Structure of single and mixed alkali Li–Rb borate glasses by neutron diffraction

We have compared the local structure of Li–, Rb– and Li–Rb diborate glasses using neutron diffraction and reverse Monte Carlo simulations. The fraction of tetrahedral boron decreases as the Rb content increases, but is similar in the single and mixed alkali glasses at diborate compositions. Rb borate glasses exhibit an extensive cation ordering with Rb–Rb correlations at 4.7 and 7 Å. The environment of alkalis is slightly modified between the single and mixed glasses while their relative distribution is essentially random. This indicates that the alkali environment is affected by the presence of a different alkali and this cation–cation interaction could be a fundamental mechanism, together with the site mismatch, to explain the mixed alkali effect.