| Autor: |
Epping JD; Institut für Physikalische Chemie, Westfälische Wilhelms-Universität Münster, Corrensstrasse 30, D-48149 Münster., Strojek W, Eckert H |
| Jazyk: |
angličtina |
| Zdroj: |
Physical chemistry chemical physics : PCCP [Phys Chem Chem Phys] 2005 Jun 07; Vol. 7 (11), pp. 2384-9. |
| DOI: |
10.1039/b502265g |
| Abstrakt: |
The spatial distribution of the sodium ions in sodium borate glasses with composition (Na2O)x(B2O3)1+x (0 < or = x < or = 0.30) has been studied by complementary high resolution and dipolar solid state NMR experiments. 23Na-23Na homonuclear dipole-dipole couplings measured via 23Na spin echo decay spectroscopy indicate the absence of cation clustering at all compositions. Consistent with this result, 11B{23Na} rotational echo double resonance (REDOR) measurements indicate that the trigonal BO3/2 groups and the four-coordinate BO4/2(-) units experience 23Na local dipolar fields of similar magnitudes. Both experiments suggest a sodium spatial distribution that is essentially statistical. The dipolar coupling data are modelled consistently on the basis of a cubic NaCl-type lattice, from which an appropriate number of sodium ions have been randomly removed to reproduce the Na+ number density of the glass under consideration. In addition, at very low Na+ concentrations (x < 0.12) a compositionally independent local sodium environment is formed, which is characterized by a significant Na+ BO4/2(-) pair correlation involving an internuclear distance of 316 pm. For higher sodium oxide contents (x > 0.20) the 23Na-23Na dipole-dipole couplings suggest a gradual transition from a random towards a more homogeneous sodium distribution. |
| Databáze: |
MEDLINE |
| Externí odkaz: |
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