-
polar coupling constant of 460 kHz (see Figure 11). If (according to XPS) each of the borate units participates in 4 P-O-B linkages, then the average number of B-O-P linkages per phosphorus atom equals 80/90 ) 0.89 as mentioned above. Various scenarios A-C regarding the distribution of P-O-B linkages can be considered, ranging from maximum dispersion to extreme clustering of P-O-B linkages. All of these scenarios take into account our MAS NMR result that no P(2) 2B units are present in these glasses. In the case of maximized dispersion
The Mixed-network Former Effect in Phosphate Glass
J. Phys. Chem. C, Vol. 112, No. 32, 2008 12537
TABLE 5: 31P Spectral Deconvolutions of (NaPO3)1-x(B2O3)x Glasses x
δ (ppm)
G/L
fwhm (ppm)
%
0 0.025
-19.2 -11.0 -19.6 -11.2 -19.1 -29.8 -10.8 -18.7 -28.5 -10.8 -18.6 -28.4 -10.6 -18.2 -28.1 -10.6 -18.2 -27.9 -10.2 -17.8 -27.9 -10.5 -18.0 -26.0 -10.4 -17.8 -24.8
0.7 0.8 0.7 1.0 0.8 0.0 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.7 0.8 0.65 0.7 0.7 0.5 1.0 0.8 0.8 0.7 0.7 0.75 0.65 1.0
13.5 8.7 8.3 8.7 9.0 9.0 8.7 8.3 9.0 8.7 10.2 9.0 8.7 11.5 9.0 8.7 11.9 9.0 8.7 12.4 9.0 9.0 12.1 12.0 9.6 11.5 12.0
100 12.3 87.7 21.8 75.1 3.1 26.2 70.6 3.2 31.9 64.9 3.2 34.2 63.4 2.4 35.6 61.2 3.2 37.6 58.4 4.1 38.9 55.5 5.6 42.6 48.8 8.6
0.050 0.075 0.1 0.125 0.15 0.175 0.2 0.3
structural groups (2) P0B (2) P1B (2) P0B (2) P1B (2) P0B (4) P4B (2) P1B (2) P0B (4) P4B (2) P1B (2) P0B (4) P4B (2) P1B (2) P0B (4) P4B (2) P1B (2) P0B (4) P4B (2) P1B (2) P0B (3) P1B (2) P1B (2) P0B (4) P4B (2) P1B (2) P0B (4) P4B
(3) + P2B (3) + P2B
(3) + P2B
(3) + P2B
Figure 11. REDOR curve of a simulated 11B-31P two-spin system, assuming a dipolar coupling constant of 1 kHz and the quadrupolar coupling constants of 465 and 500 kHz as measured in the x ) 0.10 and x ) 0.20 glasses.
(3) + P2B
(3) + P2B
(3) + P2B
(3) + P2B
(3) + P2B
(Scenario A) 80 P atoms (89%) would participate in one heteroatomic linkage and 10 (11%) in none. This model corresponds to the following description. (4) (3) (2) (2) 90 P(2) + 20 B(3) f 20 B4P + 20 P1B + 60 P1B + 10 P0B
(3) 31P-11B
In this case a rather uniform dipolar coupling would be expected for the large majority of the 31P nuclei. An alternative scenario (B) assumes that each P(3) unit is involved in two P-O-B linkages, resulting in some clustering. This model corresponds to the following description. (4) (3) (2) (2) 90 P(2) + 20 B(3) f 20 B4P + 20 P2B + 40 P1B + 30 P0B
(4) For this scenario we expect a wide distribution of 31P-11B dipole-dipole coupling strengths, as there are comparable numbers of P atoms interacting with two, one, and zero boron
Figure 12. Quantitative distribution of P-O-P, P-O-B, and B-O-B linkages in the (NaPO3)1-x(B2O3)x glass system: (a) comparison with the prediction with a random linkage scenario; (b) comparison with the scenario {1} maximizing the number of B-O-P linkages.
units, respectively. Finally, an extreme scenario (D), corresponding to even stronger connectivity clustering, can be described by the following equation. (4) (3) (2) (2) 90 P(2) + 20 B(3) f 20 B4P + 20 P3B + 20 P1B + 50 P0B
(5)
Figure 10. 31P{11B} REDOR results obtained on two representative (NaPO3)1-x(B2O3)x glasses. The parabolae show fits to the data within range ∆S/S0 < 0.2.
