“Rattling Impurities in Si and Ge Clathrates”

After an introduction to semiconductor clathrates, the results of calculations of the vibrational properties of some experimentally relevant Si- and Ge-based Type I and Type II clathrates are presented. The calculations were carried out using a first-principles, planewave, pseudopotential method. Results for the Si-based Type II materials Cs8Ga8Si128 and Rb8Ga8Si128, and for the Ge-based Type I materials Ba8Ga16Ge30 and Ba8Ga16Si5Ge25 will be discussed with an emphasis on the very low frequency rattling modes of the guest impurities.