It was recalled that KCl crystals that were doped with impurity ions such as Li+ could form atomic tunnelling centers. At very low concentrations, the Li+ ions could be considered to be isolated and their properties could be described from a macroscopic point of view. With increasing concentration, mutual interactions between the defects increasingly changed their energy spectrum and their dynamics. Various experimental results were here described in terms of 2 approximations which accounted for interacting tunnelling systems at various defect concentrations.

C.Enss: Physica B, 1996, 219-220, 239-42