Impurity Modes and Effective Force Constant in CsCl

A theoretical study of the localized modes due to ${\mathrm{H}}^{-}$ and ${\mathrm{D}}^{-}$ ($U$ centers) impurity ions and of the gap mode due to the ${\mathrm{K}}^{+}$ ion in CsCl has been made. The values of the effective force constant have been determined in different frequency regions. The values are seen to be strikingly similar in magnitude in all frequency regions.