In this model the P(3) units attract the maximum number of P-O-B linkages. The dipolar coupling distribution in this scenario would be even wider, with 22% of the P atoms each interacting with 3 or 1 boron species, whereas 55% would be unconnected to boron. The REDOR curves for all three of these scenarios were calculated using the SIMPSON code. Scenario
12538 J. Phys. Chem. C, Vol. 112, No. 32, 2008
Raskar et al.
Figure 13. 31P{11B} REDOR data on (NaPO3)1-x(B2O3)x glass (x ) 0.1). Data are compared with simulated curves for different clustering scenarios (see text).
0.804 ) 0.178. (The scaling factors used in the above calculations account for the probabilities of the corresponding isotopomers.) Calculations are based on a 11B-31P internuclear distance of 270 pm, which corresponds to an average value found over a range of different crystalline borophosphates and on angles of 109.5° between the dipolar vectors constituting the three- and four-spin systems. Figure 13 illustrates that these scenarios can be easily differentiated by REDOR. The experimental data clearly discriminate against scenario A, which incidentally is also ruled out by the 31P MAS NMR spectra, as no P(3) 1B units are detected. Furthermore, the opposite deviation of the data from scenario D suggests that the number of P(3) 3B units is probably not large. To improve the agreement with the experimental data, we (4) may assume that part of the boron species are of the B3P rather (4) than the B4P type. If the average number of P atoms linked to boron is not four but is denoted by the parameter m (3 e m e 4), then the connectivity speciation for a glass with x ) 0.1 would be given by (4) (3) (2) 90 P(2) + 20 B(3) f 20 BmP + 20 P2B + 20(m - 2) P1B + (2) (6) (110 - 20m) P0B
Figure 14. 31P{11B} REDOR data on (NaPO3)1-x(B2O3)x glass (x ) 0.2). Data are compared with simulated curves with clustered B-O-P (4) (4) linkages, assuming the formation of B3P (blue curve) and B4P units (red curve). (n) TABLE 6: Quantitative PmB Speciations of (NaPO3)1-x(B2O3)x Glasses (in % of the P Inventory)
x (2) P0B (2) P1B (2) P2B (3) P0B (3) P1B (3) P2B (3) P3B (4) P4B
m
0.000 0.025 0.050 0.075 0.100 0.125 0.150 0.175 0.200 100
84.6 10.2
74.7 14.7
57.8 25.9
46.4 31.3
36.2 35.2
28.7 36.0
19.1 38.4
11.1 38.9
5.1
7.3
13.0
19.0
26.2
32.1
38.3
44.3
4.0
3.1 4.0
3.2 4.0
3.2 3.7
2.4 3.4
3.2 3.2
4.1 3.1
5.6 3.0
A corresponds to Figure 11, scaled by the factor 0.89 × 0.804, where the latter factor accounts for the natural isotopic abundance of the 11B nucleus. The calculation for scenario B is a superposition of the REDOR curves calculated for a 31P(11B)2 three-spin system, scaled by the factor 0.222 × 0.8042 ) 0.144; the REDOR curve calculated for a 11B-31P two spin system (31P(11B10B) isotopomer), scaled by the factor 0.222 × 0.804 × 0.196 × 2 ) 0.070; and the REDOR curve calculated for a 11B-31P two spin system, scaled by the factor 0.444 × 0.804 ) 0.357. Finally, scenario (D) produces a superposition of the REDOR curve calculated for a 31P(11B)3 four-spin system, scaled by the factor 0.222 × 0.8043 ) 0.115; the REDOR curve calculated for a 11B-31P-11B three-spin system (31P(11B210B) isotopomer), scaled by the factor 0.222 × 0.8042 × 0.196 × 3 ) 0.084; the REDOR curve calculated for a 11B-31P two spin system (31P(11B10B2) isotopomer), scaled by the factor 0.222 × 0.804 × 0.1962 × 3 ) 0.020; and the REDOR curve calculated for a 11B-31P two spin system, scaled by the factor 0.222 ×
