A method to calculate the infrared absorption due to a very low concentration of defects in a diatomic cubic crystal is developed directly from the basic absorption equation in quantum mechanics when the impurity produces changes in mass and short-range force constants. It is shown that the absorption is due to the modes of T1u symmetry about the defect and is proportional to the square of the projection of the amplitudes of the ions in the defect space onto the transverse-optic modes at the zone center as determined by the perturbation. A procedure to calculate the amplitudes of the ion in the defect space for a given mode is outlined. The present method gives more physical insight into the problem than most of the Green's-function formalisms used in the field.
Atomic, Molecular and Optical Physics
Jaswal SS and Wadehra JM. Theory of impurity-induced infrared absorption in cubic crystals. Phys. Rev. B. 1975;11(10);4055-60. doi: 10.1103/PhysRevB.11.4055