Epping Jan Dirk, Strojek Wenzel, Eckert Hellmut
Institut für Physikalische Chemie, Westfälische Wilhelms-Universität Münster, Corrensstrasse 30, D-48149 Münster.
Phys Chem Chem Phys. 2005 Jun 7;7(11):2384-9. doi: 10.1039/b502265g.
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.
通过互补的高分辨率和偶极固态核磁共振实验，研究了成分（Na2O）x（B2O3）1+x（0≤x≤0.30）的硼酸钠玻璃中钠离子的空间分布。通过23Na自旋回波衰减光谱测量的23Na-23Na同核偶极-偶极耦合表明，在所有成分中均不存在阳离子聚集。与此结果一致，11B{23Na}旋转回波双共振（REDOR）测量表明，三角BO3/2基团和四配位BO4/2（-）单元经历的23Na局部偶极场大小相似。这两个实验都表明钠离子的空间分布基本上是统计性的。基于立方NaCl型晶格对偶极耦合数据进行了一致的建模，从中随机去除了适当数量的钠离子，以再现所考虑玻璃的Na+数密度。此外，在非常低的Na+浓度（x<0.12）下，形成了与成分无关的局部钠环境，其特征是存在显著的Na+BO4/2（-）对相关性，核间距为316 pm。对于更高的氧化钠含量（x>0.20），23Na-23Na偶极-偶极耦合表明从随机分布向更均匀的钠分布逐渐转变。
