Sodium distribution in mixed alkali K-Na metaphosphate glasses

The local order and distribution of Na in the mixed alkali metaphosphate glasses KxNa1-xPO3 were analyzed, with the aim to identify segregation or a random mixture of both cation species. X-Ray photoelectron spectroscopy and several nuclear magnetic resonance (NMR) techniques were applied, including P-31 and Na-23 high-resolution spectroscopy, Na-23 triple quantum-MAS NMR, rotational echo double resonance between P-31 and Na-23, and Na-23 NMR spin echo decay. The structural picture emerging from these results reveals the similarity in the local Na environments in the glasses but also subtle structural adjustments with increasing degree of K replacement. While both cations are intimately mixed at the atomic scale, the Na-23 spin echo decay data suggest a detectable like-cation preference in the spatial distribution of the ions. These structural properties are consistent with those determined in Li-Rb metaphosphates, indicating that the origin of the mixed alkali effect observed in the conductivity of Na-K metaphosphate glasses may also be explained by structurally blocked ion diffusion.