Influence of Additional Cation Impurities on Localized Vibration Modes Due toUCenters in NaCl and KCl

The frequencies of the localized lattice-vibration modes due to $U$ centers in alkali halides have been computed when one of the nearest neighbors of ${\mathrm{H}}^{-}$ ion is replaced by an additional cation impurity. The results are in good agreement with the experimental values.