Structural manifestations of the mixed-alkali effect: NMR studies of sodium rubidium borate glasses

The spatial distribution of the sodium ions in sodium rubidium borate glasses with composition [(Na2O)x(Rb2O)1−x]y(B2O3)1−y (0 ⩽ x ⩽ 1, y = 0.2 and 0.3) has been studied by complementary high resolution and dipolar solid state NMR experiments. 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, consistent with a random distribution of the overall cation population. Combined with this result, 23Na–23Na homonuclear second moments measured via 23Na spin echo decay spectroscopy reveal non-statistical mixing of sodium and rubidium ions in relation to each other, suggesting some preference of like-cation interactions. These results support the idea that the details of unlike-cation mixing are influenced by the size differences between the ions involved.