Infrared defect absorption in strontium titanate

Abstract
The infrared absorption spectrum of SrTiO3 from 0.15 to 0.5 eV has been studied as a function of temperature. Structure corresponding to two types of defects has been observed. First the 164-meV impurity and its one- and two-phonon replicas are described. It is found that the impurity is a complex whose extension is of the order of one unit cell and that the local environment is crucial to the calculation of the relative intensities of the phonon replicas. Second the 435-meV hydrogen-impurity stretching mode is studied. We have found a higher energy satellite to the structures previously observed, with the same energy dependence as a function of temperature. However, the intensity of the satellite continuously increases from room temperature to low temperature. This satellite is related to the softening of the zone-center phonon mode that does not reach zero frequency in SrTiO3.