Infrared-active gap modes in KI:Cl

Abstract

Recently some calculations of the frequencies of gap modes have indicated that, in KI:Cl, two infrared-active modes are to be expected. These calculations were based on inter-ionic potential models and obtained normal modes for large crystallites embedded in dielectric continua. Results for the same system found by using a Green function approach with the same set of potentials show only one gap mode. It is shown that, when the Green function method is applied to KI:Cl, predictions of mode frequencies and even the number of modes in the gap depend extremely sensitively on the changes in force constants introduced by the impurity. It is also demonstrated that for a system in such a critical regime some unusual isotope effects are likely.