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Solid State NMR Studies of Cation Distributions in Glasses. Hellmut Eckert Institut für Physikalische Chemie WWU Münster. network modifier. network former. B,Si,P. O. Li-Cs. directly bonded neighbors. 0.1-0.3 nm, MAS-NMR. Coordination numbers and symmetries. Site quantification.
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Solid State NMR Studies of Cation Distributions in Glasses Hellmut Eckert Institut für Physikalische Chemie WWU Münster
network modifier network former B,Si,P O Li-Cs directly bonded neighbors 0.1-0.3 nm, MAS-NMR Coordination numbers and symmetries Site quantification Short-range order in glasses
B, Si, P O Li-Cs Spatial distribution of modifiers 0.3-0.6 nm Dipolar NMR Network former-network modifier correlation Network former connectivity Medium-range order in glasses
Cation distributions in silicate glasses: MD - Simulations Huang and Cormack, J. Chem. Phys. 1990
Cation Clustering in Silicate Glasses .....also implies nbo-clustering and strong Cation-nbo interactions O Li+ SiO- O O Si Li+ -OSi O O O O Si O- Li+ O -O Si Si Li+ O O Li+ O O Si O- O O O Li+ -O Si Si Q(3) – Li interactions: strong Q(4) – Li interactions: weak
Modulation of HD under sample rotation + I - Tr S + + Tr I-channel p pulse Magic- Angle Spinning (MAS) 7Li Rotational Echo Double Resonance (REDOR) 29Si
p p/2 2 0 1 3 4 DS DS = S0 S0 p/2 p [ Tr ] REDOR Pulse Sequence 29Si 7Li - S S0 S0 29Si 7Li depends on: strength of interaction (# neighbors, distance) dipolar evolution time N . Tr
29Si-{7Li}-REDOR Curve of Li2SiO3 Crystal structure: Voellenkle, ZEKRD 154 (1981) 77-81
Compositional dependence of M2( 7Li-29Si) In lithium silicate glasses Q(3) Q(4) M2 = K Srij-6 ~ Nj <rij>-6
Combination NMR/MD Simulations (H. Lammert, A. Heuer) MD: R(Q(3)-Li) = 320 pm NMR: M2 = 12 kHz2 NLi ~ 3
Cation clustering in lithium silicate glasses O Li+ SiO- O O Si Li+ -OSi O O O O Si O- Li+ O -O Si Si Li+ O O Li+ O O Si O- O O O Li+ -O Si Si Q(3) : 3 Li neighbors at distance 320 pm Q(4) : interaction with Li negligible
Li+ O- O Li+ SiO- O O Si Li+ -OSi O O O O Si O- Li+ O -O Si Si Li+ O O Li+ O O Si O- O O O Li+ -O Si Si Lithium-lithium interaction measured by 7Li{6Li} SEDOR d(Li-NBO) = 1.93 Å 4 Li-Li distances at 2.73 Å M26Li-7Li = 9.9 kHz2
2BO3/2 + Na2O = 2 BO4/2 Na clustering implies difference in dipolar couplings for the two boron sites 23Na-11BO3/2 23Na-11BO4/2 Approach: 11B{23Na}-REDOR BO3 BO4 Anionic compensation mechanism in borate glasses
11B{23Na} REDOR in sodium borate glass (Na2O)0.2 (B2O3)0.8 BO4- BO3
11B - 23Na dipolar M2 -values in sodium borate glasses d(Na-BO4) = 316 pm
B,Si,P O Li-Cs Network modifier-network modifier correlations Spatial cation distribution
23Na spin echo decay spectroscopy M2 = 0.9562 (µo/4p)2g4h2 Srij-6 Regime of Validity:HQ(2) ~ HD << H1 << HQ(1) (selective excitation) low Temperature (~100 K) (no dynamic contribution)
Dependence of M2(23Na-23Na) on Sodium Number Density in Sodium Borate Glasses Model: Statistical Distribution of Na ions in a truncated NaCl lattice with R(Na-B) = 316 pm
Conclusions Dipolar NMR reveals new quantitative information on cation distribution in silicate and borate glasses Lithium and sodium silicate Glasses: Li -Silicate glasses: clustering at low alkali (< 33 mole% Li2O) 3 Q(3)...Li distances at 320 pm; 4 Li...Li distances 270 pm Sodium silicate glasses: clustering at < 20 mole% Na2O Lithium and sodium borate glasses: Na+....BO4- pair correlation: 316 pm Cation distribution close to homogeneous
Acknowledgments Wenzel Strojek Stefan Puls Jan-Dirk Epping Ulrike Voigt Prof. Andreas Heuer Heiko Lammert Deutsche Forschungsgemeinschaft: SFB 458 MWF Nordrhein-Westfalen