Alkali ion distribution in mixed-alkali chalcogenide glasses: Na-23-{Li-7} spin echo double resonance NMR studies of the system [(Na2S)(1-y)(Li2S)(y)](0.5)(GeS2)(0.5)

The relative arrangement of sodium and lithium ions in mixed alkali thiogermanate glasses of composition [(Na2S)(1-y)(Li2S)(y)]0.5(GeS2)(0.5) (y=0.20, 0.40, 0.50, 0.60, 0.80) has been studied by Na-23-{Li-7} spin echo double resonance (SEDOR) spectroscopy. The results indicate that lithium and sodium cations are intimately mixed and not part of microscopically or macroscopically segregated phases. In a more quantitative analysis, the experimental results are compared with various Li/Na cation distribution models. While a complete quantitative description would require independent knowledge of the overall spatial cation distribution in these glasses, the SEDOR data are found to be consistent with a distribution of the alkali ions that is close to homogeneous and not clustered. On the other hand, if clustering did occur, the SEDOR data would give proof of preferred interactions among like cations ('like cation clustering'), There is no evidence for the cation pairing models previously invoked to describe the structure of mixed alkali glasses.