Lattice Dynamics of Imperfect Alkali Halides

Abstract

In the framework of the T‐matrix formalism the generalized phonon dispersion relations for a crystal containing a finite concentration of randomly distributed defects are introduced. The Green functions and the resonance conditions are given for the nearest‐neighbor perturbation model of defects in NaCl‐type lattices. Systematic calculations concerning monovalent positive and negative defects in Na, K, and Rb chlorides, bromides, and iodides are then performed in the framework of Hardy's deformation dipole (dd) model for the host lattice dynamics, and applications to the $U$ center are given. In the same framework the nearest‐neighbor effective force constant is defined as a function of the interionic distance and calculated for all the family of alkali halides. The results for several defects in NaCl and KCl are compared and found to be in good agreement with thermal conductivity data. Predictions concerning the offcenter systems are presented. Finally, theoretical acoustic dispersion curves, phonon relaxation rates, and infrared absorption spectra for copper‐ and silver‐doped NaCl crystals are also reported.