Mixed alkaline–earth effects in ion conducting glasses

We report ionic mobilities in silicate glasses containing sodium oxide and either one or two alkaline–earth oxides, based on dynamic mechanical thermal analysis (DMTA) and electrical conductivity spectroscopy. In mixed alkaline–earth glasses, the mobilities of alkaline–earth ions are considerably lower than in the corresponding single alkaline–earth glasses. Furthermore, the glass transition temperatures of mixed alkaline–earth glasses are lower than expected from a linear extrapolation of the transition temperatures of the corresponding single alkaline–earth glasses. In order to quantify the degree of decoupling of the mobile ions from the glassy network, we define a new decoupling ratio which makes use of both mechanical and electrical data. We, thereby, show that the effect of mixing divalent ions is qualitatively similar to the effect of mixing monovalent ions. Quantitatively, the mixed alkaline–earth effect is less pronounced than the mixed alkali effect. A possible explanation for this observation is given.