Phonon band structures and resonant scattering in Na 8 Si 46 and Cs 8 Sn 44 clathrates

The low and glasslike thermal conductivity of metal-doped semiconductor clathrate compounds makes them potentially high-efficiency thermoelectric materials. The cause of this unique and remarkable property has been postulated to be due to resonant scattering of lattice phonons by localized vibrations of the dopants. We present theoretical evidence in support of this hypothesis through the analysis of electronic and vibrational interactions between dopant atoms with the host framework. In particular, the contrasting behavior of two clathrates: the glasslike thermal conductivity in Na 8 Si 46 and the normal behavior in Cs 8 Sn 44 can be rationalized.