The case m ) 4 corresponds to scenario B, whereas for m < (2) 4, the fraction of P(2) 1B units is decreased and the fraction of P0B units is increased (scenario C). Figure 13 shows that a model with m ) 3.5 provides, of all scenarios considered, the best (3) (2) match to the data. Also, the predicted (P(2) 0B + P2B)/P1B signal area ratio is consistent with the experimental result in this case (see Figure 9). Even stronger evidence for m < 4 is found at higher boron contents (x > 0.125). In this concentration region the XPS and the NMR data signify that the concentration of P-O-B linkages (4) is no longer maximized, and the formation of B3P units is more likely. This result is also clearly evident from Figure 14, which compares the expected and experimental REDOR data for the sample with x ) 0.2 assuming either four or three B-O-P linkages for boron (scenarios B and C, respectively). Consistent with the conclusions from XPS, the experimental REDOR data confirm that a description according to eq 7 provides a very good match to the experimental data. (4) (3) (2) 80 P(2) + 40 B(3) f 40 B3P + 40 P2B + 40 P1B
(7)
Site Connectivity Speciations. On the basis of reaction model 1 and combining the insights from all of the above NMR and XPS experiments, we can derive the complete site connectivity speciations of all the glasses within the concentration region x (3)(3), and P(3) units e 0.2. If we discard the presence of P(2) 2B,1BP0B 3B (consistent with experimental results) and assume that the small peak near -30 ppm arises from P(4) 4B units from BPO4 domains, then the number of P(3) 2B units follows with eq 1 directly from the boron content; (3) (4) (4) n(P2B ) ) n(BmP ) - n(P4B )
(8)
where n ) 2x. The quantity is available from the 31P MAS NMR line shape analysis. Conservation of the total number of B-O-P linkages (from the B and the P points of view) adds the expression given in eq 9. (4) (BmP )
n(P(4) 4B)
(4) (3) (4) (2) n(BmP ) × m ) n(P2B ) × 2 + n(P4B ) × 4 + n(P1B )×1
(9)
The average number m of B-O-P linkages per boron atom is now adjusted such that the quantity n (P(2) 1B) is consistent with the number derived from the deconvolution of the 31P MAS
The Mixed-network Former Effect in Phosphate Glass
J. Phys. Chem. C, Vol. 112, No. 32, 2008 12539
Figure 15. Schematic glass structure model for NaPO3-B2O3 glasses.
NMR spectrum (as calculated from the fractional area of the -11 ppm resonance), see Table 5. Finally, the number of P(2) 0B units follows from the expression: (2) (2) n(P0B ) ) 1 - 3x - n(P1B )
(10)
(n) PmB
Table 6 summarizes all the species concentrations (in percent of the P inventory). We note that the values show smooth dependences on composition, and m decreases monotonically from 4.0 to 3.0 over the compositional range studied. These m values are generally found in excellent agreement with the conclusions from the REDOR data, which provide an (3) independent check for this analysis. The absence of P(2) 2B, P0B, (3) (3) P1B, and P3B units implies that the connectivity distribution is (3) (2) highly nonstatistical and suggests that the P2B and P1B units identified here have special stability in this glass system. Conclusions The combination of advanced NMR techniques and X-ray photoelectron spectroscopy has proven to be a powerful approach for characterizing the mixed network former effect in sodium borophosphate glasses within the compositional domain (NaPO3)1-x(B2O3)x (0.0 e x e 0.3). Even though the 11B and 31P MAS NMR spectra are only poorly resolved, quantitative (4) (2) (3) BmP , PmB , and PmB site speciations can be given for all glass compositions. The structural model is schematically summarized in Figure 15. Up to levels of x e 0.2, all the boron atoms (2x) (4) are found in the anionic BmP state (m ≈ 3-4), implying that an equal number (2x) of neutral P(3) units must be present in addition to 1-3x P(2) units. The resulting linear increase in the concentration of bridging oxygen species is in excellent agreement with the concomitant increase in Tg. XPS and NMR data indicate that there is a clear preference for the formation of heteroatomic