1464
J. Phys. Chem. B 2000, 104, 1464-1472
Investigation of Sodium Distribution in Phosphate Glasses Using Spin-Echo
23Na
NMR†
Todd M. Alam,*,‡ Jay McLaughlin,§,#,¶ Carol C. Click,|,# Sam Conzone,|,# Richard K. Brow,| Timothy J. Boyle,⊥ and J. W. Zwanziger§ Sandia National Laboratories, Department of Material Aging and Reliability, Albuquerque, New Mexico 87185-1407, Indiana UniVersity, Department of Chemistry, Bloomington, Indiana 47405, UniVersity of MissourisRolla, Ceramic Engineering Department, Rolla, Missouri 65409-0330, and Sandia National Laboratories, Materials Processing Department, AdVanced Materials Laboratory, 1001 UniVersity BouleVard SE., Albuquerque, New Mexico 87106-1349 ReceiVed: September 7, 1999; In Final Form: December 28, 1999
The spatial arrangements of sodium cations for a series of sodium phosphate glasses, xNa2O‚(100-x)P2O5 (x e 55), were investigated using 23Na spin-echo NMR spectroscopy. The spin-echo decay rate is a function of the Na-Na homonuclear dipolar coupling, and is related to the spatial proximity of neighboring Na nuclei. The spin-echo decay rate in these sodium phosphate glasses increases nonlinearly with higher sodium number density, and thus provides a measure of the Na-Na extended range order. The results of these 23Na NMR experiments are discussed within the context of several structural models, including a decimated crystal lattice model, cubic dilation lattice model, a hard sphere (HS) random distribution model, and a pairwise cluster hard sphere model. While the experimental 23Na spin-echo M2 are described adequately by both the decimated lattice and the random HS models, it is demonstrated that the slight nonlinear behavior of M2 as a function of sodium number density is more correctly described by the random distribution in the HS model. At low sodium number densities the experimental M2 is inconsistent with models incorporating Na-Na clustering. The ability to distinguish between Na-Na clusters and nonclustered distributions becomes more difficult at higher sodium concentrations.
Introduction The impact of different atomic constituents on the structure of glasses, and the resulting physical properties, continues to be an area of great interest. For simple oxide glasses the current structural paradigm suggests that these constituents are either network formers (i.e., P2O5, SiO2, and B2O3), network modifiers (i.e. Li2O, Na2O, CaO), or intermediates (i.e., Al2O3).1,2 The network formers produce well-defined coordination polyhedra that are linked into a three-dimensional network through bridging oxygens, while the network modifiers depolymerize this network through the formation of nonbridging oxygens. The intermediate components generally do not form glasses individually, but have network-like structures when combined with other network formers. The large formal charge of +5 on phosphorus versus the +3 and +4 charges of boron and silicon results in distinct physical properties and structural differences for these different glass systems. For example, an anomalous minimum in the glass transition temperature (Tg) and density (see Figure 1) is observed in alkali phosphate glasses,3,4 whereas no corresponding be* Author to whom correspondence should be addressed. † Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the U.S. Department of Energy under Contract DE-AC04-94AL85000. ‡ Sandia National Laboratories, Department of Material Aging and Reliability. § Indiana University. | University of MissourisRolla. ⊥ Sandia National Laboratories, Materials Processing Department. # Work partially performed as a participant of the student intern program at Sandia National Laboratories. ¶ Present address: NIST, 100 Bureau Driver, MS 8562, Gaithersburg, MD 20899-8562.
