Infrared Absorption Induced by a Charge Defect in an Ionic Crystal

An expression is derived for the impurity-induced infrared absorption in an ionic crystal, using a slightly more general model for the defect than considered previously. This model includes not only changes in mass and force constant but also changes in the effective charge tensors of the impurity and its neighbors. A $T$-matrix formulation is used, assuming a low concentration of defects. Calculations are made to explicitly demonstrate the effects of a charge-mass-force-constant defect in NaCl. In some cases the absorption is seen to increase with decreasing effective charge. For a charge-mass defect, there exists a frequency at which the dipole moment of the crystal changes sign and at which the absorption is exactly zero.