LOCALIZED MODES ASSOCIATED WITH DEFECTS IN IONIC-CRYSTALS

Computational techniques for the calculation of frequencies of gap and local modes from interionic potentials are reviewed and compared. In a preliminary section the two types of approach, direct calculation and Green's function techniques, are applied to diatomic linear chains. The required Green's functions are obtained in analytic form by extending a recent straightforward derivation. Results of applying these methods to alkali halides and hydrides are then discussed. Critical comparisons of calculations by the different methods provide some clear answers to some methodological questions such as (a) the extent of crystal regions which have to be considered defective and (b) the consequences of ignoring frequency-dependent polarization changes introduced by impurities. For both diatomic chains and three-dimensional crystals particular emphasis is placed on the conditions required for the multiple occurrence of modes. © 1990.