Infrared Absorption due to Substitutional Impurity in Cubic Crystals

The explicit expressions have been obtained for the absorption line-shape function due to one-phonon processes in some models of cubic crystals with impurities. All of them show pronounced resonance peaks in the low-frequency region for heavy impurities. Similar resonances also occur for light impurities when the force constants are reduced. An estimate is made of the change in force constant in the case of ${\mathrm{Li}}^6$ and ${\mathrm{Li}}^7$ in KBr, based on the experimental data of Sievers and Takeno.