Localized vibrational modes near substitutional impurities in alkali halide crystals: the U-center

Abstract

Abstract The localized vibrational modes due to a substitutional, monovalent negative impurity, in an alkali halide crystal, are investigated on the basis of L ifschitz ' s theory,(1) using both point-ion(2) and polarizable-ion(3) models for the lattice dynamics. Assuming that only negative ions are polarizable, the secular equation for the frequencies of localized modes is studied taking into account the change of atomic mass, of nearest neighbor closed repulsion, of ionic polarizability and of effective electronic charge in the outer shell. The influence of these variables on the position of the frequencies of localized modes, and in particular the opportunity of using either a polarizable-ion or a point-ion model, is discussed for the case in which the localized frequency lies very near to, or very far from, the maximum frequency of the vibrational spectrum. Numerical computations using the USCC 90 Remington computer are planned for the U-center (H− substituting a halogen ion) in NaCl and KCl crystals.