Molecular dynamics simulations of the dynamics of ions in single and mixed alkali glasses

Molecular dynamics simulations were carried out at 700 K in single alkali and mixed alkali metasilicate glasses to reproduce the dramatic mixed alkali effect occurring in the dilute foreign alkali region. Previous experimental work showed that one foreign alkali appears to immobilize a large number of host ions. This large number can now be rationalized with the help of the simulations as being due to blockage by the less mobile foreign ions of the co-operative ion dynamics that originate from ion–ion interaction and correlation. Molecular dynamics simulations of Li metasilicate glass at 500 K showed that the ubiquitous nearly constant loss (NCL) comes from the motions of caged Li ions that experience ‘dynamic anharmonicity’ caused by the motions of the caging matrix atoms, particularly the oxygen atoms. The length scales of the Li ion motions are distributed according to a Lévy distribution that has a long tail, which is responsible for the peculiar frequency dependence of the NCL.