Figure 1. Variation of the glass density (b) and sodium number density, FNa (O) with Na2O mol fraction in the xNa2O‚(100 - x)P2O5 glass series. A minimum in the density was observed near x ∼ 2025%, while FNa shows a small variation in slope between 20 and 25% Na2O.
havior exists for the alkali silicate systems. In phosphate glasses this anomalous Tg and density behavior is attributed to the balance between the loss of the fully polymerized phosphate tetrahedral network with the initial addition of network modifiers (depolymerization) against the restructuring of the glass network due to the formation of oxygen-alkali bridges at higher modifier concentrations.3-5 The changes in the glass structure due to the addition of network modifiers have been investigated using a
10.1021/jp9931509 CCC: $19.00 © 2000 American Chemical Society Published on Web 02/02/2000
Sodium Distribution in Phosphate Glasses wide variety of techniques, including nuclear magnetic resonance (NMR),2,6-10,11 Raman,4,12 and infrared (IR) spectroscopies,3,12 X-ray and neutron diffraction,5,13-16 and molecular dynamic (MD) simulations.17,18 Unfortunately, the impact of cation distribution on the restructuring of the glass and the presence of extended range cation order has not been completely addressed for alkali phosphate glass systems. The role of cation distributions has been pursued in silicon and boron oxide glasses, where solid-state NMR investigations, including spin-echo, spin-echo double resonance (SEDOR), and rotational-echo double resonance (REDOR) techniques, have demonstrated that a wealth of information on modifier distribution can be obtained from these experiments. Spin-echo NMR experiments have been used to probe the Na distribution in tellurite glasses,19 along with the Na and Cs distributions in borate glasses.20 The distribution of Al in sodium aluminoborate glasses has also been investigated using 11B-27Al REDOR experiments.21 The structural impact of alkali distributions on the mixed alkali effect (MAE) in silicate glasses is perhaps the most extensively investigated area of cation distribution using NMR. In sodium silicate glasses, 23Na spin-echo decay spectroscopy has been used to show that at low alkali content an inhomogeneous cation distribution is present,22 which is consistent with molecular dynamic simulations of alkali silicate glasses.23,24 For mixed alkali silicate glasses, 7Li-23Na SEDOR and 6Li-23Na SEDOR experiments,22,25-27 along with 29Si23Na and 29Si-7Li REDOR investigations have also been used to determine cation distribution.22,28 From these NMR experiments, Eckert and co-workers22,26-28 showed that no preferential interactions between unlike cations or preferential cation pairing is observed in Na-Li disilicate glasses. While there have been numerous NMR investigations of cation distribution in silicate and borosilicate glasses, no comparable study of alkali phosphate glasses has been reported. In fact, while NMR has played a very important role in the investigation of the structure and dynamics of the phosphate backbone,6-10,29-32 details about the local environment of the network modifiers has been more limited. A 23Na magic angle spinning (MAS) NMR investigation of the Na chemical shift in Li-Na metaphosphate glasses revealed that the Na and Li cations are uniformly mixed.29 Recent 6Li MAS NMR investigations of binary lithium ultraphosphate glasses demonstrate that the cation environment does not undergo any abrupt changes as a function of Li2O concentration.33 Wenslow and Mueller have used dynamic-angle-spinning (DAS) NMR experiments on mixed alkali glasses to demonstrate that there are multiple cation environments present.34 Recently 23Na-31P crosspolarization (CP) MAS NMR experiments have demonstrated that the Na cation is associated with the fully polymerized phosphate tetrahedra in ultraphosphate glasses, presumably via coordination through terminal PdO bonds.35 The question of cation clustering and segregation, or extended long-range order within phosphate glasses remains unanswered. In the present contribution we report the first 23Na spinecho decay NMR investigation to probe directly the Na-Na distribution for a series of sodium phosphate glasses. From these studies the resulting second moment (M2), as a function of Na2O modifier concentration, are related to the Na-Na distribution. These results for phosphate glasses are discussed in terms of various cation distribution models, including random cation distributions as well as mutual cation clustering. Fundamental Concepts and Methodology The dephasing that occurs during a simple Hahn spin-echo pulse sequence, 90°-τ-180°-τ- acquire, can be used to access
