Impurity-Induced Infrared Absorption in a Monatomic fcc Lattice

Abstract

The infrared absorption by a low concentration of defects in a monatomic fcc lattice has been calculated. The host crystal was assumed to be composed of uncharged atoms interacting only with their nearest neighbors by means of central harmonic forces. The substitutional-defect atom differed from the host atoms in its mass, nearest-neighbor force constant, and effective charge. Calculations were made using effective-charge parameters based on a rigid, charged defect and uncharged host atoms and on an uncharged, deformable defect and deformable host atoms. Both the localized-mode frequencies and the band-mode absorption coefficient were expressed in terms of the Green's functions of Lifshitz. The real and imaginary parts of the 12 necessary Green's functions of the perfect crystal were calculated assuming rigid atoms. The vectorlike localized-mode frequencies were calculated for defects with various masses and force constants. The coefficient of absorption by band modes was calculated as a function of frequency for defects with various masses, force constants, and effective charges. The model gives fair agreement with exploratory absorption measurements on presumably applicable systems, i.e., Ar: Kr and Ar: Xe.