Vibrational Properties and Entropy

Abstract

Point defects have an influence on the vibrational properties of solids. They can induce localized states of vibration exactly as for the electronic spectrum. An important category of such defects are substitutional impurities with a mass lighter than the host atoms. They are called mass defects and the frequency of their localized modes gives information on the coupling between the impurity and its neighbors. Experimentally, the changes in the phonon spectrum are seen, for instance, in infrared absorption. In this chapter we discuss these properties on the basis of simple models. We also apply these models in order to estimate the vibrational contribution to the entropy, which is important for the knowledge of the thermodynamic properties of defects.