J. Phys. Chem. B, Vol. 104, No. 7, 2000 1465 the strength of the homonuclear dipolar coupling between nuclei.36 In spin-1/2 nuclei such as 31P, 15N and 13C, the use of dipolar dephasing experiments has become an important tool to probe distances in solids. For quadrupolar nuclei, (spin I > 1/2) structural information obtained from dipolar dephasing has seen limited development due to complications from first- and second-order quadrupolar perturbations and multiple transitions. Haase and Oldfield have shown that in the limit H(1) Q . Hrf . HD ≈ H(2) Q , (where Hrf is the interaction with the radio (2) frequency pulse, HD the dipolar interactions, and H(1) Q and HQ represent the first- and second-order quadrupolar interactions), the echo decay is Gaussian and allows the measurement of the average homonuclear dipolar second moment M2:37
I(2τ) ) exp[-M2(2τ)2/2] I(0)
(1)
In eq 1, I(2τ) represents the echo amplitude for the dephasing period τ, and I(0) the echo amplitude for no dephasing. Following the formulation of van Vleck,38 the spin-echo second moment is given by
M2 )
9 4
E(n) L
4
2
γ p
(1 - 3cos2 θjk)2
∑ j 0.008 Å-3, does not allow these different distribution types to be distinguished. Therefore changes in the Na-Na interaction as a function of modifier concentration at high Na2O concentrations cannot be addressed in the present investigation. Conclusions These experiments demonstrate that 23Na NMR spin-echo M2 experiment can be used to investigate cation distribution in xNa2O‚(100 - x)P2O5 (10 e x e 55) glasses. The gradual
1472 J. Phys. Chem. B, Vol. 104, No. 7, 2000 nonlinear increase in M2 as a function of Na number density illustrates that major variations in the Na-Na distribution do not directly influence the thermodynamics of the glass and do not correlate with the anomalous behavior of the glass transition temperature. A variety of model distributions were evaluated and compared to the experimental M2 values. The 23Na M2 results in these phosphate glasses are consistent with a Na-Na distribution that is disordered in comparison to the distribution in crystalline systems, and can be described by a random HS model. This HS model predicts an average closest Na-Na approach distance of 3.47 Å, and a Na-Na pair correlation function that is very similar to the Li-Li pair correlation function recently reported in MD simulations.17 A pairwise NaNa HS cluster model was found to predict M2 values that deviated substantially from those observed experimentally, especially at low FNa values. In conclusion, the spin-echo results for these Na phosphate glasses are consistent with a homogeneous random distribution of Na atoms, with no evidence for Na-Na clustering. These 23Na M2 results provide an initial basis for future evaluations and comparisons of Na distribution in phosphate glasses, including Na distributions in mixed alkali systems, which are presently being pursued. Acknowledgment. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the U.S. Department of Energy under Contract DE-AC0494AL85000. References and Notes (1) Paul, A. Chemistry of Glasses; Chapman and Hall: New York, 1990. (2) Zwanziger, J. W. Int. ReV. Phys. Chem. 1998, 17, 65. (3) Hudgens, J. J.; Martin, S. W. J. Am. Ceram. Soc. 1993, 76, 1691. (4) Hudgens, J. J.; Brow, R. K.; Tallant, D. R.; Martin, S. W. J. NonCryst. Solids 1998, 223, 21. (5) Hoppe, U. J. Non-Cryst. Solids 1996, 195, 138. (6) Brow, R. K.; Kirpatrick, R. J.; Turner, G. L. J. Non-Cryst. Solids 1990, 116, 39. (7) Brow, R. K.; Phifer, C. C.; Turner, G. L.; Kirkpatrick, R. J. J. Am. Ceram. Soc. 1991, 74, 1287. (8) Brow, R. K.; Kirkpatrick, R. J.; Turner, G. L. J. Am. Ceram. Soc. 1993, 76, 919. (9) Brow, R. K.; Tallant, D. R.; Hudgens, J. J.; Martin, S. W.; Irwin, A. D. J. Non-Cryst. Solids 1994, 177, 221.