P-O-B over homoatomic P-O-P and B-O-B linkages. We note that this situation is markedly different from the NaPO3-TeO2 system previously examined by us, where the connectivity distribution is close to statistical and Tg depends only weakly on composition.18 In the present system, the number m of B-O-P linkages per boron is close to the maximum number possible, especially at low B2O3 contents. Above x ) 0.125, m decreases gradually from 4 to 3. Finally, a closer inspection of the REDOR data reveals that the P-O-B linkages have a tendency to cluster, with each P(3) units attracting two boron next-nearest neighbors, to become P(3) 2B units. Most likely, the energetically favorable interaction of underbonded BO4-units with overbonded P(3) groups (within the framework of bond valence theory) is the main driving force of these processes. Acknowledgment. Funding by the SFB 458, administered by the Deutsche Forschungsgemeinschaft, is most gratefully
acknowledged. We thank Professor Harald Zu¨chner for making the XPS equipment in his laboratory available, Dr. Sebastian Wegner for his help with some of the SIMPSON simulations, and Wilma Pro¨bsting for the DSC measurements. References and Notes (1) Branda, F.; Constantini, A.; Fresa, R.; Buri, A. Phys. Chem. Glasses 1995, 36, 272. (2) Constantini, A.; Buri, A.; Branda, F. Solid State Ionics 1994, 67, 175. (3) Takebe, H.; Harada, T.; Kuwatara, M. J. Non-Cryst. Solids 2006, 352, 709. (4) Ahoussou, A. P.; Rogez, J.; Kone, A. Thermochim. Acta 2006, 441, 96. (5) Videau, J. J.; Ducel, J. F.; Suh, K. S.; Senegas, J. J. Alloys Comp. 1992, 188, 157. (6) Ducel, J. F.; Videau, J. J. Mater Lett. 1992, 13, 271. (7) Kumar, S.; Vinatier, P.; Levasseur, A.; Rao, K. J. J. Solid State Chem. 2004, 177, 1723. (8) Anantha, P. S.; Hariharan, K. Mater. Chem. Phys. 2005, 89, 428. (9) Beekenkamp, P.; Hardemann, G. E. G. Verres Refract. 1966, 20, 419. (10) Yun, Y. H.; Bray, P. J. J. Non-Cryst. Solids 1978, 30, 45. (11) Feng, T.; Linzhang, P. J. Non-Cryst. Solids 1989, 112, 142. (12) Villa, M.; Scagliotti, M.; Chiodelli, G. J. Non-Cryst. Solids 1987, 94, 101. (13) Koudelka, L.; Mosner, P.; Zeyer, M.; Ja¨ger, C. Phys. Chem. Glasses 2002, 43C, 102. (14) Koudelka, L.; Mosner, P.; Zeyer, M.; Ja¨ger, C., J. Non-Cryst. Solids 2003. (15) Zeyer-Du¨sterer, M.; Montagne, L.; Palavit, G.; Ja¨ger, C. Solid State Nucl. Magn. Reson. 2005, 27, 50. (a) Zeyer, M.; Montagne, L.; Ja¨ger, C. Glass Sci. Technol. 2002, 75, 186. (16) Elbers, S.; Strojek, W.; Koudelka, L.; Eckert, H. Solid State Nucl. Magn. Reson. 2005, 27, 65. (17) Zielniok, D.; Cramer, C.; Eckert, H. Chem. Mater. 2007, 19, 3162. (18) Rinke, M.; Zhang, L.; Eckert, H. Chem. Phys. Chem. 2007, 8, 1988. (19) Gresch, R.; Mu¨ller-Warmuth, W.; Dutz, H. J. Non-Cryst. Solids 1979, 34, 127. (20) Brow, R. J. Non-Cryst. Solids 1996, 194, 267. (21) Ducel, J. F.; Videau, J. J.; Gonbeau, D.; Pfilster-Guillouzo, G. Phys. Chem. Glasses 1995, 36, 247. (22) Massiot, D.; Fayon, F.; Capron, M.; King, I.; Le Calve, S.; Alonso, B.; Durand, J. O.; Bujoli, B.; Gan, Z.; Hoatson, G. Magn. Reson. Chem. 2002, 40, 70. (23) Gullion, T.; Schaefer, J. J. Magn. Reson. 1989, 81, 196. (24) Gullion, T. Magn. Reson. ReV. 1997, 17, 83. (25) Chan, J. C. C.; Eckert, H. J. Magn. Reson. 2000, 147, 170. (26) Garbow, J. R.; Gullion, T. J. Magn. Reson. 1991, 95, 442. (27) Bak, M.; Rasmussen, J. T.; Nielsen, N. C. J. Magn. Reson. 2000, 147, 296. (28) Strojek, W.; Kalwei, M.; Eckert, H. J. Phys. Chem. 2004, 108, 7061. (29) Hudgens, J. J.; Brow, R. K.; Tallant, D. R.; Martin, S. W. J. NonCryst. Solids 1998, 223, 21. (30) Yifen, J.; Xiangsheng, C.; Xihuai, H. J. Non-Cryst. Solids 1989, 112, 147. (31) Ducel, J. F.; Videau, J. J.; Couzi, M. Phys. Chem. Glasses 1993, 34, 212.
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