Infrared Absorption in Anharmonic Crystals with Impurities: An Application to a Diatomic Linear KI‐Chain

In expansion of well-known results of dynamic and optical properties of anharmonic ideal crystals, a perturbation theory, using Matsubara functions and diagrammatic treatment is carried out to describe anharmonic crystals with impurities. Taking explicity into account anharmonic contributions of the potential expansion up to fourth order, mass changes as well as force constant changes between impurity and host-lattice particles, it is shown that the anharmonic phonon propagator can be written as a sum of terms, which are added up within a Dyson equation. The corresponding proper self-energy includes impurity-activated harmonic as well as impurity-influenced anharmonic contributions. As an application the infrared absorption of a diatomic linear KI-chain with next-neighbour interaction, a single mass defect, and consideration of anharmonicities up to second order of the van Hove parameter (O(α2)) is chosen. Numerical calculations of temperature-dependent lineshapes and shifts of defect and main absorption lines are presented and discussed together with results of defect-induced in-band absorption.