Alam et al. (10) Kirkpatrick, R. J.; Brow, R. K. Solid State Nucl. Magn. Reson. 1995, 5, 9. (11) Eckert, H. Prog. NMR Spectrosc. 1992, 24, 159. (12) Meyer, K. J. Non-Cryst. Solids 1997, 209, 227. (13) Hoppe, U.; Walter, G.; Stachel, D.; Barz, A.; Hannon, A. C. Z. Naturforsch. 1997, 52a, 259. (14) Hoppe, U.; Walter, G.; Kranold, R.; Stachel, D.; Barz, A. J. NonCryst. Solids 1995, 192 & 193, 28. (15) Kranold, R.; Walter, G.; Lembke, U.; Rieker, T.; Stachel, D. J. Non-Cryst. Solids 1998, 232-234, 502. (16) Hoppe, U.; Walter, G.; Kranold, R.; Stachel, D.; Barz, A.; Hannon, A. C. Physica B 1997, 234-236, 388. (17) Liang, J.-J.; Cygan, R. T.; Alam, T. M. J. Non-Cryst. Solids, in press. (18) Speghini, A.; Sourial, E.; Peres, T.; Pinna, G.; Bettinelli, M.; Capobianco, J. A. Phys. Chem. Chem. Phys. 1999, 1, 173. (19) Zwanziger, J. W.; McLaughlin, J. C.; Tagg, S. L. Phys. ReV. B 1997, 56, 5243. (20) Ratai, E.; Janssen, M.; Eckert, H. Solid State Ionics 1998, 105, 25. (21) Wu¨llen, L. V.; Zu¨chner, L.; Mu¨ller-Warmuth, W.; Eckert, H. Solid State Nucl. Magn. Reson. 1996, 6, 203. (22) Wu¨llen, L. V.; Gee, B.; Zu¨chner, L.; Bertmer, M.; Eckert, H. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 1539. (23) Huang, C.; Cormack, A. N. J. Chem. Phys. 1991, 95, 3634. (24) Huang, C.; Cormack, A. N. J. Chem. Phys. 1990, 93, 8180. (25) Yap, A. T.-W.; Forster, H.; Elliot, S. R. Phys. ReV. Lett. 1995, 75, 3956. (26) Gee, B.; Eckert, H. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 1610. (27) Gee, B.; Eckert, H. J. Phys. Chem. 1996, 100, 3705. (28) Gee, B.; Janssen, M.; Eckert, H. J. Non-Cryst. Solids 1997, 215, 41. (29) Sato, R. K.; Kirkpatrick, R. J.; Brow, R. K. J. Non-Cryst. Solids 1992, 143, 257. (30) Alam, T. M.; Brow, R. K. J. Non-Cryst. Solids 1998, 223, 1. (31) Ruffle´, B.; Beaufils, S.; Gallier, J. Chem. Phys. 1995, 195, 339. (32) Hartman, P.; Vogel, J.; Ja¨ger, C. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 1658. (33) Alam, T. M.; Conzone, S.; Brow, R. K.; Boyle, T. J. J. Non-Cryst. Solids 1999, 258, 140. (34) Wenslow, R. M.; Mueller, K. T. J. Non-Cryst. Solids 1998, 231, 78. (35) Prabakar, S.; Wenslow, R. M.; Mueller, K. T. J. Non-Cryst. Solids. In press. (36) Engelsberg, M.; Norberg, R. E. Phys. ReV. B 1972, 5, 3395. (37) Haase, J.; Oldfield, E. J. Magn. Reson., Ser. A 1993, 101, 30. (38) van Vleck, J. H. Phys. ReV. 1948, 74, 1168. (39) McGreevy, R. L.; Pusztai, L. Mol. Simul. 1988, 1, 359. (40) McAdam, A.; Jost, K. H.; Beagley, B. Acta Crystallogr. 1968, B24, 1621. (41) McQuarrie, D. A. Statistical Mechanics; Harper & Row: New York, 1976.
