Cation environments and spatial distribution in Na2O–B2O3 glasses: New results from solid state NMR

The spatial distribution of the sodium ions in sodium borate glasses with composition (Na2O)x(B2O3)1−x (0 ≤ x ≤ 0.30) has been studied by complementary high resolution and dipolar solid state NMR experiments. 23Na–23Na homonuclear dipole–dipole couplings measured via23Na 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.